Morphological Covariance and Onset of Foot Prehensility as Indicators of Integrated Evolutionary Dynamics in the Herons (Ardeidae)

The ultimate form an organism attains is based, in part, on the rate and timing of developmental trajectories and on compensatory relationships between morphological traits. For example, there is often an inverse correlation between the relative size of an organism's head and the length of its legs. Avian examples with a disproportionately small head and long legs include ostriches (Struthionidae), flamingos (Phoenicopteridae), cranes (Gruidae), and stilts (Recurvirostridae). To determine whether a possible compensatory relationship exists between relative head size and hind-limb length in a typically long-legged family of birds—the Ardeidae—we measured and analyzed skull dimensions (length, width, and height of cranium, and bill length) and skeletal hind-limb dimensions (femur, tibiotarsus, and tarsometatarsus) of the 12 North American species (north of Mexico) and of 12 additional taxa, including the morphologically divergent Agamia and Cochlearius. We found that Ardea species exhibit the smallest relative head sizes associated with the longest legs, while Butorides, Nycticorax, Nyctanassa, and Cochlearius have among the largest heads relative to hind-limb length. Furthermore, both positive and negative allometries occur in paired comparisons between the three hind-limb bones, expressed in tall morphotypes having disproportionately short femurs while short-legged morphotypes exhibit disproportionately long femurs; we show that this relationship has implications for foraging behavior. Moreover, the nestlings of short-legged herons exhibit functional precociality of the hind limbs through an early onset of prehensile ability of the feet to grasp branches, which is later expressed in adult foraging mode. This developmentally accelerated prehensile function in small-bodied species may be attributed, in part, to selection for predator avoidance in the early nestling stage.