The evolution of enclosed nesting in passerines is shaped by competition, energetic costs, and predation threat

Many avian species breed in enclosed nests that may provide better protection against predation and climatic conditions compared to open nests and are generally associated with larger clutch sizes and slower offspring growth. Here we show that different enclosed nesting strategies are each linked to behaviors with very different costs and benefits on a macroevolutionary scale. Using a detailed dataset of nest structure and location from the order Passeriformes, we employed phylogenetic comparative methods to evaluate (1) how predation, competition, design complexity, and energetic costs have shaped evolutionary transitions between different nesting strategies, and (2) whether these strategies also have distinct relationships with life-history traits. We find that flexible strategies (i.e., nesting in both open and enclosed sites) as well as energetically demanding strategies are evolutionarily unstable, indicating the presence of underlying ecological tradeoffs between antipredator protections, construction costs, and competition. We confirm that species with enclosed nests have larger clutch sizes and longer development and nestling periods compared to open nesters, but only species that construct enclosed nests rather than compete for preexisting cavities spend more time incubating and are concentrated in the tropics. Flexible strategies prevail in seasonal environments and are linked to larger clutches-but not longer development-compared to nesting in the open. Overall, our results suggest that predation, competition, and energetic costs affect the evolution of nesting strategies, but via distinct pathways, and that caution is warranted when generalizing about the functions of enclosed nest designs in birds. center dot Many birds raise their young in enclosed nests, which likely offer protection from predators and climatic conditions. We currently know little, however, about why some species build their own enclosed nests, while others adopt existing cavities.center dot Using a broad-scale, comparative approach with data from more than 4,000 passerine species (order Passeriformes), we evaluate how predation, competition, nest complexity, and energetic costs have shaped the evolution of these different enclosed nesting strategies.center dot We find that both flexible and energetically costly enclosed nesting strategies are disfavored on evolutionary timescales. We also show that enclosed nesters have larger clutches and longer developmental periods irrespective of whether they compete for or build their nests.center dot Our study highlights that different types of enclosed nesting strategies are linked to different sets of evolutionary costs and benefits. Muchas especies de aves crian en nidos cerrados que pueden proporcionar una mejor proteccion contra la depredacion y las condiciones climaticas en comparacion con los nidos abiertos, y generalmente se asocian con tamanos de nidada mas grandes y un crecimiento mas lento de las crias. Aqui mostramos que diferentes estrategias de anidamiento en nidos cerrados estan vinculadas a comportamientos con costos y beneficios muy diferentes a escala macro-evolutiva. Utilizando un conjunto de datos detallado sobre la estructura y la ubicacion de los nidos del orden Passeriformes, empleamos metodos comparativos filogeneticos para evaluar (1) como la depredacion, la competencia, la complejidad del diseno y los costos energeticos han dado forma a las transiciones evolutivas entre diferentes estrategias de anidamiento; y (2) si estas estrategias tambien tienen relaciones distintas con los rasgos de la historia de vida. Encontramos que las estrategias flexibles (i.e., anidar tanto en sitios abiertos como cerrados), asi como las estrategias energeticamente exigentes, son evolutivamente inestables, lo que indica la presencia de compensaciones ecologicas subyacentes entre las protecciones contra depredadores, los costos de construccion y la competencia. Confirmamos que las especies con nidos cerrados tienen tamanos de nidada mas grandes y periodos de desarrollo y crianza mas largos en comparacion con las especies que anidan en nidos abiertos, pero solo las especies que construyen nidos cerrados en lugar de competir por cavidades preexistentes pasan mas tiempo incubando y estan concentradas en los tropicos. Las estrategias flexibles prevalecen en entornos estacionales y estan vinculadas a tamanos de nidada mas grandes, pero no a un desarrollo mas largo, en comparacion con el anidamiento en lugares abiertos. En general, nuestros resultados sugieren que la depredacion, la competencia y los costos energeticos afectan la evolucion de las estrategias de anidamiento, pero a traves de vias distintas, y que se debe tener precaucion al generalizar sobre las funciones de los disenos de los nidos cerrados en las aves.