Meeting Abstract
Allen’s Rule is a biogeographic rule that predicts appendage sizes of organisms vary with latitude in order to minimize or maximize exposed surface area and facilitate heat retention or dissipation in cold and hot climates, respectively. Due to its vascularization, the avian bill plays an important thermoregulatory role, and is therefore likely to conform to these predictions. While Allen’s rule has been examined in relation to bird bill sizes over broad geographic gradients, there remains little support for morphological variation over time. Over the past several decades, many regions of North America have demonstrated a consistent increase in temperature due to modern climate change. However, climate change is occurring non-uniformly through space and time, and thus, we expect its impact on populations should vary geographically. We hypothesized that Northern Cardinal (Cardinalis cardinalis) bill size and bill size variation would decrease with increases in latitude. Additionally, we hypothesized that as minimum average temperatures increase over time, Northern Cardinal bill size would also increase over time. We obtained measurements of bill surface area for Northern Cardinal museum specimens collected over an 85-year period (n =559) and tested the effects of climate variation. We found that bill sizes decreased with increases in latitude, and that the variation in bill size was greater in southerly regions than in northerly regions. We also found that bill sizes increased over time due to increasing minimum average temperatures, and that this relationship has strengthened over time. These results imply that bill size varies as a function of temperature over space and time, following the predictions of Allen’s Rule.
