Comparing Model Projections with Observations: Birds
The IPCC characterizes global warming as detrimental for most of the planet’s animals, including birds, even suggesting rising temperatures could drive many of them to extinction. When contemplating the special abilities of these winged creatures—such as the ability to fly—one would think highly mobile species such as birds could overcome whatever degree of stress a temperature increase might impose upon them, merely by moving to more-hospitable habitat, of course, or to take advantage of whatever new opportunities global warming might present for them.
In what follows, we review several studies that indicate birds do indeed respond in such a manner. The studies also show birds are able to tolerate much greater heat stress than previously thought.
Thomas and Lennon (1999) helped initiate extinction concerns about birds when they analyzed temporal trends in the spatial distributions of British birds over a 20-year period of global warming, looking for climate-induced changes in their breeding ranges. In doing so, they found the northern margins of southerly species’ breeding ranges shifted northward by an average of 19 km from 1970 to 1990, while the southern margins of northerly species’ breeding ranges shifted, in the mean, not at all. British birds have expanded their ranges in the face of global warming, clearly a positive response that makes extinction much less a possibility than it was before the warming.
Simultaneously, Brown et al. (1999) had been studying a natural population of individually recognizable, color-banded Mexican jays in the Chiricahua Mountains of Arizona (USA) over the period 1971–1998 for trends in egg-laying dates and monthly minimum air temperatures. Over the 29-year period of their study, they determined the date of first nest construction occurred 10.8 days earlier, while the date of first clutch in the population came 10.1 days earlier. These changes were associated with significant upward trends in monthly minimum temperature in the study area, of which they said that in many bird species “breeding is timed so as to have young in the nest when the principal food of the nestlings is at its peak.” With warmer minimum temperatures occurring earlier and earlier over their study period, they suggested this climatic trend could be producing an earlier abundance of such food, which would help explain the increasingly earlier egg-laying date.
The three researchers also identified a second way in which earlier-occurring warmer night temperatures might lead to earlier breeding dates in birds: by alleviating thermal stresses on females on cold nights. Citing several studies that had revealed similar breeding trends in European birds, they suggested the “recognition of similar trends on both continents in very different environments is consistent with the interpretation that some avian populations are already responding to climate changes in the last 29 years or so.” And once again, these widespread changes are positive in nature, for not only are bird ranges increasing in size as air temperatures rise, the temporal availability of food needed to sustain important life processes is advancing in synchrony with the timing of egg-laying.
Five years later in Europe, Brommer (2004) categorized birds of Finland as either northerly (34 species) or southerly (116 species) and quantified changes in their range margins and distributions from two atlases of breeding birds, one covering the period 1974–79 and one covering the period 1986–89, in an attempt to determine how the two groups of species responded to what he called “the period of the earth’s most rapid climate warming in the last 10,000 years,” citing McCarthy et al. (2001). This exercise revealed southern species experienced a mean poleward advancement of their northern range boundaries of 18.8 km over the 12-year period. However, the southern range boundaries of the northern species remained essentially unaltered. Noting similar results had been obtained for birds in the United Kingdom (Thomas and Lennon, 1999) and other species (primarily butterflies) elsewhere (Parmesan, 1996; Parmesan et al., 1999), Brommer concluded “in general, for Northern Hemisphere species, southerly range margins of species are less responsive to climate change than the northerly margins.” This demonstrates once again that the ranges of birds (and possibly other animals) in a warming world will likely increase in size, as their northern range boundaries expand poleward and upward while their southern range boundaries remain largely unaltered, which should render them less subject to extinction than they are currently or have been in the past.
Also working in Europe, and using data from the Breeding Bird Atlas of Lake Constance, which borders Germany, Switzerland, and Austria, Lemoine et al. (2007) analyzed the impact of land-use and climate changes on the region’s abundance of Central European birds between the periods 1980–1981 and 1990–1992, and between 1990–1992 and 2000–2002. This work revealed, in their words, “the total number of [bird] species in the Lake Constance region increased from 141 species in 1980 to 146 species in 1990 and to 154 species in 2000,” while “winter temperatures increased by 2.71°C and spring temperatures increased by 2.12°C over the 23 years from the first to the last census.” These and other data led them to conclude, “increases in temperature appear to have allowed increases in abundance of species whose range centers were located in southern Europe and that may have been limited by low winter or spring temperature.” In addition, they report “the impact of climate change on bird populations increased in importance between 1990 and 2000 and is now more significant than any other tested factor.” This is a very important finding because the warming has tremendously benefited European birds and helped to buffer them against extinction.
