Weather and Mountain Birds

Mountain weather has not been our ally this spring and early summer. This week's Tuesday-Thursday mist-netting stint on the Mt. Mansfield ridgeline featured the now-expected mix of swirling clouds, wind, and intermittent rain. Our session wasn't a complete wash-out, as we were able to run nets much of the time, but captures were sparse overall. Based on the condition of female incubation/brood patches, it appears that most species are still actively nesting. Winter Wrens (an early-nesting species) were the only persistent singers. Bicknell's Thrushes were very quiet, with just a brief burst of vocalizing at dusk.

So far this season, we have captured 25 individual Bicknell's Thrushes on the Mansfield ridgeline, a number that is about "normal". However, we have had fewer returning banded birds (i.e. those banded in previous years). Of 25 mist-netted individuals, 18 have been males and 7 females. There is a preponderance of yearling birds (13 males, 3 females), reflecting last summer's solid production in the absence of red squirrels and other predators (I saw the season's first squirrel on Wednesday). We recaptured one 10 year-old male thrush at the far northern edge of our study area, our first encounter with it since 2004.

Many birders, including Mountain Birdwatch field workers who are counting high-elevation birds all over the Northeast, have commented on how quiet the montane forest was in June. We've observed the same on Mansfield and suspect that the frequently inclement weather is in part responsible. Although montane forest birds and other wildlife are adapted to relatively harsh weather conditions, we can't help wondering about the effects of this year's weather on nesting success. In past years (this is our 20th on Mansfield), we have witnessed nestling abandonment and death following prolonged spells of bad weather. We assume that when adults become stressed by the elements, they may be unable balance their own energetic needs with those of their young, causing nestlings to die of exposure. We have several times found Bicknell's Thrush and Blackpoll Warbler nests with dead young, only later to confirm that the females were alive and well. When times are tough, it generally pays a relatively long-lived bird (if you consider several years to constitute a long life...) to ensure its own survival at the expense of current reproduction, so that it can live to breed again.

VCE is working with our long term datasets from Mansfield and other Vermont mountains to understand annual variation in local weather patterns, how they impact avian survivorship and productivity, and how they might relate to potential future climate changes. We'll be back on Mansfield in two weeks, hoping to observe and capture fledglings of different species, but wondering if that will be the case.

Chris Rimmer and Kent McFarland

Photo: Kent, aka Kapt. Krummholz, on a soggy Mt. Mansfield net run