Individual variation in feeding behaviour and mechanics in songbirds

Granivory in songbirds is a biomechanically challenging endeavour. Seeds need to be dehusked before consumption, requiring complex and precisely controlled beak and tongue movements. Because seeds are small, birds also need to consume large quantities to meet their dietary demands. In short, granivorous songbirds need to be fast and efficient at processing seeds. However, the biomechanics of seed dehusking are still poorly understood. In this thesis we investigated how seed feeding performance varies within a population of Domestic Canaries (Serinus canaria), and what the potential sources of this variation could be. Individual differences in beak morphology, food preference, and age may all influence dehusking performance, raising important questions about how birds can adapt to environmental changes. Using high-speed video recordings and automated beak tip tracking, we quantified feeding performance, beak kinematics, and seed handling skills of birds during feeding on two seed types. Contrary to expectations, higher beak kinematic capabilities did not result in faster or more successful dehusking. Higher maximal closing speeds even reduced success rates. Instead, efficient seed handling skills, specifically the ability to orient seeds for splitting the husk in two halves, resulted in faster feeding. Next, we explored potential drivers of variation. Beak morphology was an important factor: longer beaks showed higher opening-closing frequencies, while deeper beaks, presumably associated with higher bite forces, reduced feeding time on tougher seeds. These results align with the known force-velocity trade-off observed across bird species. Seed preference was examined through food choice trials. While individuals showed strong differences in preferences, these were not meaningfully related to improvements in seed handling or kinematic capabilities, suggesting that handling skills might be easily transferable across seed types. Finally, we studied the effects of age. Our longitudinal analysis showed that beak frequency in function of age followed a parabolic curve, increasing in youth and declining in old age. Seed handling skills improved with age in males only, and success rates showed a parabolic pattern with age, which was more pronounced in males. These findings suggest that early-life learning enhances performance, while later-life declines may stem from muscular or neuromotor deterioration. In summary, this thesis highlights the complexity of granivory in songbirds and demonstrates the importance of skills for efficient feeding on seeds. Future biomechanics research on feeding should therefore focus on identifying skills and studying underlying aspects of skill acquisition and maintenance, such as cognition and sensorimotor control.