Contemporaneously, for the portion of the United States east of the Rocky Mountains, Hitch and Leberg (2007) used data from the North American Breeding Bird Survey to evaluate shifts in the northern range boundaries of 26 species of birds with southern distributions and the southern range boundaries of 29 species of birds with northern distributions between the periods 1967–1971 and 1998–2002. They found the northern margins of the southern group of birds showed significant northward shifts that averaged 2.35 km per year for all species studied, which finding they describe as being “consistent with the results of Thomas and Lennon (1999) from Great Britain.” Also in agreement with the observations about British birds, they determined “levels of warming do not appear to be so great [that] they are forcing birds to abandon the southernmost portions of their distributions,” a finding that is being replicated around the world.
Halupka et al. (2008) documented various breeding parameters of reed warblers (long-lived passerine birds that spend their winters in Africa but breed in the reed beds of marshlands in the Palaearctic, with some of them nesting in fishponds of southwest Poland) during 12 breeding seasons (1970–73, 1980–83, 1994, 2003, and 2005–06) that encompassed the period 1970–2006. They compared trends in what they measured with concomitant trends in mean monthly temperatures. This work revealed that mean breeding season (April–August) temperature increased significantly between 1970 and 2006, as did the mean temperature of each individual month of the breeding season, with the average temperature for the May–July period rising by 2°C. They found that in 2005 and 2006, egg-laying started three weeks earlier than in 1970 (as assessed by the first-egg date of the earliest pair of breeding birds), and the median first-egg date shifted forward in time by 18 days.
The end of egg-laying, however, did not change significantly in either direction, so there was a corresponding increase in the length of the egg-laying period, and with this longer laying period available to them, more birds were able to rear second broods. In the 1970s and 1980s, for example, the Polish researchers report, “only about 0–15% of individuals laid second clutches,” but “between 1994 and 2006 up to 35% of birds reared second broods.” In addition, they report, “during seasons with warm springs, early nests were better protected by being hidden in newly emerged reeds” and “as a result, these nests suffered fewer losses from predation.” They concluded, “the studied population of reed warblers benefits from climate warming.”
Another case in point was described by Jensen et al. (2008), who wrote, “global climate change is expected to shift species ranges polewards, with a risk of range contractions and population declines of especially high-Arctic species,” citing contentions of the Arctic Climate Impact Assessment (ACIA, 2005). To evaluate this claim, they constructed species distribution models for the Svalbard-nesting pink-footed goose (Anser brachyrhynchus), in order to “relate their occurrence to environmental and climatic variables.” They then used the most parsimonious of these models to “predict their distribution under a warmer climate scenario,” based upon “mean May temperature, the number of frost-free months and the proportion of moist and wet moss-dominated vegetation in the area,” the latter of which is “an indicator of suitable feeding conditions.”
The results of this exercise indicated, in the words of the six scientists, that global warming “will have a positive effect on the suitability of Svalbard for nesting geese in terms of range expansion into the northern and eastern parts of Svalbard which are currently unsuitable.” They also noted this result does not even consider the likelihood that glaciers will decrease in size and expose still more potential nest sites. Thus they concluded their paper by stating increased temperatures could release the population of pink-footed geese from the “present density-dependent regulation during the nesting period,” and “elongation of the frost-free season in Svalbard may relax their dependence on the acquisition of body stores before arrival (so-called ‘capital’ breeding, sensu Drent and Daan, 1980), so that geese will have more time to acquire the necessary resources upon arrival and still breed successfully,” noting “both factors are likely to have a positive effect on the population growth.”
In another relevant study, based on bird-ringing records covering a time span of 41 years (1964–2004), Husek and Adamik (2008) “documented shifts in the timing of breeding and brood size in a long-distance migrant, the red-backed shrike (Lanius collurio) from a central European population,” after which they compared their results with the climatic history of the region over the same time period. They thus determined temperatures in May significantly increased over the period of their study, and they state, “in line with this increasing May temperature” there was “a 3– to 4–day shift towards earlier breeding.” This pattern, in their words, “is consistent with the results of similar studies on other long-distance migrating songbirds (e.g., Dunn, 2004).” In addition, they report, there was “an increase in brood size by approximately 0.3 nestlings since 1964.” And of this latter finding they state, “given that early broods are usually larger (Lack, 1968; this study) and that they have a higher nest success (Muller et al., 2005), this may have a positive effect on future population increases as the temperature continues to rise.”
