Birds that breed exclusively on islands have smaller clutches

Ornithology, 2024, 141, 1–8 https://doi.org/10.1093/ornithology/ukae005 Advance access publication 25 January 2024 Research Article Birds that breed exclusively on islands have smaller clutches Michał T. Jezierski* Edward Grey Institute of Field Ornithology, Department of Biology, University of Oxford, Oxford, UK Corresponding author: * The “island syndrome” refers to similarity in the biology of island organisms, but its generality is questionable, as the scope of species and traits examined are often limited. Here, I show that birds breeding exclusively on islands (breeding island endemics) evolved smaller clutches, using a dataset of 4,530 bird species. Using an inclusive definition of a breeding island endemic, which also encompasses migratory species and seabirds, I examine the evolution of clutch sizes in island breeding species using phylogenetic generalized linear models. Across disparate phylogenetic hypotheses, and after accounting for biological and geographical co-variables, I show that breeding island endemic landbirds (470 species) evolved smaller clutch sizes than continental breeding species (3,818 species). I show that the evolution of clutch size follows the expectations of the island syndrome, as among breeding island endemic landbirds there is a positive relationship between clutch size and breeding range area. Finally, I reinforce the view that the island syndrome is a general pattern in birds, spanning diverse phylogenetic and ecological groups, by showing that in a seabird-only dataset (242 species), breeding island endemic seabirds show evolution of smaller clutch sizes. In a model of the full dataset of both landbirds and seabirds (4,530 species) there was no evidence of an interaction of being a seabird with breeding island endemicity, showing that seabirds and landbirds respond in the same way. This study, using more than 40% of all bird species, provides the first evidence of a general evolutionary response in a life history trait, clearly showing the island syndrome as a general evolutionary tendency associated with island environments. Keywords: birds, clutch size, islands, island syndrome, latitude, life history, phylogenetic, seabirds How to Cite Jezierski, M. T. (2024). Birds that breed exclusively on islands have smaller clutches. Ornithology 141:ukae005. LAY SUMMARY • The island syndrome is a phenomenon of similarity in morphology, behavior, and life history between island organisms. • Even among birds, its extent is not fully studied; it is not clear which traits (particularly behavioral and life history traits) are involved, and how generally it applies across the whole diversity of birds (more than 10,000 species). • Using a dataset of 4,530 species with established molecular phylogenies, I examined whether birds that nest exclusively on islands consistently differ from continental-breeding species in terms of a key life history trait: clutch size. • I found that breeding island endemic landbirds have smaller clutches. Among island breeding endemic landbirds, breeding range area was positively correlated with clutch size, as expected in the island syndrome. Seabirds show the same evolutionary response in clutch size as landbirds. • This study shows that a life history trait is part of the island syndrome across a representative sample of the avian phylogeny, and shows that seabirds also exhibit the island syndrome. Las aves que se reproducen exclusivamente en islas tienen camadas más pequeñas RESUMEN El “síndrome de la isla” se refiere a la similitud en la biología de los organismos de las islas, pero su generalidad es cuestionable, ya que la amplitud de especies y rasgos examinados a menudo es limitada. Aquí, muestro que las aves que se reproducen exclusivamente en islas (reproductoras endémicas de islas) evolucionaron nidadas más pequeñas, utilizando un set de datos de 4.530 especies de aves. Utilizando una definición inclusiva de reproductoras endémicas de islas, que también abarca especies migratorias y aves marinas, examino la evolución de los tamaños de nidada en especies reproductoras de islas mediante modelos lineales generalizados filogenéticos. A través de diversas hipótesis filogenéticas, y después de tener en cuenta las covariables biológicas y geográficas, demuestro que las aves terrestres reproductoras endémicas de islas (470 especies) evolucionaron tamaños de nidada más pequeños que las especies continentales (3.818 especies). Demuestro que la evolución del tamaño de la nidada sigue las expectativas del síndrome de la isla, ya que entre las aves terrestres reproductoras endémicas de islas hay una relación positiva entre el tamaño de la nidada y el área de reproducción. Finalmente, refuerzo la idea de que el síndrome de la isla Submission Date: August 30, 2023. Editorial Acceptance Date: January 13, 2024 © American Ornithological Society 2024. Published by Oxford University Press for the American Ornithological Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial License ((https://creativecommons.org/ licenses/by/4.0/)), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact ABSTRACT 2 Island bird clutches M. T. Jezierski es un patrón general en las aves, abarcando diversos grupos filogenéticos y ecológicos, al mostrar que en un set de datos exclusivo de aves marinas (242 especies), las aves marinas reproductoras endémicas de islas muestran la evolución de tamaños de nidada más pequeños. En un modelo del set completo de datos de aves terrestres y marinas (4.530 especies), no hubo evidencia de una interacción entre ser un ave marina y ser un ave reproductora endémica de islas, mostrando que las aves marinas y las aves terrestres responden de la misma manera. Este estudio, que utiliza más del 40% de todas las especies de aves, proporciona la primera evidencia de una respuesta evolutiva general en un rasgo de la historia de vida, mostrando claramente el síndrome de la isla como una tendencia evolutiva general asociada con ambientes isleños. Palabras clave: aves, aves marinas, filogenético, historia de vida, islas, latitud, síndrome de la isla, tamaño de nidada INTRODUCTION Geography determines the distribution of species diversity and trait occurrence on Earth (Violle et al. 2014), generating patterns of biological variation following basic features like latitude or insularity (Willig et al. 2003, Whittaker et al. 2023). On islands, area limitation and isolation result in decreased species diversity, and communities with vacant niches (MacArthur and Wilson 1967, Whittaker et al. 2023). These island communities are thought to have relaxed predation and inter-specific competition, and higher population densities compared to mainland counterparts (MacArthur and Wilson 1967, Whittaker et al. 2023). As these conditions are repeated (...truncated)