Abstract
Recent changes in global climate have been linked with changes in animal body size. While declines in body size are commonly explained as an adaptive thermoregulatory response to climate warming, many species do not decline in size, and alternative explanations for size change exist. One possibility is that temporal changes in animal body size are driven by changes in environmental productivity and food availability. This hypothesis is difficult to test due to the lack of suitable estimates that go back in time. Here, we use an alternative, indirect, approach and assess whether continent‐wide changes over the previous 100 years in body size in 15 species of Australian birds are associated with changes in their yellow carotenoid‐based plumage coloration. This type of coloration is strongly affected by food availability because birds cannot synthesize carotenoids and need to ingest them, and because color expression depends on general body condition. We found significant continent‐wide intraspecific temporal changes in body size (wing length) and yellow carotenoid‐based color (plumage reflectance) for half the species. Direction and magnitude of changes were highly variable among species. Meta‐analysis indicated that neither body size nor yellow plumage color showed a consistent temporal trend and that changes in color were not correlated with changes in size over the past 100 years. We conclude that our data provide no evidence that broad‐scale variation in food availability is a general explanation for continent‐wide changes in body size in this group of species. The interspecific variability in temporal changes in size as well as color suggests that it might be unlikely that a single factor drives these changes, and more detailed studies of museum specimens and long‐term field studies are required to disentangle the processes involved.
Highlights
Recent anthropogenic-induced changes in climate have led to a broad range of biological responses, including changes in the timing of major life events such as breeding and migration and shifts in species’ distributions (Walther et al, 2002)
Our study aims to determine whether temporal changes in size and carotenoid-based plumage colors of Australian birds are correlated within species, consistent with the idea that wide-ranging changes in food availability could be driving large-scale morphological changes over time
We assessed the possibility that temporal changes in body size could be driven by changes in food availability by comparing temporal tends in size with temporal trends in a condition-dependent trait: carotenoid-based coloration
Summary
Recent anthropogenic-induced changes in climate have led to a broad range of biological responses, including changes in the timing of major life events such as breeding and migration and shifts in species’ distributions (Walther et al, 2002). Changing food availability may help to explain observed variation in contemporary body size changes: changes in primary productivity will vary across landscapes due to the considerable variation in rainfall patterns that strongly determine primary production (Rosenzweig, 1968). Testing this hypothesis directly over long time periods, proves difficult as we lack long-term records of primary productivity (Roxburgh et al, 2004). Our study aims to determine whether temporal changes in size and carotenoid-based plumage colors of Australian birds are correlated within species, consistent with the idea that wide-ranging changes in food availability could be driving large-scale morphological changes over time
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