About the same time, Seoane and Carrascal (2008) wrote, “it has been hypothesized that species preferring low environmental temperatures, which inhabit cooler habitats or areas, would be negatively affected by global warming as a consequence of the widely accepted increase of temperature during the last two decades,” while noting “this effect is assumed to be more intense at higher latitudes and altitudes because these areas seem to be changing more rapidly.” They devised a study “to assess whether population changes agree with what could be expected under global warming (a decrease in species typical of cooler environments),” focusing on birds.
Working in the Spanish portion of the Iberian Peninsula in the southwestern part of the Mediterranean Basin, the two researchers determined breeding population changes for 57 species of common passerine birds between 1996 and 2004 in areas without any apparent land-use changes. This work revealed, in their words, that “one-half of the study species showed significant increasing recent trends despite the public concern that bird populations are generally decreasing,” while “only one-tenth showed a significant decrease.”
In discussing their findings, Seoane and Carrascal state, “the coherent pattern in population trends we found disagrees with the proposed detrimental effect of global warming on bird populations of western Europe.” They are not the only ones to have come to this conclusion. They noted, for example, “one-half of terrestrial passerine birds in the United Kingdom exhibited increasing recent trends in a very similar time period (1994–2004),” citing Raven et al. (2005), and they explained “there is also a marked consistency between the observed increasing trends for forest and open woodland species in the Iberian Peninsula and at more northern European latitudes in the same recent years,” citing Gregory et al. (2005). Likewise, they indicated “Julliard et al. (2004a), working with 77 common bird species in France, found that species with large ecological breadth showed a tendency to increase their numbers throughout the analyzed period.”
Commenting further on their findings, Seoane and Carrascal state that in their study, “bird species that inhabit dense wooded habitats show striking patterns of population increase throughout time.” Noting “this is also the case with those bird species mainly distributed across central and northern Europe that reach their southern boundary limits in the north of the Iberian Peninsula,” they theorize “these short- to medium-term population increases may be due to concomitant increases in productivity.” In support of this notion they cite the thinking of Julliard et al. (2004b) and the empirical observations of Myneni et al. (1997), Tucker et al. (2001), Zhou et al. (2001), Fang et al. (2003), and Slayback et al. (2003), whose work figured prominently in establishing the reality of the late twentieth-century warming- and CO2-induced greening of the Earth phenomenon, which has produced, in the words of the Spanish scientists, “an increase in plant growth or terrestrial net primary production in middle latitudes of the Northern Hemisphere since the 1980s, particularly in forest environments.”
It should be abundantly clear from these several observations that the supposedly unprecedented warmth of the past two decades has not led to what Seoane and Carrascal call “the proposed detrimental effect of global warming on bird populations of western Europe.”
After observing two second clutches in a newly established population of tree swallows in the Shenandoah Valley of Virginia (USA), Monroe et al. (2008) monitored all late nests in the following two breeding seasons to see what they could learn about the phenomenon. This surveillance revealed, “among all females nesting in the early breeding rounds of 2006 and 2007, 5% of birds with successful first clutches later laid second clutches.” In addition, they report the mean productivity for double-brooded females for 2006–2007 was 4.4 ± 1.3 fledglings from first clutches and 3.4 ± 0.8 from second clutches, so “double-brooded females significantly increased their total annual productivity compared to birds nesting only in the early rounds of breeding.” In fact, the productivity of the double-brooded females was approximately 75 percent greater than that of the single-brooded females. And in summarizing their findings in the concluding paragraph of their paper, Monroe et al. write, “in general, late summer and fall nesting among North American birds is underappreciated and may be increasing due to global warming,” citing the work of Koenig and Stahl (2007).
