One of the great horror stories associated with predictions of CO2-induced global warming is the claim that the number of birds and their habitat areas will decline. Some climate alarmists go so far as to contend that global warming will result in the extinction of a number of bird species. Although some changes in bird populations and their habitat areas have been documented in the literature, linking such changes to global warming remains an unsettled matter; and when there have been changes, they are not nearly as horrific as climate alarmists make them out to be. In fact, much of the change could, and should, be viewed as positive.
In the Chiricahua Mountains of Arizona, Brown et al., (1999) discovered that Mexican jays constructed their nests 10.8 days earlier in 1998 than they did in 1971. Additionally, they report that the date of first clutch occurred 10.1 days earlier by the end of their study. But are the changes in these parameters the result of global warming? The authors note that "breeding is timed so as to have young in the nest when the principal food of the nestlings is at its peak." The obvious conclusion to be drawn from this observation is that something has been causing such food to become available at an increasingly earlier date over the past three decades; and that something is probably a combination of earlier-occurring spring warmth and the ongoing rise in the air's CO2 content [see Trees (Early Spring Growth) in our Subject Index].
Bird breeding ranges are also changing. In an analysis of many bird surveys conducted along the Thelon River and its tributaries in the Canadian Northwest Territories from the 1920s through much of the 1990s, Norment et al. (1999) found three bird species to have expanded their breeding ranges southward, nine northward, and sixteen were observed to be new to the area. The authors note that the primarily northward range expansions may be explained by "a recent warming trend at the northern treeline during the 1970s and 1980s." Alternatively, they note that the influx of new species may also be due to "increasing populations in more southerly areas." In either case, we have a situation where birds and mammals appear to be faring quite well -- could it actually be said they are thriving? -- in the face of increasing temperatures in this forest-tundra landscape, a phenomenon that also appears to be operative in Norway (Saether et al., 2000).
Another example of changes in bird breeding ranges comes from the study of Thomas and Lennon (1999). From 1970 to 1990, the authors observed that the northern breeding range boundaries of southern species in Britain shifted northward by an average of 19 km, while the southern boundaries of the ranges of northern species shifted not at all. We view the northward range expansions as opportunistic responses to regional warming and the constancy of southern boundaries as a consequence of the ongoing rise in the air's CO2 content neutralizing what would otherwise have been an impetus for the southern boundaries of the plant-insect associations to which the birds are adapted to shift northward as well [see Growth Response to CO2 with Other Variables (Temperature) in our Subject Index].
Another interesting observation to come out of Britain concerns bird population changes. Krebs et al. (1999) discovered that 13 species that live exclusively in farmland declined by an average of 30% between 1968 and 1995, but that 29 species of habitat generalists increased by an average of 23%. The authors attribute the 13-species decline to the intensification of agriculture that has led to the loss of heterogeneous landscapes that are particularly beneficial to these species. Although they are silent on the population increases in the 29 generalist species, we believe they are responding to a temperature- and CO2-induced increase in natural (non-farmland) vegetative productivity [see, again, Growth Response to CO2 with Other Variables (Temperature)].
Moving to colder regions of the earth, if the future is reflective of the past, seabirds of Greenland would likely welcome global warming. According to Wagner and Melles (2001), a significant number of seabirds inhabited the area around Liverpool Land, on the east coast of Greenland, during the Medieval Warm Period (900-1300 AD), yet there was little to no (inferred) bird presence for a several-hundred-year period prior to this time (Dark Ages Cold Period) and another significant absence of birds thereafter during the Little Ice Age, which marked "the coldest period since the early Holocene in East Greenland." As temperatures have risen over the course of the past 100 years, however, seabirds have once again begun to inhabit the area.
A warmer climate has also benefited birds on Australia's Heard Island, some 4000 kilometers southwest of Perth. Over the past five decades, during which time this sub-Antarctic island experienced a local warming of approximately 1°C, bird populations have expanded significantly. One of the real winners has been the King penguin, whose population "exploded from only three breeding pairs in 1947 to 25,000" in recent years (Pockely, 2001).
Penguins in Antarctica have also fared well in warmer climates. In response to dramatic warming observed on the western Antarctic Peninsula over the past several decades, the penguin population there has become more diverse as chinstrap and gentoo penguins have begun to take up residence among the long inhabiting Adelie penguin population (Smith et al., 1999). Additionally, a study of penguin populations on the Ardley Peninsula of maritime Antarctica by Sun et al. (2000) found that over the past 3,000 years the penguin population was lowest from 1,800-2,300 years BP during a period of low temperature.
