What is Gloger's Rule? Studies of the Relationships Among Climate, Coloration and Life History in Birds

Ecogeographic rules, the consistent patterns of morphological variation along environment and climatological gradients, have fascinated biologists for almost two hundred years. While some are the subject of renewed research interest, others have remained largely unstudied. Gloger's Rule, the pattern of organisms being darker in warm and wet environments, is one such understudied pattern. In this dissertation, I address three unknowns: what are the specific relationships between climate variables and coloration, what life history traits predict adherence to Gloger's Rule, and how are birds changing coloration with changes in climate. I address the first two unknowns in a comparative context comparing adherence to Gloger's Rule across species in Chapters 1 and 2; I study the third unknown using Song Sparrows (Melospiza melodia) in Chapter 3.;In Chapter 1, I test the different relationships between climate variables and coloration that had been proposed to define Gloger's Rule. Using coloration data from thousands of museum specimens from 99 species, I compare the explanatory power of each climate variable to determine which climate variable is most supported in birds. I show Gloger's Rule is not a simple relationship between one climate variable and coloration, rather different species have closer relationships to different climate variables. This finding highlights the need to understand Gloger's Rule in terms of entire climate, not a single climate variable. Additionally, I find life history traits and phylogenetic relationships are not good predictors of which climate variable explains the most variation in a species.;In Chapter 2, I examine whether life history traits are related to how species follow Gloger's Rule in a comparative study of 101 species. The hypotheses proposed to explain Gloger's Rule (Crypsis, Feather-degrading Bacteria, Promotion of Evaporation, Thermoregulation, and Conserved Physiological Processes) predict relationships with life history traits, supported climate variables relationships, or phylogenetic patterns. We find significant relationships between life history traits related to the Crypsis Hypothesis and Feather-Degrading Bacteria Hypothesis and adherence to Gloger's Rule. We find no support for the predictions of the other three hypotheses as universal selection pressures underlying Gloger's Rule, though they may still be at work in individual species.;Lastly in Chapter 3, I study the effects of global climate change on bird coloration, examining if Song Sparrows (Melospiza melodia) have changed color consistent with Gloger's Rule. I find that models containing climate change information are well supported compared to models that only have historical climate information, supporting the conclusion that birds have changed coloration with global climate change. Surprisingly and contrary to Gloger's Rule, birds have become lighter in areas that are becoming wetter and/or warmer. I hypothesize this may be due to changes in land use concurrent to climate change. Together these three chapters help to forward our understanding of Gloger's Rule, a very common but poorly understood pattern.