Winter Robin (Photos by Don Reimer)
Winter Robin (Photos by Don Reimer)
You may have noticed scores of American Robins throughout the midcoast this winter. Sizeable numbers were encountered on most of December’s local Christmas bird counts as well. Although robins are considered traditional harbingers of spring, many spend their whole winter in their northern breeding range when ample food is available. Our January temperatures averaged warmer than usual and regional snowfalls were relatively light until February. Outside of the nesting season, robins join roosting winter flocks that may contain thousands of birds. Research shows that robins are most likely to stick around when snow depths average a foot or less.

Subsisting on a diet of invertebrates and fruit, summer robins eat different types of food depending on the time of day: more earthworms and small snails in the morning and more fruit later in the day. Because they forage largely on lawns, some robins are also vulnerable to pesticide poisoning.

Currently robin flocks are focused on cleaning up remnant patches of berries, rose hips and frozen apples. Apple trees are swarmed and foraged over until all remaining fruit is stripped away. Once these more desirable staples are depleted, robins turn to staghorn sumac fruits as a late-season alternative.

Robins are prolific breeders capable of producing three broods in one year. So why aren’t we overrun with robins? Mortality is one factor. Only 40 percent of nests successfully produce young, and roughly 25 percent of fledged young survive beyond November. It is estimated that half of the robins alive in any year will make it to the next year.

Last week I observed a vocal group of eight Eastern Bluebirds feasting on fruits in Warren village. Small contingents of bluebirds frequent Maine throughout most winters. Found only in North America, this iconic, once-common cousin of the robin suffered declines during the past century as farm acreage and orchard nest sites gradually diminished. Introduction from Europe of the House Sparrow (in 1851) and Starling (1890) led to further declines as these aggressive species competed for nesting cavities. Thankfully, bluebird populations have rebounded, due substantially to “trails” of bluebird nest boxes established across open grassy terrains.

If you’ve hosted bluebirds in your yard, you may find them slightly fickle in claiming a nest box. Bluebird pairs may investigate potential nest sites but then vacate the premises for a spell. Meanwhile, nesting rivals such as Tree Swallows (an early spring migrant) and House Sparrows arrive to occupy these sites. This situation can lead to intense battles over permanent residency when the tardy bluebirds eventually return. Both bluebird mates typically enter a nest box during the courtship phase, but the female makes the final choice.

Here are some helpful hints if you’re considering a bluebird nest box this year: Bluebird boxes should be constructed of wood, not metal. Cedar and plywood work well. Light- colored or natural wood exteriors are best; dark-colored houses can get too hot inside. The nest hole opening should measure about 1 to 11⁄2 inches wide to exclude starlings and other larger intruders. The box opening should face away from direct sun. In some instances, wintering bluebirds may huddle inside existing nest boxes to stay warm during harsh weather. Remember to clean out the boxes prior to the spring nesting season. And if you lack dimensional blueprints or general interest to construct your own bluebird house this spring, Mid-Coast Audubon has plenty of custom-quality housing stock available.