If you live in eastern North America., you have probably seen an eastern bluebird! Males and females both have a blue head, back and wings with an orange chest, though females have a slightly duller color. They are often found sitting on telephone wires, fence posts and nest boxes. Unlike bluebirds, tree swallows (Tachycineta bicolor) migrate long distances. They winter in the southern U.S., Mexico and northern Central America and breed throughout central and northern North America. Nest boxes have helped expand their large breeding range as far south as northern Georgia.
Keeping Up with the Cavity Nesters
We monitor the nest boxes around SCBI weekly from March to the end of August. We check on the progress and health of chicks, note whether nests succeed or fail, watch for signs of predation and address pests (like wasps, ants and blowflies). We also check for harmful invasive species, like house sparrows, and remove their nests before they lay eggs. House sparrows are aggressive and known to outcompete North America’s native birds. They will kill adults and chicks, destroy eggs and then build their nests on top.
When we check a nest box, we first try to figure out which birds are using it. Even if the adults are out collecting construction material, we can guess what species might be nesting there. One indication is the style of nest. Invasive house sparrows typically build messy nests, filling the boxes with grass and bits of trash. House wrens fill their nest boxes with sticks. Tree swallow nests are typically made of dried grasses, with the nest cup full of soft feathers from other birds. Eastern bluebirds build nests of dried grasses and pine needles but don’t usually use feathers.
Eggs inside a nest can also provide clues. Tree swallow eggs are small and white, whereas an eastern bluebird’s eggs are a little larger and rounder. Bluebird eggs are almost always sky blue, though on rare occasions, they can lay white eggs. They usually lay three to five eggs, while tree swallows lay four to seven.