Rusty Blackbirds (Euphagus carolinus) appear to exhibit predator satiation with a resident raptor species

Abstract

Predator satiation is an understudied topic among vertebrate organisms, especially avian species. In Anderson, South Carolina, a species of migratory black bird, Euphagus carolinus (Rusty Blackbird) was observed roosting seasonally in a patch of Phyllostachys aurea (Golden bamboo) in large numbers. A resident avian predator, Buteo lineatus (Red-shouldered Hawk was seen preying on these roosting blackbirds; however, when this hawk would attack, pin down, and eat a blackbird, the rest of the flock would not react or fly away. Upon initial observation, we questioned if these roosting blackbirds exhibited predator satiation as a protection mechanism against avian predation. When E. carolinus flocked to roost, their numbers were visually counted and compared between at least two members of the research team. After allowing the birds to settle between fifteen seconds and one minute, a B. lineatus call was played with speakers to the flock. Simultaneously, a regulation-sized football was tossed to simulate the hawk attacking a member of the flock. The number of birds that flew away was recorded. Results between roosting numbers and flight numbers upon disturbance were analyzed to see if roosting group size correlates with evasiveness in birds. Results indicated that as blackbird group size increased, the total percentage of escaping birds decreased. Statistical analysis showed the leaving percentage versus the natural logarithm of group size was highly statistically significant (p < 0.001) and had a moderately negative overall correlation (rs = -0.62). Our results suggest that these blackbirds feel safer in large groups instead of small groups during predation events when roosting in dense brush. LAY SUMMARYO_LIThere is little research involving predator satiation tendencies in birds. C_LIO_LIPredator satiation is a strategy developed by prey animals to cope with predators, relying on sheer numbers to avoid predation instead of fight or flight. C_LIO_LIWe observed roosting Rusty Blackbirds in Anderson, SC displaying predator satiation-linked tendencies under avian predation stress when in large groups. C_LIO_LIWe tested the behavior of Rusty Blackbirds by simulating a hawk attack, noting the group size and the number of birds escaping. C_LIO_LIOur results showed that as group size increased, flight percentage decreased under a simulated predator attack. C_LIO_LIRusty Blackbirds seem to use the predator satiation strategy when roosting. C_LI