|"Soaring" (1950) by Andrew Wyeth. Shelburne Museum, Shelburne, VT.|
What I found puzzling is that the number of vultures counted at the roost tree never varied by more than several birds on each of my visits conducted from late spring through the summer. This seemed odd being they appeared to be all adults and I thought that they would have been incubating eggs and caring for young for much of that time rather than hanging out at a communal roost. For a plausible explanation I delved into several references on vulture life history.
According to the species account given on the EOL website, Turkey Vultures at northern temperate latitudes lay eggs (typically 2) between May and June. The incubation period is 32 to 41 days (average 35 days) and the young fledge the nest at 70 to 80 days of age. There is little data from Vermont establishing when nesting typically begins; however, assuming a date of May 10, egg hatch would be expected to take place about June 14 with young fledged some time between late August and early September. Both parents share in egg incubation and the care of the young. This eliminates the possibility that the roost is comprised of mostly post-mating males. Rather, sexual maturity is not attained until 3 years of age which suggests the vultures seen at the roost through the summer were likely sub-adult birds with the possibility of a few mature individuals that either failed to mate or experienced nest failure.
On the morning of September 30, my last count of the year, a total of 36 vultures were counted at the roost. A few weeks later the roost tree was abandoned as the birds undertook fall migration to points south for the winter. Sometime around the last week of February or first week of March the first Turkey Vultures of the New Year can be expected to return to Vermont on steady outstretched wings riding spring air currents with seemingly little to no effort as captured by Wyeth.