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Brood sex ratio modulates the effects of extra food on parental effort and sibling competition in a sexually dimorphic raptor

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Abstract

Conflicts are inherent to family systems and may occur at three levels. First, each parent benefits if its mate takes the greater share of parental investment. Second, offspring try to manipulate their parents into devolving more resources than it is optimal for them. Third, siblings compete for resources. Food availability can affect the dynamics of each level of interaction. By means of a food supplementation experiment, we assessed how the initial availability of extra food during breeding affects later parental effort, sibling competition, and parent-offspring interactions in a small dimorphic raptor, the lesser kestrel (Falco naumanni). Being the larger sex, female nestlings are likely to have higher energy requirements. Female-biased broods had a higher rate of aggressive interactions and were fed more frequently in control nests. Food was not shared evenly among broodmates, and daughters tended to receive more feedings than sons. In young broods, parents controlled food allocation by entering the nest, whereas in older broods, offspring controlled food allocation by monopolizing the nest site entrance. Extra food induced male parents to reduce the rate of feedings delivered by entering the nest. Additionally, extra food improved nestling growth in male-biased broods, leading to an increase in the frequency of parental feedings and aggressive interactions, likely due to faster growth rates. These findings reveal a key effect of brood sex ratio in determining family interactions in a species with reverse sexual size dimorphism and suggest that all levels of conflict between family members should be considered simultaneously when investigating the evolution of parental care.

Significance statement

Breeding poses energetic costs on parents, and environmental resources are usually limited. To successfully breed, parents coordinate their efforts and manage the allocation of resources to offspring, while offspring communicate their needs. Conflict can arise among family members: each parent benefits if the other takes on most of the workload, offspring ask for more care than what parents can provide, and siblings compete for food. Food availability affects these interaction levels separately, but they have rarely been integrated. By providing extra food to lesser kestrel (Falco naumanni) nests, we show that parental provisioning behaviour and sibling competition are simultaneously affected by initial resource availability during breeding. Although lesser kestrel nestlings show only moderate sex differences in body size (and therefore energy requirements), the effect of extra food on family interactions primarily depended on the sex ratio of the brood.

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Data availability

The dataset analysed in this study is available in Figshare, DOI: https://doi.org/10.6084/m9.figshare.13502553

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Acknowledgements

This study is dedicated to the memory of the late Matteo Griggio, who greatly contributed to its conceptualization and realization, but suddenly and unexpectedly passed away before it could be completed. We thank E. De Capua (Provincia di Matera), M. Cristallo (Genio Civile Matera), and the Comune di Matera, Ufficio Scuole, for authorizing and tolerating our work and presence at the breeding colonies, as well as for constant support during field activities. We also wish to thank C. Bonaldi, A. Curcio and S. Podofillini for assistance during fieldwork, and the associate editor, Prof. M. Leonard, and two anonymous reviewers for constructive criticism that greatly improved a previous version of the manuscript.

Funding

The study was supported by institutional funding from the Università degli Studi di Milano (to DR), Università degli Studi di Padova (to CS), and the Istituto Nazionale per la Protezione e la Ricerca Ambientale (ISPRA) (to JGC). Nestboxes were installed with support from the LIFE Project ‘RapaciLucani’ (LIFE05NAT/IT/00009) and were designed by G. Ceccolini.

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Authors and Affiliations

  1. Centre for Evolutionary Biology, School of Biological Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, Western Australia, 6009, Australia

    Camilla Soravia

  2. Area Avifauna Migratrice, Istituto Superiore per la Protezione e la Ricerca Ambientale (ISPRA), via Ca’ Fornacetta 9, I-40064, Ozzano Emilia, BO, Italy

    Jacopo G. Cecere

  3. Dipartimento di Scienze e Politiche Ambientali, Università degli Studi di Milano, via Celoria 26, I-20133, Milano, Italy

    Diego Rubolini

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  1. Camilla Soravia
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Correspondence to Camilla Soravia.

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Handling of wild birds was performed by the Italian Institute for Environmental Protection and Research (ISPRA) under the authorization of Law 157/1992 [Art. 4 (1) and Art. 7 (5)]. No bird was injured by the handling procedure. All applicable international, national, and/or institutional guidelines for the use of animals were followed.

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Soravia, C., Cecere, J.G. & Rubolini, D. Brood sex ratio modulates the effects of extra food on parental effort and sibling competition in a sexually dimorphic raptor. Behav Ecol Sociobiol 75, 62 (2021). https://doi.org/10.1007/s00265-021-02970-0

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