Warmer temperatures have also been shown to benefit bird populations on shorter time scales. According to Brichetti et al. (2000), statistically significant differences exit in survival rates of the Mediterranean waterbird, Cory's Shearwater, between La Niņa (cooler) years and El Niņo (warmer) years, with survival rates of this species being greater during strong El Niņo years. On the other hand, Sillett et al. (2000) have reported that the annual survival of black-throated blue warblers in their tropical winter quarters in Jamaica is lower during El Niņo years and higher in La Niņa years. However, they say this result "is best explained by the impact of ENSO on local climate and a concomitant change in food availability for overwintering birds." Specifically, they note that reduced rainfall during El Niņo years in Jamaica "leads to a decreased amount of food available for warblers in the winter dry season and, hence, to lower survival." Back in New Hampshire, they also found that black-throated blue warbler fecundity is limited by food availability in much the same way, via its effects on fledgling weight. Specifically, when adults are feeding nestlings and dependent juveniles in the summer (when food is most limited), lepidopteran larval biomass (the warblers' primary prey in summer) is positively correlated with the Southern Oscillation Index; and with less larval biomass available in El Niņo years than in La Niņa years, "fledglings weighed less in El Niņo years relative to La Niņa years," and their survival was likewise less.
Finally, Lloyd et al. (1998) studied relationships between bird abundances and a number of large-scale vegetation features, including the density and distribution of mesquite trees, at the Buenos Aires National Wildlife Refuge in southeastern Arizona, in an effort to understand changes that occur within bird communities as a result of changes in ecosystem composition due to what we and many others believe to be the historical CO2-induced expansion of trees and other woody plants into this and other similar regions [see Trees (Range Expansions) in our Subject Index]. Although some individuals feel that woody-plant range expansions are due to overgrazing by cattle, Brown and Archer (1999) recently demonstrated that honey mesquite expansion into the southwestern United States occurred at a rapid rate, regardless of grazing pressure and soil moisture content. Thus, some other factor -- notably the increasing CO2 content of the air -- must have been responsible for the observed expansion of honey mesquite trees in this region. Be that as it may, the results of Lloyd et al.'s analysis showed that, of all the many variables they examined, only the density and distribution of mesquite trees were found to influence bird populations, with total bird abundance increasing with increasing mesquite density. In addition, they note that "greater bird species richness [was] found on plots with higher mesquite densities." Thus, the modest increase in the air's CO2 content over the past two centuries has likely had a pronounced positive effect on local bird biodiversity in the American southwest.
In conclusion, it is interesting to note that so many papers report positive impacts of warming -- and increasing atmospheric CO2 concentrations (via their effects on plants and the insects that feed on them) -- on bird populations. But, of course, such is only to be expected, as warmer temperatures expand the area of land over which birds can safely subsist, thereby offering them new and expanded opportunities for colonization.
References
Brichetti, P., Foschi, U.F and Boano, G. 2000. Does El Niņo affect survival rate of Mediterranean populations of Cory's Shearwater? Waterbirds 23: 147-154.
Brown, J.R. and Archer, S. 1999. Shrub invasion of grassland: Recruitment is continuous and not regulated by herbaceous biomass or density. Ecology 80: 2385-2396.
Brown, J.L., Shou-Hsien, L. and Bhagabati, N. 1999. Long-term trend toward earlier breeding in an American bird: A response to global warming? Proceedings of the National Academy of Science, U.S.A. 96: 5565-5569.
Krebs, J.R., Wilson, J.D., Bradbury, R.B. and Siriwardena, G.M. 1999. The second silent spring? Nature 400: 611-612.
Lloyd, J., Mannan, R.W., Destefano, S. and Kirkpatrick, C. 1998. The effects of mesquite invasion on a southeastern Arizona grassland bird community. Wilson Bulletin 110: 403-408.
Norment, C.J., Hall, A. and Hendricks, P. 1999. Important bird and mammal records in the Thelon River Valley, Northwest Territories: Range expansions and possible causes. The Canadian Field-Naturalist 113: 375-385.
Pockely, P. 2001. Climate change transforms island ecosystem. Nature 410: 616.
Saether, B.-E., Tufto, J., Engen, S., Jerstad, K., Rostad, O.W. and Skatan, J.E. 2000. Population dynamical consequences of climate change for a small temperate songbird. Science 287: 854-856.
Sillett, T.S., Holmes, R.T. and Sherry, T.W. 2000. Impacts of a global climate cycle on population dynamics of a migratory songbird. Science 288: 2040-2042.
Smith, R.C., Ainley, D., Baker, K., Domack, E., Emslie, S., Fraser, B., Kennett, J., Leventer, A., Mosley-Thompson, E., Stammerjohn, S. and Vernet M. 1999. Marine ecosystem sensitivity to climate change. BioScience 49: 393-404.
Sun, L., Xie, Z. and Zhao, J. 2000. A 3,000-year record of penguin populations. Nature 407: 858.
Thomas, C.D. and Lennon, J.J. 1999. Birds extend their ranges northwards. Nature 399: 213.
Wagner, B. and Melles, M. 2001. A Holocene seabird record from Raffles So sediments, East Greenland, in response to climatic and oceanic changes. Boreas 30: 228-239.