Noting “climate envelopes (or the climatic niche concept) are the current methods of choice for prediction of species distributions under climate change,” Beale et al. (2008) remind us that “climate envelope methods and assumptions have been criticized as ecologically and statistically naive (Pearson and Dawson, 2003; Hampe, 2004),” and “there are many reasons why species distributions may not match climate, including biotic interactions (Davis et al., 1998), adaptive evolution (Thomas et al., 2001), dispersal limitation (Svenning and Skov, 2007), and historical chance (Cotgreave and Harvey, 1994).” To shed more light on the subject, they evaluated the degree of matchup of species distributions to environment by generating synthetic distributions that retained the spatial structure of observed distributions but were randomly placed with respect to climate. More specifically, “using data on the European distribution of 100 bird species, [they] generated 99 synthetic distribution patterns for each species,” and “for each of the 100 species, [they] fitted climate envelope models to both the true distribution and the 99 simulated distributions by using standard climate variables.” They then determined the goodness-of-fit of the many distribution patterns, because, as they note, “there has been no attempt to quantify how often high goodness-of-fit scores, and hence ostensibly good matches between distribution and climate, can occur by chance alone.”
In a rather surprising result, the three U.K. researchers determined “species-climate associations found by climate envelope methods are no better than chance for 68 of 100 European bird species.” And, they write, “because birds are perceived to be equally strongly associated with climate as other species groups and trophic levels (Huntley et al., 2004),” their results “cast doubt on the predictions of climate envelope models for all taxa.” They conclude, “many, if not most, published climate envelopes may be no better than expected from chance associations alone, questioning the implications of many published studies.” The bottom line with respect to our stewardship of the Earth is thus well described by their conclusion: “scientific studies and climate change adaptation policies based on the indiscriminate use of climate envelope methods irrespective of species sensitivity to climate may be misleading and in need of revision,” as is also evident from the results of the many other studies reviewed in this brief analysis of the subject.
Grandgeorge et al. (2008) analyzed population sizes and phylogenetic and spatial structures of British and Irish seabirds based on “(1) presence or absence of the seabird species in the different counties of Britain and Ireland between 1875 to 1900 and 1968 to 1972, (2) seabird breeding censuses of Britain and Ireland from 1969 to 1970, 1985 to 1988 and 1998 to 2002, (3) at-sea abundance and distribution surveys of seabirds in the North Sea from 1980 to 1985 and 1990 to 1995, and (4) a bioenergetics model to estimate energy expenditures for 40 seabird species.” This work revealed, in their words, “a marked expansion in the breeding range of seabirds in Britain and Ireland between 1875 and 1972.” In addition, they report total seabird numbers “increased at an average rate of 1% per annum between 1969 and 2002, with a related increase of 115% in predicted total seabird predation.” What is more, they state, “between 1875 and 1972 no seabird species was lost and there was an overall expansion in breeding range of the seabird population of Britain and Ireland, with the number of counties occupied increasing from 31 to 47.”
In light of these findings, the six scientists concluded, “the seabird community of Britain and Ireland has been remarkably resilient to environmental change in the 20th century.” In fact, it “prospered during the 20th century,” and “significantly raised ocean temperatures in the North Sea (Beaugrand, 2004)” may even have “created more favorable environmental conditions for some seabird species,” citing the work of Thompson (2006). These conclusions are of course very different from the “end of the world” scenarios painted by the IPCC.
In much the same vein, Brommer (2008) wrote that a “population-level change expected under a climate-warming scenario is a poleward shift in the distribution of organisms,” and he stated it is thus believed by many that birds that “do not shift their range margin consist of species that are declining, and would therefore be of particular management concern.” A few years earlier, Brommer (2004) had measured the range sizes and northern range margin locations of 116 bird species with a predominantly southern distribution in Finland, and of those species he noted “the trend slope describing the change in their abundance for the period 1983–2005 was calculated for 53 species by Vaisanen (2006).” This, he noted, resulted in “the largest dataset available of the long-term trends in population numbers of Finnish birds that is comparable across species, because it has both been gathered and analyzed using the same procedures.” Therefore, to complete the behavioral picture of the latter 53 species, Brommer (2008) determined the concomitant changes in their northern range margins.
The Finnish bird specialist found “species foraging in wet habitats had experienced strong range margin shifts as compared with other feeding ecologies.” But he said he found “no evidence that those feeding ecological groups that showed a relatively small shift in range margin had experienced low population growth or a population decline.” Therefore, in discussing “the lack of correlation between the shift in range margin of the different feeding ecologies and the change in their mean abundance,” Brommer stated this real-world finding “is contrary to expected under a climate-change scenario, because, all else being equal, a clear range-margin shift should indicate a good capacity to track climatic change, which should result in a more positive trend in abundance if climate change is indeed the main driver of population-level change.”
