Tuesday, June 18, 2013

ACCIPITERS IN A MUSEUM


Sharp-shinned Hawk (Accipiter striatus), Cooper’s Hawk (A. cooperii) and Northern Goshawk (A. gentilis) are three hawks of different sizes that are basically quite similar to one another. They are at home in wooded country, where they nest and usually forage. However, during migration, all three can be seen in open country, and all three can be seen at any time flying overhead, above the forest canopy.

All three eat primarily birds, and all three have relatively short, rounded wings and relatively long tails, useful aerodynamically as they chase their prey through the vegetation. All fly with a rapid flapping flight, interspersed with short glides.

The two smaller species, Sharp-shinned and Cooper’s, have become relatively common in urban/suburban settings, where they find abundant bird life, especially where people concentrate the birds by feeding them. So the hawks that appear in people’s yards are often one of these two species. The larger Northern Goshawk tends to stay in large tracts of conifer forests in wilderness areas.

People have trouble distinguishing the species, especially the two smaller ones, and there have been volumes written about the identification of accipiter hawks in North America. These Slater Museum specimens are put forward to furnish would-be identifiers a better idea of the relative sizes of the sexes and species. The study skins were chosen to be representative.


The first photo shows immatures of both species, the smaller males on the left and females on the right for each species. The size differences are quite apparent, and a female Cooper’s is very much larger than a male Sharp-shinned, but there is a steady gradient from one sex and then one species to the next. You can see that a female Sharp-shinned is as close in size to a male Cooper’s as it is to a male Sharp-shinned.

Immatures of the two species differ on average in the markings on their underparts, with Cooper’s tending to have finer, more distinct streaks and Sharp-shinned broader, blurrier streaks, often with some barring on the sides. Nevertheless, you can see that there is much variation. The best mark, if there is any doubt about the size, is the tail, more graduated in Cooper’s (outermost feathers substantially shorter than the central ones).

The second photo shows the same species as adults, in this case with only one female Cooper’s. Note first the difference in the tail shape. There is really no difference in the ventral color pattern. The final photo shows the upper sides of the same birds. Note the difference in color in the sexes, the males with distinctly bluer upperparts, as well as the better-defined dark cap of the Cooper’s. The male Cooper’s on the left is growing in a new central rectrix.

Dennis Paulson

1 comment:

wanderflechten said...

Lets hope so. With 100% survival we would actually be knee-deep in Mallards within 13 years. After 27 years the mass of Mallards would equal that of the earth.

(using: survival 9/year, average Mallard pair 3.3kg, average density of Mallard 0.5g/cc, volume from earth's surface (land and ocean) to knee-high (≈50cm) 2.6x10^8km^3, mass of earth 5.97x10^24kg)