Avian morphometrics and climate change

Abstract

Climate has changed rapidly since the end of 19th century due to increased emission of greenhouse gases into the earth's atmosphere. These changes have resulted in species changing their distributions and their phenology. Recent studies have also demonstrated that climate change underlies changes to morphological traits in endotherms. Climate related variations in morphological traits across geographic space have long been recognized. For instance, Bergman's and Allen's rule describe latitudinal patterns in morphological traits; larger body sizes and smaller appendage sizes relative to body size in colder, higher latitudes. One explanation for these rules is based on the heat exchange capacity of the trait and its adaptiveness to the thermal environment. Recent studies on morphological responses to climate change mostly focused on changes to body size and there is little understanding of what other morphological traits are responding to climate change and which particular changes in climate the morphological traits are sensitive to. In my thesis, I focused on birds in the Australasian group Meliphagides, and investigated how their bills, which are a structure important in heat exchange, have responded to changes in climate in recent years.

In Chapter 2, I used microCT technology to confirm that the formula for surface area of a cone is a valid proxy for measurement of bill surface area. This formula was used to estimate bill size of individuals used in this study i.e. 9847 museum specimens of Meliphagides from 79 species. In Chapter 3, I examined the geographic patterns of bill size and then its associations with climate to generate predictions on bill size response to climate change. I extended the conventional modelling to include temperature extremes associated with physiological limits related to thermal performance, as well as other climate variables that may influence thermoregulation of endotherms such as humidity and rainfall. Latitudinal patterns of bill size conformed to Allen's rule with large bill sizes in warmer low latitudes. I found a significant interaction between frequency of exposure to summer extremes (days equal to or above 35 degrees) with rainfall and humidity, with large bill sizes in hot, dry and, hot, humid environments, which most likely optimizes thermal performance via improving dry heat loss. However, the non-linear association between bill size and summer extremes indicated that bills are small in populations that are frequently exposed to summer extremes, possibly a mechanism to balance heat loads at temperatures exceeding body temperature. The significant interaction between winter extremes (days <5 degrees) and winter rainfall indicated that the bills are small in populations in cold wet environments, which is most likely an adaptation to reduce heat loss at colder temperatures. In Chapter 4, I showed that changes in temperature extremes that cause changes to thermoregulatory demands drive recent changes in bill size of Meliphagides, in line with the predictions based on spatial patterns. I demonstrated that the bill size response may vary in terms of both magnitude and direction based on the local climate experienced by the population and other environmental perturbations, such as landscape change. In Chapter 5, I investigated the associations between bill size change, and species' traits and found no evidence of strong associations between them. Only 11.1% of variation in bill size change observed was explained by species' traits, suggesting that other factors such as differences in climate exposure may account for morphological responses to climate change. Overall, my thesis contributes evidence to show morphological traits other than body size are responding to climate change, specifically changes in temperature extremes that known to cause changes to thermoregulatory demands. However, whether this change represents a genetic adaptation or phenotypic plasticity is yet to be explored.