Black-throated Blue Warbler nest with 4 eggs (photo by S. Sillett).

The nests of songbirds are vulnerable to predators because nestlings cannot run or fly until they are just about to leave the nest, and even their parents can’t prevent most predators from eating their eggs and nestlings. Common nest predators for small songbirds include bigger birds like jays and hawks, small mammals like squirrels, chipmunks, and mice, and reptiles like snakes and lizards. Birds build their nests in many different types of places, from the ground to the treetops, but no nest site is completely safe from the many species of mammals, snakes, lizards, and birds that feast on protein-rich eggs and nestlings. Nest predators are the dominant cause of nest failure for songbirds, and so the types and abundance of nest predators can have a huge effect on how successful birds are in raising their young.

The numbers of nest predators can vary a lot over time and between different locations, so differences in nest success between years or between different habitats can be due to differences in the number or types of nest predators. For example, squirrel populations change between years depending on how many acorns and other seeds the trees produced the year before. So, even though small birds don’t eat acorns, the number of acorns can actually end up having a huge effect on their reproductive success! The way humans affect natural landscapes is also having a big effect on nest predator communities. Roads and development lead to habitat fragmentation, which is the breaking up of large pieces of habitat into smaller pieces. Studies have shown that habitat fragmentation can lead to higher numbers and different species of nest predators, causing higher nest predation rates. Differences in nest predation rates between habitats can also occur naturally, without any human impacts, simply if the types or number of predators is different, or if the predators have more or less other food to eat.

Click here to see a video of a predation event in action!

Learning module developed by K. Langin, H. Sofaer and S. Sillett for the Hubbard Brook Research Foundation (2009).