In another revealing study, Maclean et al. (2008) analyzed counts of seven wading bird species—the Eurasian oystercatcher, grey plover, red knot, dunlin, bar-tailed godwit, Eurasian curlew, and common redshank—made at approximately 3,500 different sites in Belgium, Denmark, France, Germany, Ireland, the Netherlands, and the United Kingdom on at least an annual basis since the late 1970s. They did this in order to determine what range adjustments the waders may have made in response to regional warming, calculating the weighted geographical centroids of the bird populations for all sites with complete coverage for every year between 1981 and 2000.
This work revealed, in the words of the seven scientists, that “the weighted geographical centroid of the overwintering population of the majority of species has shifted in a northeasterly direction, perpendicular to winter isotherms,” with overall 20-year shifts ranging from 30 to 119 km. In addition, they report, “when the dataset for each species was split into 10 parts, according to the mean temperature of the sites, responses are much stronger at the colder extremities of species ranges.” In fact, they found, “at warmer sites, there was no palpable relationship between changes in bird numbers and changes in temperature.” They concluded, “range expansions rather than shifts are occurring” as the planet warms.
In discussing the significance of their findings, the members of the international research team state the commonly used climate-envelope approach to predicting warming-induced species migrations “essentially assumes that as climate alters, changes at one margin of a species’ range are mirrored by those at the other, such that approximately the same ‘climate space’ is occupied regardless of actual climate,” whereas the evidence suggests “that this may not be the case: climate space can also change.”
In further discussing their important finding, Maclean et al. write, “it is actually not surprising that responses to temperature appear only to be occurring at the colder extremities of species ranges,” for “it has long been known that it is common for species to be limited by environmental factors at one extremity, but by biological interactions at the other,” citing the work of Connell (1983) and Begon et al. (2005). They concluded it is likely “the warmer extremities of the species ranges examined in this study are controlled primarily by biotic interactions, whereas the colder margins are dependent on temperature.”
Dyrcz and Halupka (2009) examined long-term responses in the breeding performance of Great Reed Warblers (living on fish ponds near Milicz in southwest Poland) during various years from 1970 to 2007 (1970–1974, 1981–1984, 1997, and 2004–2007), over which period mean temperatures during the egg-laying months of the species (May–July) rose by a remarkable 2.2°C, from 15.3 to 17.5°C. The two researchers found a “significant advancement in both earliest and annual median first-egg-laying dates” that “correlated with temperature increases early in the season.” Latest first-egg-laying dates remained unchanged, as did several other breeding statistics, including clutch size, nest losses, and number of young per nest. Consequently—and contrary to a Bavarian population of Great Reed Warblers that also advanced its latest first-egg-laying date—the Polish bird population expanded its breeding season in response to regional warming, whereas the Bavarian birds merely shifted theirs, as documented by Schaefer et al. (2006).
The two researchers thus concluded, “the studied population does not benefit from climate warming (as found in Bavaria), but apparently does not suffer,” reiterating “the Great Reed Warbler has adapted well ... by shifting the timing of breeding.” The results of their study, they state, “do not confirm the prediction of Bairlein and Winkel (2000) that long-distance migrants would suffer due to climate change.” In addition, they write, a comparison of their data with that of the Bavarian population “provides evidence that different populations of the same species can adapt in different ways to climate change,” noting “this was also previously found for woodland species,” citing the work of Visser et al. (2002) and Sanz (2003).
Moving from Europe to Asia, Qian et al. (2009) compiled a comprehensive dataset of bird species richness in China—based on pertinent scientific literature published over the past three decades—for 207 localities (the vast majority of which were national nature reserves with a mean area of 3270 km2), which they then analyzed for their relationships to 13 different environmental variables. In the words of the authors, “of all environmental variables examined, normalized difference vegetation index [NDVI], a measure of plant productivity, is the best variable to explain the variance in breeding bird richness.” More specifically, they determined that four of the 13 variables they tested explained 45.3 percent of the total species richness variance, with 21.2 percent being accounted for by NDVI, 12.5 percent by elevation range, and 11.6 percent by annual potential evapotranspiration and mean annual temperature together. In addition, they note the two most important predictors of their study (NDVI and elevation range) “have been found to be major predictors for breeding bird richness in other regions and the whole of the globe, indicating that the finding of [their] study at a smaller scale is to a large degree consistent with those of previous studies of breeding birds at larger scales.”
