This may be a portent of global warming: a new paper in the bird journal Oikos, based on 46 (!) years of research, shows that migratory birds in the U.S. are getting smaller over time.
One of the generalizations about biogeography that I teach my students is “Bergmann’s Rule,” the observation that within species of mammals and birds, populations from more northerly locations have larger body sizes than those from further south. This has classically been attributed to selection to conserve heat: if you’re twice as large (in terms of doubling every linear dimension), you increase your body volume by a factor of 23, or 8. Thus the amount of heat you generate, which is based on body mass, increases eightfold. Body surface area, however, is proportional to the square of linear dimension, and doubling that would increase surface area by 4. Thus, by doubling body size, the ratio of heat lost/heat produced would be halved (4/8). In other words, by getting bigger, you conserve heat more efficiently.
This, at least, is the explanation we give students when describing Bergmann’s rule.
But there’s a problem with this explanation. Bergmann’s rule holds not only for endotherms (warm-blooded animals like birds and mammals), but also for ectotherms—cold-blooded creatures like insects and amphibians, who don’t generate their own body heat. In one of my old papers, for instance, I that found Bergmann’s rule was scrupulously obeyed by fruit flies.
Another problem is that we don’t know whether a pattern of more northerly populations being larger reflects true evolutionary change (evolved body-size differences attributed to genetic differences), a developmental response to temperature (you get bigger if you’re born and grow up in a cold climate), or both. In fruit flies, I found that both factors were at work, but certainly there had been some evolutionary change within the species I studied (Drosophila melanogaster). But it’s unclear why, for a fly, it’s good to evolve a bigger body in a colder climate. They don’t have to conserve body heat.
In the Oikos paper, van Burkirk et al. combined bird data collected since 1961 at the Powdermill Nature Reserve in Pennsylvania. Birds were trapped during spring and fall migrations (local residents were trapped, too) and were measured for weight, “fat score,” and “wing chord,” a measure of wing size. In toto, they measured nearly half a million birds from 102 species.
Upshot: most of the species got smaller over the 46 years of study.
Of 65 species breeding in the study area, 51 got smaller as measured by body mass Of 83 species caught migrating north during the spring, 60 got smaller. And of 75 species caught migrating south in fall, 66 got smaller. All of these trends over time were statistically significant. For the birds that bred locally, and hence for which local temperature could be measured, the decline in mass was significantly correlated with an increase in temperature. This of course reflects the fact that temperature has been going up since 1961.
The decline in body mass wasn’t large: for spring migrants, for example, mass decreased only 1.3% over 46 years. However, this is a fairly large change over evolutionary time. The thing is, we don’t know if this change, even if related to temperature, is due to evolution of the birds (genes for smaller body size have replaced those for larger size), developmental change (birds simply grew up smaller as their environment got hotter) or both. This could be tested by rearing the offspring of birds under constant laboratory conditions, but that would be onerous. The rather small decrease in mass means that very large laboratory samples would be needed to detect such a small change. And it’s no picnic to rear wild birds of even a single species in captivity, much less the dozens and dozens it would take to see if the change of mass over time is an evolutionary change.
While these differences might reflect declining “conditions” in the birds’ habitats (i.e., less food), the authors address this by looking at population densities at the birds’ breeding grounds. Presumably bad conditions should be reflected in lower densities. But they found no association between bird density and bird body size.
Regardless of whether the birds’ change in body size reflects genetic change, developmental plasticity, or both, it does indicate that organisms have responded to a long-term increase in temperature. The authors don’t say a lot about global warming, but do raise the issue at the very end of the paper:
Of course, we have long known that evolved changes are an inevitable consequence of almost any human activity that modifies the environment and thereby influences the selective regime experienced by organisms. Classic examples include adaptation to urbanization and contaminated soils (Bradshaw and Jain 1966, Partecke and Gwinner 2007). Similar responses to climate change may be on-going and widespread;whether they will prove to be adequate remains to be seen. Particularly salient and sobering, however, should current trends continue unabated, is the immense biological scope and geographic scale of changes that are taking place compared with the limited information and resources we presently have for measuring, understanding and mitigating those changes.
h/t: Matthew Cobb
van Buskirk, J., R. S. Mulvihill and R. C. Leberman. 2010. Declining body sizes in North American birds associated with climate change. Oikos, early view doi: 10.1111/j.1600-0706.2009.18349.x
Coyne, J. A., and E. Beecham. 1987. Heritability of two morphological characters within and among natural populations of Drosophila melanogaster. Genetics 117:727-737 (couldn’t resist).