In the now persistently delivered songs of Northern Cardinalsand Song Sparrows is evidence, for some birds at least, that subtle but forceful endogenous changes, however incongruous they may be with the surrounding winter scenery, have been taking place.  The lively signature of tiny tracks across a skim of snow on an ice patch near one of our feeders looked like it could only have been left by small birds and mammals feeling the first stirrings of spring!

In the Dark-eyed Junco pictured below (detail photo of the wings folded over its back), the existence of a molt limit between the more worn, brownish unmolted, retained juvenal wing feathers and the more recently molted gray and black feathers is accentuated by the fact that its first prebasic molt (always partial or incomplete in this species) was asymmetrical.  Last fall, when it was a hatching year (HY) bird, it replaced the two innermost flight feathers (secondaries 8 & 9) on its left wing, but only the smaller innermost one on its right wing.  When the calendar year changed on January 1st, this individual automatically moved from the HY to the SY (second year) age class.   Because any after hatching year (AHY) juncos last fall underwent a complete prebasic molt, they have no molt limits.  Therefore, juncos clearly lacking any molt limits at this time of year are certainly after second year (ASY) birds.

In the fall of its hatching year, a woodpecker ordinarily does not molt any of its juvenal primary coverts; in its second fall, when it is aged SY, it molts up to several outer primary coverts, retaining its now very brownish juvenal inner primary coverts; it is not until its third fall, when it is a TY bird, that it replaces any inner primary coverts, and, as with the bird pictured below, a few middle  juvenal primary coverts (now extremely brown and worn in comparison to the recently molted primary coverts on either side) sometimes are not replaced.  So, with the change of the calendar year to 2003, the bird below became an ATY, specifically a 4Y (alive in its fourth calendar year), individual.

One of these recaptures was a second year (SY) Fox Sparrow originally banded on 1/08/03.  It had maximum fat deposits then and weighed a whopping 49.0 grams (the species weighs 35.0g, on average, without fat).  This is very near the heaviest FOSP that we have ever recorded, the maximum being 49.4g for an HY bird banded on 11/23/92.  Although observed at our feeders almost daily thereafter, this FOSP was not recaptured until 2/20, when it was caught in a mist net.  Apparently, following its original capture in a wire Potter trap, this bird became literally "trap shy;" our switch to mist nets on 2/20 apparently fooled it!  The question now is, will it become both trap and net shy?  At the recapture, it still had very heavy fat deposits, but weighed almost 5g less (44.1g), likely due to the much milder weather conditions leading up to the day and time of its recapture.
 

It may seem remarkable that a tiny insectivorous bird like the GCKI can survive winter conditions at Powdermill and elsewhere in the Northeast (as far north as Maine).   Several factors likely contribute to its ability to do this.  First, although its small size means that it has a low volume to surface area ratio (i.e., a relatively large surface area across which its body heat can be lost) and, consequently, a higher per gram metabolic rate than larger species, its diminutive size also means that its total caloric demand is relatively low and that it can satisfy this with comparatively very small absolute quantities of food.

Another factor contributing to this species' survival in winter may be its reported ability to become hypothermic (significantly reducing its body temperture and metabolic rate) overnight, when fasting is unavoidable and survival depends on the adequacy of stored fat reserves for meeting the overnight energy demand.  Researchers Charles Blem and John Pagels (1984, Condor 86:491-492) estimated that the maximum amount of fat stored by a GCKI would be insufficient to meet its overnight caloric demand at freezing temperatures unless it underwent nocturnal hypothermia.

Lastly, GCKIs are morphologically and behaviorally specialized for foraging in conifers, whose dense evergreen foliage both harbors the various suspended life stages of overwintering insects that they prey upon and creates a much more sheltered (i.e., energy-conserving) microclimate in which they can feed and roost.
 

New (unbanded, that is!) birds have been captured far less frequently in recent weeks as the local bird population shifts to winter "residents" such as this male White-breasted Nuthatch photographed December 7th.