These findings make a good deal of sense, for in a major review of plant-animal interactions in 51 terrestrial ecosystems conducted 20 years earlier, McNaughton et al. (1989) found the biomass of plant-eating animals is a strongly increasing function of aboveground primary production, and in a subsequent review of 22 aquatic ecosystems, Cyr and Pace (1993) found the herbivore biomass of watery habitats also increases in response to increases in vegetative productivity. As such, it should be abundantly clear that greater plant productivity—both terrestrial and aquatic—leads to greater populations of plants and the animals that feed upon them, which should therefore lead to greater ecosystem biodiversity, because each species of plant and animal must maintain a certain “critical biomass” to sustain its unique identity and ensure its long-term viability. And that’s where atmospheric CO2 enrichment enters the picture: It increases plant productivity, which supports more animal life, which leads to greater animal biodiversity, which is good for the planet and good for mankind, the stewards and beneficiaries of all life upon it.
Moving on from Asia to Africa, Hockey and Midgley (2009) write, “in the influential fourth assessment report of the Intergovernmental Panel on Climate Change, Rosenzweig et al. (2007) tested several thousand time-series data sets for changes in species behavior and geographic range consistent with climate change, reaching the conclusion that it is very likely that climate change is driving changes in natural biological systems.” However, they state “the use of such large data sets in meta-analyses may discourage the close inspection of observations and result in naively misattributing observed shifts to climate when other explanations may be more parsimonious.”
To test this hypothesis, Hockey and Midgley “collated information about recent range changes in South African birds, specifically indigenous species that have colonized the Cape Peninsula, at the south-western tip of Africa in the Western Cape province, since the 1940s,” where they state there have been “widespread anthropogenic changes of many kinds to the landscape, including urbanization, commercial afforestation and the introduction and spread of invasive alien trees, most of which occurred before climate change accelerated in the 1970s.”
The two researchers found the colonization events “concur with a ‘climate change’ explanation, assuming extrapolation of Northern Hemisphere results and simplistic application of theory,” but “on individual inspection, all bar one may be more parsimoniously explained by direct anthropogenic changes to the landscape than by the indirect effects of climate change.” Also, “no a priori predictions relating to climate change, such as colonizers being small and/or originating in nearby arid shrub-lands, were upheld.”
In discussing their findings, the South African scientists state their work suggests “observed climate changes have not yet been sufficient to trigger extensive shifts in the ranges of indigenous birds in this region, or that a priori assumptions are incorrect.” Either way, they continue, “this study highlights the danger of naive attribution of range changes to climate change, even if those range changes accord with the predictions of climate-change models,” because “misattribution could distract conservationists from addressing pressing issues involving other drivers of biodiversity change such as habitat transformation, and obscure important lessons that might be learned from the dynamics that pertain to such changes.”
Also in 2009, but farther south in the Southern Hemisphere, Huang et al. (2009) evaluated paleo-evidence for penguin populations at Gardner Island in East Antarctica. According to the five researchers, penguins colonized the site shortly after it became ice-free 8,500 years ago. A pronounced population peak is also evident in the data from about 4,700 to 2,400 BP, which corresponds closely to a substantially warmer period at this site. While this is interesting in and of itself, the authors document four other studies (all of the studies conducted to date) showing a penguin optimum roughly 3,000 to 4,000 years ago and coinciding with notably warm conditions. Together, these five studies encompass East Antarctica, the Ross Sea region, and the West Antarctic Peninsula. Studies of elephant seals (Hall, 2006) show they, too, were found closer to the South Pole during past warmer periods. And since all data currently available point to penguins having been most abundant during the warmest period of the Holocene several thousand years ago, it would seem reasonable to presume that penguins would respond positively, not negatively as the IPCC contends, to any future warming that may occur.
Additional support for this thesis comes from Carey (2009), who notes “organisms living today are descended from ancestors that experienced considerable climate change in the past,” and she thus suggests “species that persist into future climates may be able to do so in part owing to the genetic heritage passed down from ancestors who survived climate changes in the past.” She also states, “if climate change were the only new challenge facing birds, one might imagine that many species could become adapted to new conditions and survive with existing population variability and the genetic information that their ancestors used to survive past climate change.”
