Body Design or Behavior? What Explains the Performance of Slender-Billed Gulls (Chroicocephalus genei) Feeding on...

(1) Background: Understanding the evolution of the form-function relationship requires identifying the selection pressures acting on individuals. The paradigm of Arnold provides a useful framework to infer how the natural selection acting on phenotypic traits can modulate individual fitness. Despite the theoretical advance, experimental studies of individual performances that explicitly address form, i.e., the phenotypic integration of functional morphology (body design and mechanics) and of behavior, are still rare. (2) Methods: Slender-billed gull food acquisition behaviors were video recorded in the salt pans of Salin de Giraud, Camargue, where brine shrimp are their main prey. We averaged the food intake rate over 21 individuals. We computed the mean hourly energy intake of an average gull by multiplying the mean hourly prey intake rate by the weight and energy content of a brine shrimp. We used this mean hourly energy intake to investigate the time needed by an average slender-billed gull individual to acquire the energy required to achieve their daily field metabolic rate. We computed the food metabolic rate of slender-billed gulls by using the model of Dunn et al. In addition, using slow-motion video sequences, we perform a functional and integrative analysis of three performances associated with food acquisition behaviors, i.e., locomotion, food capture, and food transport. (3) Results: We demonstrate that the energy assimilated by this performance is sufficient to cover less than 6 h of an adult gull's metabolic food rate during its breeding season. We show that brine shrimp capture by gulls does not involve the use of specialized morphological structures but rather involves a particular behavioral sequence that invariably associates a mode of locomotion, a mode of capture and a mode of transport of the prey from the beak to the pharynx. The comparison of this sequence to the register of food acquisition behaviors used by other Charadriiformes reveals its similarity with behaviors that are used by two shorebird species (Phalaropus fulicarius and P. lobatus) also feeding on prey captured from saltwater surfaces. (4) Conclusions: Altogether, our study supports (1) a causal chain in which performance results from the interaction between morphological structures and behaviors and (2) the idea that the performance peak of a realized phenotype can be reached by using the best combination of behaviors, either by convergent evolution or by their conservation among those available in a phylogenetically determined register.