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Range Expansion (Animals - Birds) -- Summary
Could global warming decimate earth's biosphere? Climate alarmists have long claimed that the increase in temperature predicted to result from the ongoing rise in the air's CO2 content will be so great and occur so fast that many species of plants and animals will not be able to migrate poleward in latitude or upward in elevation rapidly enough to avoid extinction, as they are forced to seek cooler living conditions. However, as we explain in our major report The Specter of Species Extinction, there are many reasons for rejecting this contention, one of the most powerful being that increases in the air's CO2 content generally enable plants to endure warmer weather, with the result that there is little or no need for them to shift the heat-limiting warm-temperature boundaries of their ranges as temperatures rise. At the cold-limiting cool-temperature boundaries of their ranges, on the other hand, warming provides opportunities for plants to push poleward and upward, which results in expansions of their ranges and reductions in their risk of extinction. In this Summary we thus review a number of studies of birds, which suggest they may respond in like manner, mimicking the respective shifts and non-shifts of the cold- and heat-limited boundaries of the ranges of the plants and associated herbivore life forms on which they depend for food.

Thomas and Lennon (1999) analyzed the distributions of British birds over a 20-year period of global warming, looking for climate-induced changes in their breeding ranges. They found that 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 not at all, in the mean. This finding was stated by them to be similar to the results obtained for European butterflies, "for which the northern margins have expanded more than the southern margins have retracted," and they attributed both the bird results and the butterfly results to the effects of global warming as experienced in the regions studied.

Contemporaneously, Lloyd et al. (1998) evaluated the relationship between bird abundances and a number of large-scale vegetation features at the Buenos Aires National Wildlife Refuge in southeastern Arizona (USA) in an effort to understand changes within bird communities caused by changes in ecosystem composition, and, more specifically, changes associated with the documented expansion of trees into this region. Among the following variables, for example -- grass, herb and shrub cover, percent cover of native grasses, percent cover of an introduced grass, average size of mesquite trees, and the density of mesquite trees -- only the density and distribution of mesquite trees were found to influence bird populations; and in this regard, both total bird abundance and bird species richness were observed to increase with increasing mesquite density, as the trees expanded their ranges into what formerly were grasslands, which phenomenon was likely prompted by the historical and ongoing rise in the air's CO2 content (see Range Expansion (Plants) in our Subject Index).

Along the Thelon River and its tributaries in the Northwest Territories of Canada, Norment et al. (1999) compared the results of many surveys of bird populations conducted from the 1920s through much of the 1990s. Over this period, they found that three bird species expanded their breeding ranges southward, nine expanded northward, and sixteen were observed to be new to the area, concluding that the primarily northward range expansions were due to a warming trend at the northern treeline during the 1970s and 1980s. Alternatively, they opined that the influx of new species may have been due to increasing populations in more southerly areas. In either case, the region's birds appeared to be faring better than ever, helped along by both rising temperatures and atmospheric CO2 concentrations.

In a somewhat similar situation, Saether et al. (2000) studied the responses of a small passerine songbird of southern Norway (Cinclus cinclus) to variations in mean winter temperature and precipitation over a period of twenty years. After developing a model based on their observations, they used it to determine the response of the birds to a 2.5C increase in air temperature, such as is predicted to occur by several climate models in response to a doubling of the air's CO2 concentration. This work revealed that the Cinclus cinclus population of the region would likely increase by 50% or more, much as has been found to have already occurred, but to a lesser degree, among numerous species of butterflies and birds throughout Britain and other parts of Europe in response to the lesser warming of the 20th century.

In a study that called for caution in modeling bird responses to global warming, Visser et al. (2003) examined laying dates of 24 populations of Parus major and P. caeruleus in six European countries from 1979 to 1998, over which period several, but not all, of the locations studied exhibited increases in near-surface air temperature. They report finding that "the phenological response to large-scale changes in spring temperature varies across a species' range, even between populations situated close to each other." What is more, they found that "this variation cannot be fully explained by variation in the temperature change during the pre- and post-laying periods." Noting that their results "show the value of replicating population studies across parts of a species' range, as the effects of climate change may differ, even within a single species, on a small geographical scale," they suggest that great caution be exercised so as not to conclude too much from studies of bird responses to warming that are not massively replicated across large areas. Nevertheless, we note that the modeling study of Saether et al. produced results that were harmonious with what has been observed throughout most of Europe and elsewhere.

Also working in Europe with a bird described as "a topic of particular interest to ornithologists of the 19th and 20th century," due to "the rapid expansion of its range in historical times," Kinzelbach (2004) reexamined all records of the serin (Serinus serinus) in 16th-century Europe, including "both those already known and some that have been newly discovered." This work confirmed the findings of Mayr (1926), i.e., that "north of 48N there were no free-living populations of Serinus serinus in the 16th century." During that period, the serin may have attempted to expand its range, but Kinzelbach says it "was halted by colder periods of the Little Ice Age after 1585, only resuming a rapid expansion at the beginning of the 19th century," after which it was "able to expand its range from the Mediterranean region throughout large areas of Central Europe within a mere 200 years," over which time the temperature history of Esper et al. (2002) depicts post-Little Ice Age Northern Hemispheric warming beginning at about the same time the serin began expanding its range northward.

