Are long‐term widespread avian body size changes related to food availability? A test using contemporaneous changes in carotenoid‐based color

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.

Carotenoid‐based plumage colour saturation increases with temperature in Australian passerines

“…whether a realistic spatial gradient leads to discriminable differences in colour while being aware of the limitations of the models, (Kemp et al, 2015)). We have used these models to quantify variation in carotenoid-based colours before (Little et al, 2017) and the output of these models correlates well with other measurements indicating carotenoid deposition in the plumage (see below).…”

“…PC1 correlated positively with a measurement of chroma or saturation (the distance to the achromatic centre, Endler & Mielke, 2005;Stoddard & Prum, 2008)) in all species (mean r = 0.97, SD = 0.11). Higher values of saturation or chroma are usually correlated with increased deposition of carotenoids in the plumage (Mcgraw & Gregory, 2004;Saks, Mcgraw, & Hõrak, 2003), and following (Little et al, 2017). we therefore used PC1 as an index of carotenoid deposition in the plumage, whereby high deposition of carotenoid is indicated by higher values of PC1.…”

“…Beyond collinearity issues, we did not use other approaches such as model selection or path analysis to identify the best set of environmental predictors of carotenoid-based colour in each species because the meta-analytical approach used (see below) requires having effects for all variables and species, irrespective of their statistical significance. Each linear model also included the following covariates: collection year (centred) to account for potential temporal differences in carotenoid-based coloration due to fading, dirt accumulation and temporal variation(Little et al, 2017), and a spatial autocorrelation vector to account for non-independence between geographically close specimens followingCrase, Liedloff, and Wintle (2012).…”

A review of Gloger's rule, an ecogeographical rule of colour: definitions, interpretations and evidence

“…Future tests of Gloger's rule should aim at collecting new data on colour variation (for example using museum specimens; Little et al , ) via randomised designs to avoid choosing specific taxa or specimens. The colour variation measured should be consistent with variation in melanin deposition, and ideally melanin concentrations should be measured directly (Singaravelan et al , ; Galván & Wakamatsu, ).…”

Darker where cold and wet: Australian birds follow their own version of Gloger's rule

“…A similar prediction could be tested in birds by comparing temporal distributional changes with contemporaneous changes in temperature. Similarly, temporal changes in coloration could be assessed for selected species based on museum specimens (Galeotti et al , Roulin , Campbell‐Tennant et al , Little et al ). If the links between temperature and plumage coloration represent adaptive variation, the potential to adapt to climatic change will depend on the potential for colour change.…”