Avian body size changes and climate change: warming or increasing variability?

Abstract

There has been a growing interest in whether established ecogeographical patterns, such as Bergmann's rule, explain changes in animal morphology related to climate change. Bergmann's rule has often been used to predict that body size will decrease as the climate warms, but the predictions about how body size will change are critically dependent on the mechanistic explanation behind the rule. To investigate change in avian body size in western North America, we used two long-term banding data sets from central California, USA; the data spanned 40 years at one site and 27 years ) at the other. We found that wing length of birds captured at both sites has been steadily increasing at a rate of 0.024-0.084% per year. Although changes in body mass were not always significant, when they were, the trend was positive and the magnitudes of significant trends were similar to those for wing length (0.040-0.112% per year). There was no clear difference between the rates of change of long-distance vs. short-distance migrants or between birds that bred locally compared to those that bred to the north of the sites. Previous studies from other regions of the world have documented decreases in avian body size and have used Bergmann's rule and increases in mean temperature to explain these shifts. Because our results do not support this pattern, we propose that rather than responding to increasing mean temperatures, avian body size in central California may be influenced by changing climatic variability or changes in primary productivity. More information on regional variation in the rates of avian body size change will be needed to test these hypotheses.

Severe recent decrease of adult body mass in a declining insectivorous bird population

“…Here, we found that wing length actually increased at a rate corresponding to 0.5% per year in females and 0.2% in males. Goodman et al [42] postulated that longer wing length might be a response to greater climatic variability or changes in primary productivity, both often described as consequences of global change that vary among biomes. Conversely, a recent study reported that cliff swallows (Petrochelidon pyrrhonota) nesting alongside a highway had reduced their wing length in a period of 30 years in order to favour agility and avoid vehicles [45], showing that selective pressures can also be local and come from unexpected sources.…”

“…At this point, it remains unclear why the studied population showed a different trend from that of tree swallow spring migrants captured in Pennsylvania [41], and the causes of this change remain highly speculative. Nonetheless, the mechanisms proposed by Goodman et al [42] (e.g. to better resist extreme weather events such as severe storms during migration) constitute an avenue for further research.…”

Consequences of altered temperature regimes for emerging freshwater invertebrates

“…Long-term records have been crucial for revealing ecological patterns in response to climate change, including alterations in species' phenologies (e.g., Parmesan 2006;Pau et al 2011), changes in body size distributions (e.g., Daufresne et al 2009;Goodman et al 2012;Sheridan and Bickford 2011), and alterations in rates of ecosystem function (e.g., Traill et al 2010). Observational data have also been used to forecast the effect of climate change on biota by modelling changes in distributions under climate change scenarios (Walther et al 2002).…”

Morphological Change to Birds over 120 Years Is Not Explained by Thermal Adaptation to Climate Change

“…Therefore, the assumption of unidirectional changes in morphology as a response to a single environmental factor (temperature) across a wide range of species with different ecologies may be questioned, and a review of recent studies examining trends in body size produced conflicting results [16]. Birds and mammals have shown trends consistent with [12], [17], [18], and contrary to [19], [20], the prediction of decreasing body size with warmer temperatures and lower latitudes, or no clear trend [21]–[23]. Some studies of multiple species experiencing the same climate regime have found opposing directions of change among them [11], [16], [23], [24].…”