Working in Finland, Brommer (2004) categorized birds 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 calls "the period of the earth's most rapid climate warming in the last 10,000 years which started in 1976 (McCarthy et al., 2001)." This effort indicated that the southerly group of bird species experienced a mean poleward advancement of their northern range boundaries of 18.8 km over the 12-year period of supposedly unprecedented warming. However, the southern range boundaries of the northerly group of bird species were essentially unmoved by the skyrocketing temperature; and this result, in the words of the author, "did not change when raptors and threatened species were omitted from the analyses."

Noting that 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 that "in general, for northern hemisphere species, southerly range margins of species are less responsive to climate change than the northerly margins." Consequently, it can be appreciated that the ranges of such species in a warming world will actually 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.

Across the Atlantic Ocean, and for the portion of the United States located 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, when climate alarmists claim the earth warmed at a rate and to a level they describe as unprecedented over the past one to two millennia. In doing so, the two researchers determined that the northern margins of the southern group of birds showed significant northward shifts that averaged 2.35 km/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 behavior of British birds, they found that "the levels of warming do not appear to be so great they are forcing birds to abandon the southernmost portions of their distributions," so that once again, warming resulted in an expansion of bird ranges and a lessening of their extinction risk.

One year later, Beale et al. (2008) quantified "the match of species distributions to environment" by generating 99 synthetic species distributions for each of 100 species of birds -- based on data taken from the European Breeding Bird Atlas -- that retained "the spatial structure in the observed distributions but were randomly placed with respect to climate," after which they "fitted climate envelope models to both the true distribution and the 99 simulated distributions by using standard climate variables" and evaluated the results via a number of statistical tests. This exercise revealed that species-climate associations found by the climate envelope methods typically employed by climate alarmists "are no better than chance for 68 of 100 European bird species," which finding led them to state that "the distributions of most birds in our study are not strongly associated with the climate variables currently available." Hence, they concluded that "many, if not most, published climate envelopes may be no better than expected from chance associations alone, questioning the implications of many published studies." As a result, they also concluded that the climate envelope model "is certainly not a model that should inform policy," especially, we might add, as used by Al Gore and James Hansen.

Reinforcing this conclusion, 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 3500 different sites in Belgium, Denmark, France, Germany, Ireland, the Netherlands and the United Kingdom on at least an annual basis since the late 1970s. This they did in order to determine what range adjustments the waders may have made in response to concomitant regional warming, calculating the weighted (by count) 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 that "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 that "at warmer sites, there was no palpable relationship [our italics] between changes in bird numbers and changes in temperature." Hence, they concluded that "range expansions rather than shifts [our italics] are occurring" as the planet warms.

In discussing the significance of their findings, the members of the international research team state that the commonly used climate-envelope approach to predicting warming-induced species migrations (such as employed by climate alarmists) "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," but they say that their work suggests "this may not be the case: climate space can also change."

In further discussing their important finding, Maclean et al. say "it is actually not surprising that responses to temperature appear only to be occurring at the colder extremities of species ranges," for they note "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). Hence, they conclude it is likely that "the warmer extremities of the species ranges examined in [their] study are controlled primarily by biotic interactions, whereas the colder margins are dependent on temperature." We further note, in this regard, that Beale et al. (2008) report that "climate envelope methods and assumptions have been criticized as ecologically and statistically nave (Pearson and Dawson, 2003; Hampe, 2004)," and that "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)."

In conclusion, it is clear that where there has been confirmed regional warming over the time periods investigated, the vast majority of studies of bird range adjustments have revealed opportunistic poleward expansions of their cold-limited boundaries with little to no change in the locations of their (supposedly) heat-limited boundaries. This behavior is not one of birds rushing, or even inching, towards extinction, as many climate alarmists claim is occurring. Rather, it is one where they are doing just the opposite and fortifying themselves against the possibility of extinction.

References
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Begon, M., Townsend, C. and Harper, J. 2005. Ecology: From Individuals to Ecosystems. Blackwell, Oxford, UK.

Brommer, J.E. 2004. The range margins of northern birds shift polewards. Annales Zoologici Fennici 41: 391-397.

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Davis, A.J., Jenkinson, I.S., Lawton, J.H., Shorrocks, B. and Wood, S. 1998. Making mistakes when predicting shifts in species range in response to global warming. Nature 391: 783-786.

Esper, J., Cook, E.R. and Schweingruber, F.H. 2002. Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability. Science 295: 2250-2253.

Hampe, A. 2004. Bioclimate envelope models: what they detect and what they hide. Global Ecology and Biogeography 13: 469-471.

Hitch, A.T. and Leberg, P.L. 2007. Breeding distributions of North American bird species moving north as a result of climate change. Conservation Biology 21: 534-539.

Kinzelbach, R.K. 2004. The distribution of the serin (Serinus serinus L., 1766) in the 16th century. Journal of Ornithology 145: 177-187.

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.

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Mayr, E. 1926. Die Ausbreitung des Girlitz (Serinus canaria serinus L.). Ein Beitrag Zur Tiergeographie. Journal of Ornithology 74: 571-671.

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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.

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Last updated 20 May 2009