Tweet or treat: unveiling the effect of brood size on begging behaviour in a songbird and the importance of feeding effort and diet quality

Abstract

Understanding the dynamics of parent-offspring communication is essential to comprehend how resources are allocated and how family interactions are shaped. In altricial birds, begging behaviour is a crucial mechanism through which nestlings signal their needs to parents, thus influencing food provisioning and overall parental investment. However, nestling begging and parental responsiveness can covary differently depending on multiple factors, such as brood size and the frequency and quality of food provided to nestlings. Still, the combined role of these factors in regulating begging signals remains largely misunderstood. This study aims to explore the role of brood size in shaping nestling begging time, parental feeding rate, and diet composition in a population of blue tits (Cyanistes caeruleus). To this end, we manipulated the brood size, producing reduced, control, and enlarged broods, and used micro-cameras to record begging behaviour and parental responses. Our results showed that nestlings in reduced broods begged for significantly less time than in control and enlarged broods, which showed similar begging time. Despite this, the begging time for the whole brood was increased in enlarged broods and decreased in reduced broods, regarding controls, hence supporting the hypothesis that brood size affects begging dynamics. A key finding was the combined effect of feeding rate and diet composition, since nestlings in broods with higher feeding rates and lower proportions of spiders relative to caterpillars begged more time, whereas nestlings in broods with lower feeding rates displayed reduced begging, which was less dependent on diet composition. This finding suggests that nestling food demand may be addressed toward specific types of food and conditioned by nutritional needs, highlighting the complexity of parent-offspring communication. This study contributes to a broader understanding of signalling theories, showing that begging behaviour is influenced by both environmental conditions and family context.