In another study exploring bird responses to past periods of climate change, Tyrberg (2010) compared fossil avifaunas of the Last Interglacial (LIG), about 130,000 to 117,000 years ago, from multiple sites around the world to the modern avifaunas found in those locations. During much of this time interval, the globe was about 2°C warmer than it is today, and it was up to 10°C warmer in much of the Arctic. For fossil faunas, however, only species that still exist were included in the comparisons, because during the cold period of the last glacial, which followed the LIG, many species went extinct due to the cold, and climate tolerance can be determined reliably only for living species. Based on the areal distributions of fossil avifaunas in different parts of the world, therefore, regions were delineated in which many of the identified species coexisted, and if it was found the same sets of species share the same common ranges today, it was concluded that the avifauna, as a whole, did not respond to any significant degree to the warmer temperatures of the LIG.
For sites that were about 2°C warmer during the LIG—including four sites in Florida, one in Alaska, two in Germany, and one in New Zealand—species present during the LIG were found to be the same as the species that inhabit those regions today. At a site in Wales, however, where LIG temperatures were a full 4°C warmer than today, the fossil avifauna was similar to the current avifauna of Spain and Portugal, indicating the fossil avifauna had indeed located themselves further northward during the LIG in response to the much greater warmth of that period. And in another exception to the study’s primary findings, the LIG avifauna at a site in North Africa (which is now desert with no birds present) was similar to that of the area south of the desert today, indicating—in light of the fact that during the LIG the Sahara desert received much more rainfall than it does currently—precipitation was the overriding factor determining both the current and fossil avifauna choice of territories.
In light of these and other findings, Tyrberg concludes “as for the effect of the generally warmer climate during the LIG it seems clear that differences on the order of 2°C or less, both on land and in sea-surface temperatures, are barely, if at all, detectable in the avifaunas.”
Another concern about the effects of potential global warming on birds is that various links of certain food chains may not respond in a compatible manner in terms of the temporal development of the different stages of their life cycles, leading to a serious mismatch among the unique needs of different ecosystem trophic levels that could well spell disaster for some species. This concept has been said by Visser and Both (2005) to constitute an “insufficient adjustment” to climate change.
In a study designed to explore this phenomenon for certain elements of an important ecosystem of Central Europe, Bauer et al. (2010) examined the responses to 47 years of warming (1961–2007) of (1) the time of leafing-out of dominant English oak (Quercus robur) trees at four different research sites in the Czech Republic that are located in full-grown, multi-aged floodplain forests that had been under no forestry management; (2) the time of appearance of the two most abundant species of caterpillars in the floodplain forests—the winter moth (Operophtera brumata) and the tortrix moth (Tortrix viridana); and (3) the first and mean laying dates of two of the ecosystem’s most common birds: great tits (Parus major) and collared flycatchers (Ficedula albicollis).
According to the researchers, “mean annual temperature showed a significant increase of 0.27–0.33°C per decade, with approximately the same magnitude of change during spring at all sites.” They also found, “on average (all four sites), the bud burst date for English Oak has advanced by 7.9 days and full foliage by 8.9 days, with approximately the same shifts being recorded for the peak of the beginning and end of frass for herbivorous caterpillars,” which was the observational variable they used to characterize the caterpillars’ presence. Last, they determined “the first laying date of Great Tits has advanced by between 6.2 to 8.0 days,” while “the mean laying date has advanced by 6.4 to 8.0 days.” Likewise, they found the “Collared Flycatcher first laying date has advanced by 8.5 to 9.2 days over the past 47 years, and the mean laying date by 7.7 to 9.6 days.”
With respect to the importance of their findings, Bauer et al. state that because “trends in the timing of reproduction processes of both bird species are coherent with the trends in development of English Oak and with peak herbivorous caterpillar activity,” it is readily apparent that in this specific food chain the common shifting of the different organisms’ phenological stages toward the beginning of the year “does not appear to have led to mistiming in the trophic food chain.” Hence, there is reason to believe other food chains also may not be as seriously disrupted by global warming as many have postulated they could be. Of course, much more work of this nature is needed before any generalities are warranted.
In a contemporaneous study, Van Buskirk et al. (2010) write, “recent climate change has caused comparatively rapid shifts in the phenology and geographic distributions of many plants and animals,” but “there is debate over the degree to which populations can meet the challenges of climate change with evolutionary or phenotypic responses in life history and morphology,” which for a warming climate includes a reduction in body size. They devised an experimental strategy to further explore the issue. Specifically, they studied the body sizes of birds captured in mist-nets and traps between June 1961 and November 2006 at the Powdermill Nature Reserve—a field station operated by the Carnegie Museum of Natural History in Pennsylvania (USA) at a location that is broadly representative of bird communities in the Appalachian region of eastern North America. At this location, (1) 35 mist nets were operated five to six days per week during spring and autumn migrations, (2) a reduced number of nets was used during summer, and (3) birds for winter banding were caught in wire traps when the temperature was below freezing.
