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

Abstract

Changes in morphology have been postulated as one of the responses of animals to global warming, with increasing ambient temperatures leading to decreasing body size. However, the results of previous studies are inconsistent. Problems related to the analyses of trends in body size may be related to the short-term nature of data sets, to the selection of surrogates for body size, to the appropriate models for data analyses, and to the interpretation as morphology may change in response to ecological drivers other than climate and irrespective of size. Using generalized additive models, we analysed trends in three morphological traits of 4529 specimens of eleven bird species collected between 1889 and 2010 in southern Germany and adjacent areas. Changes and trends in morphology over time were not consistent when all species and traits were considered. Six of the eleven species displayed a significant association of tarsus length with time but the direction of the association varied. Wing length decreased in the majority of species but there were few significant trends in wing pointedness. Few of the traits were significantly associated with mean ambient temperatures. We argue that although there are significant changes in morphology over time there is no consistent trend for decreasing body size and therefore no support for the hypothesis of decreasing body size because of climate change. Non-consistent trends of change in surrogates for size within species indicate that fluctuations are influenced by factors other than temperature, and that not all surrogates may represent size appropriately. Future analyses should carefully select measures of body size and consider alternative hypotheses for change.

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

“…Finally, local responses to small‐scale environmental changes might also be important contributors to size changes (Ozgo & Schilthuizen, ). Hence, studies like ours should be complemented by detailed monitoring of phenotypic and environmental change at the local or regional scale (Gardner, Amano, Mackey, et al., ; Salewski et al., ). Preferably, such studies would be replicated across the continent, but unfortunately, long‐term studies in general, and of bird color variation in particular, are rare (but see Karell et al., for an example).…”

“…Potential alternative explanations, in particular for increases in body size, include the selective pressures exerted by extreme climatic events (McKechnie & Wolf, 2010) and changes in migratory behavior (Salewski et al, 2014). Extreme climatic events include heat and cold waves and periods with extreme low or high rainfall (Bailey & Pol, 2016).…”

“…Recent studies demonstrate that declining body size appears to be more common than increasing body size, particularly in the case of birds (Gardner, Amano, Backwell, et al, 2014;Sheridan & Bickford, 2011;Teplitsky & Millien, 2014;Van Buskirk, Mulvihill, & Leberman, 2010;Yom-Tov & Geffen, 2011). However, while body size declines may be frequent, they are certainly not universal: even closely related species from the same bioregion may show increases, decreases, and no change in size (e.g., Gardner, Heinsohn, & Joseph, 2009;Gardner, Amano, Backwell, et al, 2014) and patterns of change within species may vary for different morphological size traits (Salewski, Siebenrock, Hochachka, Woog, & Fiedler, 2014).…”

Disentangling the effects of spring anomalies in climate and net primary production on body size of temperate songbirds

“…The few existing studies focusing on temporal change in body size showed contrasting trends between species (Gardner et al 2014a, Salewski et al 2014, but see Gardner et al 2009, Van Buskirk et al 2010) and/ or between sites (Meiri et al 2009, Collins et al 2017). The few existing studies focusing on temporal change in body size showed contrasting trends between species (Gardner et al 2014a, Salewski et al 2014, but see Gardner et al 2009, Van Buskirk et al 2010) and/ or between sites (Meiri et al 2009, Collins et al 2017).…”

“…This disagreement is reinforced by a probable publication bias towards cases exhibiting significant changes (Meiri et al 2009). Moreover, some studies were based on museum data (Salewski et al 2014) and may have been prone to temporal collection and curation biases. Moreover, some studies were based on museum data (Salewski et al 2014) and may have been prone to temporal collection and curation biases.…”