Wednesday, March 7, 2012

The Nests of Winter

A well preserved nest of a Red-eyed Vireo. This nest is attached to its branch by the rim
It may be counterintuitive, but winter is the easiest time of year to find bird nests; and yes, this is obviously long after the builders are gone and the nests have become inactive. It’s only when the leaves are off the trees, that these nests, which are normally camouflaged or concealed by foliage, become visible to us. Of course, this is true only for nests that are placed well above the ground. Those on or near the ground may be covered with snow, or remain concealed by vegetation.
Song Sparrow nest hidden in a Hemlock & protected by a tree fence 
I admit to some frustration as these nests are revealed and I see how close they were to the areas that I frequented during the breeding season. Apparently,  I walked right by some of these nests without realizing they were there. A winter walk with me is likely to be peppered with statements like: “So that’s where that tanager nest was! … Dam it! Why didn’t I notice that last summer?” On one occasion during the summer, I remember sitting on a bench, working on a report and being scolded by a pair of Song Sparrows. Evidently, they were protecting a nest, but I had no clue where it was. Only now, more than 6 months later, did I discover its location; I found their long abandoned nest –tucked inside the base of a wire tree protector –not 10 feet away from my bench.
A very well preserved (if short necked) Baltimore Oriole nest
Many birds have distinct building styles –they use different building materials; they build at different elevations or attach their creations in ways indicative of their species.  As the winter progresses, these nests disintegrate and thus the identity of the builder becomes harder and harder to determine. However, some, like the woven nest of the Baltimore Oriole, remain quite recognizable even as spring comes closer and a new breeding season looms. The Baltimore Oriole’s nest usually looks a little like a baseball hanging inside of a gray stocking. The rim of the “stocking” is attached to an isolated bough of a tall shade tree. The carefully woven structure and its overall elasticity enabled the nest to stand up pretty well to the elements. Of course, these nests will not be used a second time –at least not by the birds. Most birds do not reuse their nests, not even within the same season, let alone after an entire year has gone by. Some birds like the Eastern Phoebe, will build a new nest on top of the remains of last year’s nest. Other birds –including cavity nesters like Tree Swallows, may, after deconstructing an old nest , create a new one in the same cavity, utilizing some of the salvaged materials. Some Raptors will reuse nests, or will refurbish their old ones, thus saving time and energy.
Ruins of a Catbird nest, capped and now inhabited by a mouse 

Many nests of the Gray Catbird become visible in winter but by then they are usually just barely recognizable; they certainly don’t hold up as well as the Oriole nests. As they are subjected to the elements, they begin to resemble a heap of twigs, haphazardly piled into a bush. But the relatively large size and composition of the nest ruins, together with its low height, tell us who the likely builder was.
A Catbird nest filled to the brim with mouse provisions

I’ve found many instances of bird nests being reused by Field Mice –sometimes as a winter home and sometimes as a storehouse for berries and seeds. A lid made from leaves and cottony plant fibers secured to the top of an old nest is a sure sign that a mouse has taken possession of it. 

A Field Mouse is the winter tenant of this Bluebird Box
A surprisingly large portion of our bluebird boxes become filled with mouse nests over the winter. This month as I begin cleaning the boxes before the upcoming breeding season, I will have to carefully probe dozens of mouse nests in order to determine if they are still active. I never evict an active mouse nest; instead, I wait until they are finished raising their families and then remove the nest and ready it for the birds.

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