Constraints on body mass and ecomorphological evolution in birds

A number of constraints have been hypothesised to have affected avian evolutionary history. Testing these hypotheses is key to understanding the causal processes behind observed patterns of diversity and disparity. Four topics on this theme are examined here. The first is that egg shell strength relative to incubator body mass limits body mass. Size variation in the giant, extinct flightless bird Genyornis newtoni is used to determine whether sexual dimorphism may have compensated for any mismatch between mass and eggshell strength in this species. Secondly, limits to leg bone scaling are tested for using quadratic regressions of leg bone measurements from 58 species of flightless birds. Comparisons with non-avian theropods are made to see whether patterns of scaling seen in birds demonstrate limits to leg bone allometry in birds. Thirdly, the evolution of leg bone proportions in 38 species of flightless birds is explored. Factor analyses are used to quantify changes in proportions from ancestral morphologies modelled on volant relatives. Multiple regressions are used to see if these changes are related to shifts in body size, the amount of time since loss of flight, and the size of the land area upon which each lineage evolved. In Chapter 4, the hypothesis of competition between Mesozoic birds and pterosaurs during the Mesozoic is investigated using multivariate analysis of functionally analogous traits. The results of these studies provide support to hypotheses of constrained leg bone allometry, with increases in femur length relative to girth being restricted, related to posture in birds and of ecological separation rather than competition between Mesozoic birds and pterosaurs. It is also shown that the evolution of the leg bones after the loss of flight is not limited by any universal selection pressures, with changes in the proportions of these bones showing no consistent patterns through avian phylogeny. Together, these studies provide new insights into the potential constraints that have affected avian evolution for the past 160 million years.