Red-winged blackbirds adjust aggression with brood value and predator type, but show broadly similar responses to snake predators of differing risk

Abstract Parental investment theory predicts parents should scale investment in defending offspring with the offspring's value and the risk a predator poses to offspring versus adults. While this framework has been widely applied to nest defense against avian and mammalian predators, how it extends to snake predators—which endanger both nests and adults, and vary in their capacity to envenomate—remains unexplored. We examined how red-winged blackbirds (Agelaius phoeniceus) respond to nest predators, including venomous and non-venomous snakes, testing whether aggression intensity and type varies across predators and nest stage. Parental investment theory predicts that defense intensity increases with brood value, and that birds adjust the type of aggression used depending on whether predators pose higher risk to the adults, such as reducing physical aggression toward higher-risk predators. We found that blackbirds displayed stronger aggression toward predators compared to the control, and that aggression toward brood parasites declined during the nestling stage, consistent with decreasing parasitism risk, whereas responses to nest predators increased with nest age. Type of aggression was broadly consistent across predators, including 2 snake stimuli, suggesting blackbirds did not strongly alter overall defensive behavior toward snakes despite their elevated potential risk to adults. However, birds delivered fewer physical strikes to the rattlesnake than the gopher snake, indicating that particularly risky defensive behaviors may be modulated selectively. Responses to different threats were similar between sexes but reflected sex-specific differences in defense and parental care. Neighboring birds frequently joined defenses, suggesting cooperative risk management. Together, our findings suggest that blackbirds adjust overall defense intensity according to offspring value and predator context, while selectively modulating particularly costly defensive behaviors.