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Body Design or Behavior? What Explains the Performance of Slender-Billed Gulls (Chroicocephalus genei) Feeding on Brine Shrimp (Artemia sp.) in Salt Pans?
by
and
Maud de Saint Seine
1, 
Lyse Hannier
1,
Vincent Bels
2,
Nicolas Schtickzelle
1 and
Michel Baguette
2,3,* 
1
Earth and Life Institute, UCLouvain, 1348 Louvain-la-Neuve, Belgium
2
Muséum National d’ Histoire Naturelle, Institut de Systématique, Évolution, Biodiversité, UMR 7205 Muséum National d’ Histoire Naturelle, CNRS, Sorbonne Université, EPHE, Université des Antilles, F-75005 Paris, France
3
CNRS, Station d’Écologie Théorique et Expérimentale, UAR 2029, F-09200 Moulis, France
*
Author to whom correspondence should be addressed.
Biology 2025, 14(10), 1331; https://doi.org/10.3390/biology14101331 (registering DOI)
Submission received: 10 July 2025
/
Revised: 19 September 2025
/
Accepted: 23 September 2025
/
Published: 26 September 2025
(This article belongs to the Special Issue The Future of Marine Megafauna)
Simple Summary
How animals perform vital tasks for their survival and reproduction is a crucial question to understand their current lifestyle and its evolution. Here we focus on the food acquisition by adult gulls feeding on small shrimp in salt pans in Camargue, France. The rate at which these gulls capture and ingest shrimp is extremely high. We calculate that this rate is high enough that it covers the energy requirement of an adult gull at its peak during the breeding season in less than 6 h. By analyzing slow-motion videos of feeding gulls, we were able to show that this performance is not achieved using particular, specialized morphological structures but rather by optimizing the suite of behaviors associated with locomotion, food capture, and food transport. This suite of behaviors is comparable to that used by two species of phalaropes that are not gulls but shorebirds, which also feed on prey captured at the saltwater surface. This raises the question of the similarity of these food acquisition behaviors in species belonging to different lineages, either due to their convergent evolution in the face of common environmental constraints or due to their maintenance during the diversification of lineages along the tree of life.
Abstract
(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.