The three researchers report migrating birds captured at the banding station “have steadily decreasing fat-free mass and wing chord since 1961, consistent with a response to a warmer climate” and confirming that “phenotypic responses to climate change are currently underway in entire avian assemblages,” where “size was negatively correlated with temperature in the previous year, and long-term trends were associated with the direction of natural selection acting on size over the winter.” In addition, they note “species undergoing the strongest selection favoring small wing chord showed the most rapid long-term declines in wing [size],” which suggests, as they describe it, that “phenotypic changes are therefore in line with the prevailing selection regime.” Noting “in summer, 51 of 65 breeding species had negative slopes of mass against year, 20 of 26 wintering species had negative slopes, 60 of 83 spring migrants had negative slopes, and 66 of 75 autumn migrants had negative slopes,” Van Buskirk et al. state their results “offer compelling evidence that climate change has already produced observable adaptive shifts in morphology, behavior, and phenology of a great many species,” which suggests these birds have evolved a capacity for rapid phenotypic shifts to optimum body mass in response to climate fluctuations.
Popy et al. (2010) employed data from two bird atlas surveys performed on a 1 km by 1 km grid (the first in 1992–94 and the second in 2003–05) in an alpine valley in the Italian Piedmont to see if there was any evidence for an upward shift in the ranges of 75 bird species (68 of which were detected in both surveys) over this period, during which time the region’s mean air temperature rose by 1.0°C. Their results indicated “the number of species whose mean elevation increased (n = 42) was higher than the number whose mean elevation decreased (n = 19), but the overall upward shift [29 m] was not significantly different from zero.” In addition, they state even the 29 m increase was “smaller than would be expected from ‘climatic envelope’ models,” as the “1.0°C increase in temperature would be equivalent to c. 200 m in elevation, based on an average gradient of -0.5°C per 100 m.” In addition, they indicate, “at the European scale, no overall expansion or contraction of the distributions of the studied species was detected.” In light of their findings, as well as those of others they cite, Popy et al. thus conclude, “until a better understanding of the underlying mechanisms is achieved, predictions based only on ‘climate envelope’ models should be either validated or considered cautiously.”
In one final study to be considered here, Thomas et al. (2010) write, “the timing of annual breeding is a crucial determinant of reproductive success, individual fitness, and population performance, particularly in insectivorous passerine birds,” because “by synchronizing hatching with the narrow time window of maximal food abundance, parents can enhance their reproductive success through an increase in offspring growth rate and body condition, survival to fledging, and subsequent recruitment into the breeding population.” Many people worry, in this regard, that global warming may upset such biological synchronizations, leading to downward trends in the populations of many species of birds and other animals.
Thomas et al. studied this situation using “confirmatory path analysis and data on laying date” for two populations of blue tits in northern Corsica (Muro and Pirio) in order to determine “how laying date is related to spring temperatures and vegetation phenology”—as these two factors figure highly in determining the peak period of blue tit food abundance (in this case caterpillars)—in order to provide “critical information on how passerine birds may adjust breeding in the face of directional climate change [such as regional warming] by identifying the causal paths that link laying date and environmental cues.” The French and Canadian researchers discovered, in their words, “Blue Tits use a cue system that is context specific to fine-tune laying dates to match local conditions both on a spatial (habitat) scale and on a temporal (interannual) scale,” and their “reliance on both temperature and phenology when breeding late in the season, as occurs in most populations where tits have been intensively studied north of the Mediterranean region, satisfactorily explains how these populations can advance breeding in response to rising spring temperatures while maintaining a relatively large variation in the onset of breeding on a local spatial scale.”
In discussing their findings, Thomas et al. acknowledge that “if a single environmental feature [such as temperature] were responsible for the timing of breeding, climate change could cause a severe decline in breeding success, with negative demographic consequences.” However, they state they “have not detected any consistent mismatch between Blue Tit breeding dates and caterpillar peak [abundance] dates over the 14 and 21 years for which they have data for Muro and Pirio, respectively.” Their findings, they conclude, “offer some hope that breeding populations will respond well to global warming.”
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