What’s up with the sharp-shinned hawk?

By BILL CHAISSON
Of a Feather
There are a number of bird species that are declining in numbers and explanations have been found for some of them. For example, migrants to the neotropics (northern South America, Central America, and Mexico) have for many years increasingly often found their wintering grounds to have been logged or slashed-and-burned and converted to agriculture. This has caused many wood warbler populations to decline. In my youth (the 1970s) spring migration was an overwhelming flood of birds moving north. This has now been reduced to a relative trickle.

At the other end of the equation are the birds who are adapted to open fields. They flourished through the 19th and early 20th centuries, when enormous tracts of land were devoted to agricultural uses. After World War II larger scale farming that was more highly mechanized and “chemicalized” reduced the suitability of ag land for birds, but then fields and pastures began to be abandoned and reverted to forest. Species like the dickcissel, the upland plover, and the bobwhite (among many others) have all declined as a result, particularly in the Northeast.

But then there are the enigmas: species that are seem to be less common, but may or may not be. One of these is the sharp-shinned hawk, which has shown decreasing numbers in migration since the 1980s.

Sharp-shins are the smaller of two very similar looking accipiters, the other being the Cooper’s hawk. Accipiters have long narrow tails and short, rounded wings. They are built for maneuverability, to dart rapidly among tree branches in pursuit of their principal prey, other birds. When they cross an open area, they fly with a characteristic pattern of several rapid wing beats and a short glide.

The phenomenon of their apparent decline attracted attention first during spring and fall migrations. These birds nest in forests and are secretive and hard to count most of the year. But in the spring and fall they join the cavalcade of raptors that move north and south with the seasonal change and birders sit on hills, ridges, and beaches and count them by the dozens as they fly by in their easily identifiable beat-and-glide manner.

Catherine Viverette of the Hawk Mountain Sanctuary in Pennsylvania and several other authors published a paper on the sharp-shinned populations in the January 1996 volume of The Auk, a professional journal for ornithologists. In it they gathered together several years of data from many counting locations along the coastal U.S. and inland along the Appalachians. She and her co-workers documented numerically what birders had claimed anecdotally, that the sharp-shins were declining more rapidly at the coastal migration sites than at the inland ones. They looked at Hawk Mountain and Cape May (the southern tip of New Jersey) in particular.

They also looked at Christmas Bird Count data over the years and uncovered a striking inverse correlation. As the number of sharp-shins observed in migration declined at Cape May, the number of hawks seen at bird feeders north of Cape May and Hawk Mountain increased through the years with the phenomenon being more pronounced in coastal states. According to Viverette, Christmas Bird Counts in New England between 1972 and 1992 showed a 500 percent increase in the number of sharp-shins. The hawks were staying up north for the winter.

At this point you might be wondering when I’m going to type the words “climate change.” Well, there you have it. The International Panel on Climate Change (IPCC) report in 1992 documented a series of especially warm years from the early 1980s into the 1990s. At the time they were the warmest years on record since the middle of the 19th century. These 1980s temperatures have, of course, in the last decade been very much superceded in the record books.

Cornell Laboratory of Ornithology’s Feeder Watch program has documented that sharp-shinned hawks kill more birds at feeders than do domestic cats. The Feeder Watch program data show that sharp-shin numbers decline at northern feeders after December, which suggests a much more protracted migration period than in former years, partly explaining the apparent decline.

More recent publications and information, however, still express concern over the low numbers of sharp-shinned hawks in some places. The site of the Connecticut Department of Energy and Environmental Protection blames the popularity of plate glass windows and doors. Hawks cannot see them and too often break their necks in pursuit of their passerine prey (which often die at the same window). The sharp-shin is classified as endangered in Connecticut.

A New Hampshire Audubon publication from 2009, “The State of New Hampshire’s Birds,” collected no data at all on sharp-shinned hawks. The taxon was in all the tables, but on one could say whether it was increasing or decreasing in abundance, as none were reported. This is not indicative of the national evaluation of the species though. The Cornell “Lab of O” site reports that the North American Breeding Bird Survey shows the numbers of this accipiter remaining stable between 1966 and 2015. The one subspecies that is considered endangered is the Puerto Rico sharp-shin, which has been placed on the federal endangered species list.