Tuesday, June 18, 2013


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

Tuesday, June 4, 2013


The islands off Yaquina Head in Oregon have always been just right for nesting seabirds. Isolation from the mainland gives them safety from mammalian predators, a very important feature for colonial birds. Colonies of nesting birds would be sure to attract predators such as coyotes, foxes, raccoons, and other carnivorous mammals, but apparently most of these mammals are not inclined to swim to offshore islands, so birds that nest on these islands are relatively safe.

These are scenes from Yaquina Head on 6 May 2011. A great colony of Common Murres has been there for many years, occupying the tops of several of the islands. In addition, large numbers of Brandt’s Cormorants nest among the murres and on lower ledges, and Pelagic Cormorants and Pigeon Guillemots fill in other parts of the islands.

The colony was flourishing during our visit, probably several thousand murres and good numbers of the other species. But the colony faced threats like never before. Bald Eagles have continued to increase every year since the banning of DDT in North America, and they are proving very effective predators on colonial nesting birds. Once the birds began to lay eggs, eagles disturbed the colony daily, taking both adult and young murres as well as eggs, and the massive disturbances caused by their presence enabled crows, ravens and gulls to take additional eggs and young.

Unexpectedly, Brown Pelicans proved almost as great a threat. Immature birds, not old enough to return to breeding colonies, stayed at Yaquina Head through the summer and visited the murre colony regularly. Walking and flapping through the colony, they picked up dropped fish and then began picking up young birds to get them to disgorge their fish. If that wasn’t enough, they also began to swallow the chicks whole. Many more chicks fell from their nesting ledges and drowned in the surf. Further disturbance was caused by Turkey Vultures that visited the colony.

During 372 hours of monitoring the colony in 2011, observers from the Hatfield Marine Science Center recorded 186 disturbance events, during which 1034 eggs, 142 chicks, and 70 adult murres were taken. Depredation rates were three to ten times higher than in previous years. Researchers estimated that no more than 28% of murre pairs successfully raised chicks to fledging age. Cormorants, much more spread out and with larger chicks, appeared to suffer much less mortality.

Interestingly, not only Bald Eagles but also Brown Pelicans and Turkey Vultures are increasing in the Pacific Northwest in recent years, perhaps all as a result of the removal of DDT from the environment. A new balance may be struck as one set of species becomes less common as previously rare species increase. How will we protect the murres now that we have protected their predators?

Dennis Paulson
Nature Blog Network