VCE collected small amounts of blood and feathers, which will be subjected to mitochondrial DNA analysis to document the bird’s maternal roots. This will provide insight (although not conclusive proof) into whether in fact its parents crossed species boundaries and mated. It seems likely that the father was a Veery, the mother a Bicknell’s Thrush. This would help explain the bird’s song, which was predominantly that of a Veery (young birds learn songs in part by imprinting while in the nest).
Assuming the bird is a hybrid, the question naturally arises as to how these individuals of two ecologically separate species found one another. The two do not typically overlap in habitat use. Bicknell’s Thrush is specialized on spruce-fir forests that grow only on mountaintops above ~3000 feet elevation in the northeastern US, Quebec, and Canadian Maritimes. Veery is a species of lowland hardwoods forests, primarily second growth and floodplain. Does occupancy of mountaintop forests by a Veery signal the upslope creep of deciduous forests due to climate change, as has been shown by recent UVM research? Possibly, but it is far too early to make that case. It is but one of several possible explanations for why a Veery and Bicknell’s Thrush might have paired up.
The most reasonable explanation for this presumed hybrid mating is one of serendipity. Birds are mobile creatures, and these two species are, after all, closely related, with similar plumage and vocalizations. Hybridization among birds is widespread, although uncommon, having been recorded in more than 2,000 unique species pairs. Blue- and Golden-winged Warblers provide a familiar example of two species that regularly interbreed and produce offspring that themselves may be fertile. In fact, introgression of Blue-winged Warbler genes into Golden-winged Warbler populations has been implicated as a cause of Blue-wingeds’ range expansion at the expense of Golden-wingeds. Most avian hybrids, however, show reduced or no fertility, such that hybridization is rarely significant in a biological or evolutionary sense. In the case of the Stratton “thrush”, it is best viewed as a scientific curiosity for now.
Recent birding reports have been dominated by accounts of juveniles and family groups. As always in mid-late summer, fledglings abound and are quite conspicuous as they beg from their parents. These young, often unwary birds can provide great opportunities for close-up viewing and becoming familiar with the short-lived juvenile plumage. Many songbirds are now feeding fledged young, and the early migrants (e.g., some flycatchers and warblers) will soon be dispersing southward. The vanguards of shorebird migration have begun to appear en route south from their arctic and subarctic breeding grounds—a Greater Yellowlegs was observed in South hero on July 20th and a Least Sandpiper in
Two breeding species recently removed from
You can explore all the birds reported last week in Vermont and add your own sightings at Vermont eBird.-by Chris Rimmer