Our last post shared some pretty intense videos of predation events at nests. In both videos all of the chicks got eaten, and the parents either fled or did not approach the nest while the predator was there. The badger and the 13 lined ground squirrel weren’t challenged as they made off with nestlings. It’s not always like this, though. It turns out that bird parents respond differently depending on who is trying to eat their chicks.
Mammals are extremely strong in comparison to birds. Whenever a mammal stops by a nest, the adults generally steer clear. Sometimes they will shout alarm calls from a safe distance, or swoop down close to the predator and pull up at the last second. But generally they don’t make physical contact with the predators.
Snakes are a different story. Another study that my advisor conducted also used video cameras to observe grassland bird nests (Ellison & Ribic, 2012). In this study, of the nests where adult birds happened to be with their chicks when a snake came a-calling, 76% of the parents defended their nest. This means they used the non-aggressive tactics I described earlier (alarm calling and feinting swoops) or they attacked aggressively and make contact with the snakes. 47% of these parents physically attacked the snakes.
I’ll let that sink in for a moment.
Imagine it: attacking an opponent that weighs more than you do, and is significantly stronger than you -- all in hopes of driving them away from your kids. Sure, you can fly… but you have no real weapons, you’re a songbird!
Most of the birds who chose to defend their nests aggressively pecked at the snakes. These were usually eastern meadowlarks and bobolinks -- our bigger, stronger grassland birds. Two meadowlarks also ran around the snake with their wings outstretched, as though trying to appear larger to intimidate the snake. Three bobolinks used aerial assaults and struck at the snakes while on the wing.
Birds take on some pretty serious risks when defending their nest. Adult birds may become injured, which could impact their ability to feed themselves or their young. Injury may also make them more vulnerable to predators in the future if they are slower or less agile. Adults could be killed by the predator themselves. Even if they successfully drive the predator away, chicks could be injured or killed by the scuffle. Many birds may choose to use non-aggressive defense (much safer for them), and then renest if their clutch is lost. They’re trading the short-term genetic gain of their current clutch for long-term success of future nests.
So if nest defense can be so risky, why do birds attempt it? Remember, most nest defense is non-aggressive. Adults mostly stay out of the way of the predators. They try to lure them away from nestlings, especially if their chicks are old enough to scramble out of the nest and hide. They dive at the predator and peel away at the last second. Unless adults are making contact with the offender, they’re likely pretty safe. Noisy, angry parents may also attract other birds to the scene creating a mob. While a snake or skunk may put up with one bird harassing it, they may flee if a flock of birds start dive-bombing!
Birds are more likely to attack snakes than mammals for a few reasons. Snakes take a lot longer to eat an egg or nestling than most mammals do- so there is a greater chance of an adult bird witnessing the meal. Snakes have fewer weapons than mammals: they use their mouth to grab or bite, and possibly wrap their bodies around birds. Mammals have feet and claws that birds must avoid too.
I think nest defense behavior is fascinating. Having had the experience of walking by a red-winged blackbird nest, I knew that birds attempted to drive predators away from their chicks. But I didn’t realize they would stick around to fight a predator that could be dangerous to them too! After seeing the intense footage below, I have a new appreciation for just how far birds will go to protect their offspring.
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This summer, to celebrate Year of the Bird and 100 years of bird conservation under the Migratory Bird Treaty Act, we’ll be posting regular articles about grassland bird nesting ecology. My M.S. thesis focused on grassland bird nesting ecology, and I’m excited to share my knowledge—and stories from the field- with you! We’ll go into the nest to learn about chick behavior, adult sleep habits, feeding and fledging. We’ll discuss predation and learn about how adult birds respond to different predators. You’ll get to see beautiful photos of nests, eggs, and chicks, as well as video footage straight from the nest! Best of all, the next time you’re out hiking in your favorite Wisconsin prairie, you’ll feel a bit closer to the birds you love.
If you’re interested in reading more and can’t wait for the next post, you can read more about my thesis work here.
Stay tuned for our next edition of Into the Nest, coming soon!
Written by Carolyn Byers, Madison Audubon education director
Ellison, Kevin and Ribic, Christine, "Nest Defense- Grassland Bird Responses To Snakes" (2012). USGS Northern Prairie Wildlife Research Center. Paper 252. http://digitalcommons.unl.edu/usgsnpwrc/252
Cover photo credit: Ribic research lab