The perfect wing, The perfect trade-off? : What implements the main selection pressure on wing morphology?

Abstract: Selection pressure is a constant force in evolution, pushing birds and their wings towards an optimal shape and structure, were increasing performance, and minimizing the costs is central. But even though the science of aerodynamics can provide calculations of the optimal construction for the wing in different situations this rarely directly correspond to what is observed in nature. Since the optima are not the same for all birds due to different specifications and ecology this optimum becomes harder to determine and different functions can even have different optima, resulting in selection conflict. In the genius of birds there is an immense variation between species and their wings in everything from size, shape, and function.  The aim of this study is to investigate how wing morphology over a large phylogeny of bird species correlates to migration and habitat/ecology. Many studies have been done focusing on the effect of migration on the wing morphology, and some have been done focusing on other parameters such as display or daily usage. But by including the bird’s ecology and habitat related information with migration and morphology and looking at the selection from a broad perspective, can we uncover something more? The morphology of the wing cannot provide a perfect optimum for all circumstances since they require different specifications. What then, has the largest impact on the wing’s morphology? And does the relative length of the tail provide any correlations with its habitat and performance?  1185 birds of 137 species were caught at Ottenby, Öland and information on age, weight, sex, and tail length was collected for each individual bird. Photographs were taken of the back of the bird with the left wing outstretched 90 degrees from the body and analyzed in ImageJ to calculate aspect ratio and wing loading. Data on migration distance, foraging behavior, diet, and habitat density was then added for each of the species. Mean values of all parameters was calculated on species level creating a strong dataset with 137 data points. The species mean values dataset was used to test the interspecific effects and the dataset with all individuals was used to test intraspecific effects. ANOVAs, ANCOVAs, correlations tests and random slopes mixed models were performed revealing significant connections between wing morphology, migration, and habitat density. Correlations could also be observed between wing morphology, diet, and foraging behavior. Habitat density revealed the highest correlation with wing morphology, demonstrating a greater significance than migration and the other parameters. Effects that at first sight looked significant could later be excluded as they turned out to be dependent on other variables. The study therefore also highlights the importance of including alternative parameters for reliable conclusions.