Adaptive value of habitat-dependent male provisioning behaviour under predation risk in a sexually dichromatic species

Sexual dichromatism is often assumed to increase dependent offspring's depredation risk when conspicuous adults attend the den or the nest, yet empirical evidence shows mixed, context-dependent outcomes. Facultative male provisioning, where most males do not provide food for nestlings but a minority provision conditionally, is a behavioural rarity across birds and is often obscured by typological classifications of ‘female-only’ versus ‘biparental’ care. In red-winged blackbirds, Agelaius phoeniceus , adult males are conspicuous, females are cryptic, and male provisioning to nestlings is facultative, providing a useful system for testing how parental phenotype and nest microhabitat might jointly influence the offspring's depredation risk. We combined detailed observations with a controlled field experiment to examine how microhabitat-dependent depredation gradients shape both male care and nest depredation. Across 78 natural nests, males were more likely to provision, and provisioned at higher rates, at nests with reduced predator access (i.e. nests over deeper water). To test how adult coloration and nest placement influence depredation directly, we conducted a paired artificial nest experiment using 3D-printed male and female models in upland (land) and wetland (over water) microhabitats. Nests over land and nests with conspicuous male models experienced significantly higher depredation rates, whereas nests over water and those with cryptic female models were less frequently depredated; however, the statistical interaction between adult phenotype and microhabitat was not significant. Together, these results indicate that parental sexual dichromatism and nest microhabitat both structure nest depredation risk, and that conspicuous males provision more often where that risk is intrinsically lower. Because facultative male care is rare and often overlooked in comparative work, our findings illustrate how a relatively uncommon, flexible parental strategy can mediate the ecological costs of sexual dichromatism and refine our understanding of how depredation and parental behaviour interact to shape the evolution of dimorphic plumage patterns.