CORVALLIS, Ore. – In a rare study of snakes as nest predators, scientists found nesting birds were loath to strike rattlesnakes even though they likely had no previous experience with vipers.
The research, published in Behavioral Ecology , examined the response of red-winged blackbirds to a host of western Oregon nest predators that also included scrub jays, ground squirrels and nonvenomous gopher snakes.
The predators in the study, which was led by Shelby Lawson of Oregon State University, were inanimate but lifelike facsimiles placed on platforms a few meters from the nests of blackbirds, known to be aggressive nest defenders.
The blackbirds were also exposed to cowbirds, a brood parasite – they lay their eggs in someone else’s nest, forcing another species to expend resources raising them – as well as song sparrows (as a control species; sparrows don’t prey on blackbird nests) and rattlesnakes.
Rattlesnakes are native to western Oregon but were largely extirpated west of the Cascade Range in the 1800s, meaning modern-day blackbirds in the region are probably not encountering them. Nevertheless, in tests at dozens of nests – roughly half featuring eggs and half containing nestlings – blackbirds physically struck a rattlesnake just once, compared to more than 80 gopher snake strikes.
“We don’t know whether that’s because they recognized the danger or if there's something about the viperid features or the rattle that gave the birds pause,” said Lawson, a postdoctoral researcher in David Kikuchi’s integrative biology lab in the OSU College of Science.
The project expands understanding of a biological concept known as parental investment theory, which seeks to explain how and when animals might take more risks or put more energy into protecting their young, versus choosing self-preservation instead.
Blackbirds’ parental investment has been well tested, Lawson said. They are known to adjust their aggression and nest defense tactics depending on how vulnerable their nest is to the predator they detect, and whether that same predator poses a risk to the parents as well.
“For example, they might physically beat up an egg-eating jay or squirrel near their nest, but in response to something like a hawk that could eat them, they might choose to keep their distance and alarm-call instead,” she said.
The scientists found that blackbirds consistently responded more aggressively to predators than controls, and that they weren’t more cautious in general around the snakes, even though snakes typically pose greater potential danger to bird parents than the other predators in the study.
Usually, Lawson said, blackbirds and other passerines – perching birds, as opposed to raptors, waterfowl, hummingbirds, penguins, etc. – show heightened caution toward predators that directly endanger them, but display strong, direct aggression toward those that mainly threaten their offspring like crows and squirrels.
“In our experiment, the gopher snakes and rattlesnakes represented medium-sized adults that would be unlikely to pursue healthy adult blackbirds but definitely able to injure them during close-range nest defense,” Lawson said. “And obviously a rattlesnake’s venom makes it inherently more dangerous to deal with.”
Nevertheless, including swooping at the rattlesnakes, the blackbirds responded to the vipers basically the same way they did to gopher snakes, minus the strikes.
“Neither snake was struck during incubation, but during the nestling stage the focal birds – the ones whose nest it was – routinely struck gopher snakes, and neighboring blackbirds regularly got in the act too,” she said.
“This is the first paper to test responses of breeding birds to both nonvenomous and venomous snakes within a parental investment theory framework, and it shows that we need to be including snakes in these kinds of tests more often,” Lawson added. “We still don’t fully know how birds recognize or make decisions regarding snakes despite them being a major threat to their nests, and this study is a good step toward understanding that.”
Also contributing to the study were OSU’s Angela Zbaracki, Russell Campbell and Eleanor Fraser.
Behavioral Ecology
Experimental study
Animals
Red-winged blackbirds adjust aggression with brood value and predator type, but show broadly similar responses to snake predators of differing risk
29-Jul-2026