[cobirds] Re: Lamar BBS Route (Prowers and Kiowa) on 2June2017

If Swainson's Hawks are like other raptors, it probably takes a year or two before they attempt to breed. Rather than considering these flocks as late migrants, it's much more likely that they're subadult non-breeders that are spending their first/second summer on the breeding grounds without trying to breed. I came across a few flocks like these last summer when conducting raptor counts on Meadow Springs Ranch in NE Larimer County.

Walter Wehtje

Fort Collins

On Saturday, June 3, 2017 at 2:01:18 PM UTC-6, Dave Leatherman wrote:

Ran the Lamar Breeding Bird Survey (BBS) route yesterday with assistance from Janeal.  The route starts at Prowers MM/13 northeast of town, goes north to Prowers SS then east and north, ending at Kiowa 61/K several miles sse of Chivington .  Had black rail calling on SS about a mile e of 13, a first for the route.  Other interesting species were dickcissels, good numbers of Cassin's sparrows, orchard orioles on a few riparian stops, and this: 168 Swainson's Hawks over the last 22 stops, with 106 of them on one stop!  I do not know how to interpret this.  It would seem to be one of two things: a very late kettle still coming north because of the late spring storms OR families populated with already-fledged young staging for their return flight south later this summer.  Great, great majority of birds were immatures.  Timing seems odd for either scenario.  Maybe analysis of eBird data would shed light on which it is.  The birds were mostly on the ground in fallow wheat and/or corn fields.  There is currently a notable shortage of grasshoppers on the eastern plains of CO.  This may change as the season progresses but it could be that at present this largely insectivorous buteo is being forced into habitats mostly free of vegetation in order to make available prey items (including big tenebrionid beetles) more visible (similar to what mountain plovers do with their habitat selection).  Of note, we detected zero mountain plovers on the entire route and they are usually present on 5-6 stops.

If anyone has input on the Swainson's hawk phenomenon, I'm all ears.  Please share with the whole group.  Thank you.

Dave Leatherman

Fort Collins

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