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Archive for July, 2011

Northern Limits

10 July 2011
Northern Limits
By Keith Bildstein, Ph.D.

The geographic range of Turkey Vultures extends south to Tierra del Fuego in southernmost South America, and north to south-central Saskatchewan, Canada, in North America.  Hawk Mountain has been working with Dr. Stuart Houston and other collaborators in Saskatoon, Saskatchewan, on vultures since 2004, tracking their migrations by satellite and examining the extent of their breeding home ranges in Canada.  Until earlier this month, however, we had not assessed the distribution and abundance of the Canadian populations to which these birds belong. 

All of that changed, however, when Hawk Mountain Research Associate, Dr. Marc Bechard, and I spent eight days traveling through parts of northern Minnesota, southern Manitoba, and southern Saskatchewan looking for individual vultures between 29 June and 6 July 2011.  The field work involved daytime counts along more than 1700 miles of roadways through wooded, wetland, and agricultural landscapes, searching for perched and flying birds.  The weather was perfect—most days were in the 70s and 80s, and the skies were clear-blue or, at worst, with few clouds.  Even so, our sightings of 50 flying and 1 perched vultures—which confirmed what we thought might occur–represent a modest showing by a typically common scavenger.

Ornithologists usually do their field work “ where the birds are,”  that is to say in areas where the birds are common and abundant, typically near the center of a species’ range.  And most of what we know about Turkey Vultures follows this general pattern.  In many instances, however, the real “conservation action” happens at the limits of a species’ range, where contractions or expansions of populations often presage overall declines and abundances elsewhere in the range.

Last week’s road surveys in central Canada, at the edge of the species’ northern range in south-central, together with Hawk Mountain collaborator Stuart Houston’s detailed studies of the species breeding biology there, offer insights into what is now happening this part of the bird’s range, and also serves as a benchmark for future studies and surveys.

A technical paper due to appear in the Wilson Journal of Ornithology this autumn, exemplifies the Sanctuary’s “edge-of-the-range” approach to vulture study.  The article, written by Stuart Houston and his coworkers, together with  Marc Bechard, David Barber and me, details the sizes of the breeding home ranges of satellite-tagged Turkey Vultures in central Saskatchewan. 

Simply put, the results indicate that the sizes of breeding home ranges there differ little from those that occur farther south, suggesting that Turkey Vultures behave similarly both toward the centers and peripheries of their geographic range.  This, together with preliminary data from Houston’s careful breeding studies–indicating relatively good productivity among these northern-most breeders–suggest that the vultures in central Saskatchewan are faring as well as those farther south, at least during the breeding season. 

The former, however, migrate much farther than the latter, and our work with satellite-tracked Canadian birds indicates that long-distance migration can be hazardous at times, with birds succumbing to road-side crashes with vehicles, collisions with high-tension power lines, and even a bombing death on a military artillery range. 

How all of this balances out in the end is something we are now looking into.

Although last-week’s surveys yielded sightings of only 51 Turkey Vultures, when viewed as part of a much larger whole, our Canadian surveys represent an important, if not essential, aspect of our long-term studies of this important sentinel scavenger.

Next week I leave for the Falkland Islands for four weeks of studies there.  Expect to hear more from me about that work soon.

Keith

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