Trees are not the only organisms displaying signs of renewed life this spring. The last few weeks have also heralded the beginning of the mating season for many of our bird species, a few of which are utilizing the new nest boxes that staff installed earlier this year. Below is a snap shot of the nesting activity recorded in our boxes so far.
Black-capped chickadees search for nest cavities and begin building their nests well before migrating birds return. Indeed, a pair of chickadees clambered inside one of our chickadee tubes five minutes after we installed it in early March.
Although both the male and female search for nest sites, only the female chickadee builds the nest. She starts by gathering several inches of moss and lichen. Next she adds softer materials, such as feathers, soft grasses, bullrush fluff, and fur. Finally, she creates a cozy nest cup lined with even more animal fur. The result is a snug bed that will keep the eggs, and hatchlings, nice and warm.
Like most song birds, female chickadees lay one egg a day and only start incubating once they have laid the entire clutch. At the time of writing incubating females were in 6 of our 7 chickadee tubes, sitting on clutches of between 7 and 8 eggs. All of these eggs should hatch by Memorial Day weekend.
Unfortunately, no eastern bluebirds have moved into our bluebird boxes this season. This may be because graceful tree swallows quickly claimed the real estate (4 of our 6 boxes) instead. Beautifully iridescent, these smalls birds perform aerial acrobats over Kent Field and the Ponds every afternoon.
Male tree swallows find and defend nest sites. Once they claim a site, they begin courting females, who build the nests alone. The female builds her entire nest using dry grasses. The nest is not complete, however, without a layer of feathers, preferably white, which surround and cover the eggs. So far, our tree swallow nests each hold between 3 and 5 eggs. Since females have not started incubating, the hatch date is still weeks away.
Eastern Screech Owl Boxes
Although the screech owl boxes and great horned owl platforms were not installed in time for the owl mating season, the space has not been wasted. A male house wren (a forest bird smaller than a chickadee) is building a huge nest in one of the boxes. Unlike chickadees and tree swallows, male house wrens build the nest themselves, using only twigs. If this nest impresses a female, she will top it off with a nest cup of soft, dry grass to lay her eggs.
Cavity dwelling birds generally prefer nest sites with entrances just wide enough for them to fit through. This reduces competition from larger, stronger species. The tiny house wren is a feisty exception to this rule and often evicts the larger chickadees, tree swallows, and eastern bluebirds from their nests. That the smallest bird chose the nest box with the largest entrance hole is fitting for a species ready for confrontation.
Finally, house sparrows. This species is a non-native nest cavity competitor, and we remove or sabotage any nests found on the grounds. Fortunately, these are fairly easy to identify. House sparrows build large bower shaped nests with both dry and green grasses. They often incorporate pieces of plastic and other trash as well.
Visitors may see nest boxes topped with flashy streamers. These are sparrow spookers, which scare male house sparrows and prevent them from landing on the box.
We install sparrow spookers on active tree swallow boxes, after the female tree swallow lays her first egg. The spookers will come down as soon as her young leave the nest. This prevents house sparrows from becoming accustomed to them, prolonging their effectiveness.
Interested in learning more about nest boxes? Stop by the Visitor Center in the Hunnewell Building to see a few on display. Interested in birding at the Arboretum? Check our calendar for upcoming bird walks led by Arboretum docent Bob Mayer.