Do hosts of avian brood parasites discriminate parasitic vs. predatory threats? A meta-analysis

Many animals have evolved fine-tuned enemy recognition (the ability to discriminate between threat types) and respond to threats based on their particular impact on survival and/or fitness. Birds represent an important and tractable behavioral study system to explore hypotheses of enemy recognition in detail: in addition to predation risk to adult and nest survival, up to 17% of avian species also face reproductive threats from brood parasitism, whereby parasites lay their eggs in other species' nests. While nest predation is detrimental to progeny fitness throughout the reproductive cycle, brood parasitism can carry different costs depending on the host's nest stage and whether the host rejects parasitic eggs or chicks. We conducted a literature review and a formal meta-analysis of studies that conducted model presentation experiments to compare aggression levels of hosts toward brood parasites vs. predators, and synthesized up-to-date findings on such avian enemy recognition patterns. We focused on whether hosts are more aggressive toward brood parasites during the high-cost laying and incubation stages compared to the low-cost nestling stage, whereas responses to nest predators were predicted to be consistently strong or even increasing toward latter nesting stages. We also evaluated whether these front-loaded defenses prior to the brood parasite's access to nests are modulated by hosts' foreign egg ejection responses (accepters vs. rejecters), brood parasitic offspring strategy (nestmate-evictors vs. nest-sharers), and host-brood parasite geographic overlap (sympatry vs. allopatry). As predicted, hosts responded more aggressively toward models of brood parasites during the laying and incubation stages compared to the nestling stage. In turn, host aggression toward nest predators increased in intensity level during the nestling stage. We also found support that host type mediates anti-brood parasitic responses, in that accepters were generally more aggressive to brood parasites than rejecters. We did not find evidence that geographic overlap significantly affected anti-parasitic responses, but we did find differences based on the brood parasite's nestling strategies (evictor vs. non-evictor). These findings indicate that avian hosts of brood parasites make adaptive decisions regarding costly nest defensive behaviors to protect their offspring depending on the type and cost of the threat.