Covariation in Shapes Between the Sternum and Pelvis in Aquatic Birds with Different Locomotor Modes

Birds associated with aquatic environments have diverse locomotor and foraging strategies. We hypothesize that aquatic birds have different covariation patterns in the shapes of the sternum and pelvis depending on the locomotor mode and the predominant use of wings or hind limbs when moving through the water. The study was conducted on 26 bird species, among which we identified three ecological categories: surface swimmers, wing- and foot-propelled divers. The last two categories included only species that dive from the water surface. Geometric morphometric methods were applied to analyze shapes, and covariance was investigated with two-block PLS analysis. We show that wing- and foot-propelled divers have different patterns of covariation between sternum and pelvis shapes. A narrower, elongated and flattened sternum correlates with a shortened postacetabular region of a pelvis in wing-propelled divers. A widened and shortened sternum with a deep keel correlates with an elongated post-acetabular region of a pelvis in foot-propelled divers. Surface swimmers have a wide variety of combinations of sternum and pelvis shapes, which is apparently explained by their lower specialization for specific aquatic locomotion, and by the influence of diverse ecological factors.