Broad-winged Hawks of the Pacific Flyway

The Broad-winged Hawk (Buteo platypterus) is one of North America's smaller Buteos being about two thirds the size of a Red-tailed Hawk.  It is common and wide spread in the eastern half North America with an estimated breeding population of at least 1.7 million individuals.  It breeds throughout deciduous and mixed conifer-deciduous forests and hunts mostly small mammals and reptiles, but also includes the occasional bird, amphibian, or even more occasional insect.  Breeding densities have been estimated to range from 1 pair every 2 to 5 square kilometers.  However, breeding bird surveys appear to be inadequate at detecting Broad-winged Hawks do to how secretive they are when on their nesting territory.  Migration has proved to be a better point in their annual cycle to monitor population levels.

Along with the Swainson's Hawk (Buteo swainsoni), the Broad-winged Hawk is one of the raptor species that migrates the longest distances between its breeding grounds and non-breeding grounds which stretch from Mexico to Brazil.  As might be expected from the combination of how common they are and this long migration distance, this species is a very common member of fall hawkwatch counts in the eastern USA and in Central America.  Numbers in the 10s of thousands are not unusual at many sites (such as Hawk Mountain PA) and several sites have counts of 100s of thousands (such as Corpus Cristi, TX) and even over 1 million (such as Vera Cruz, MX).  It is unusual for a raptor in that groups of these birds migrate in flocks frequently forming large kettles that fill the sky as they move south.  But these are all eastern sites.  Do Broad-winged Hawk occur in the western half of the continent?

Before the 1980s the answer would have generally been no, but during the 1980s something started to change.  Sightings during migration have been increasing in many western states including Idaho, Montana, Wyoming, Colorado, New Mexico, and Arizona.  This suggests that the breeding range of the Broad-winged Hawk is extending to the west into Alberta and British Columbia.  The Golden Gate Raptor Observatory (GGRO) in California have been seeing them regularly since that fall migration was first discovered in 1972.  It remains the best place to spot a Broad-winged hawk west of the Rocky Mountains.  This year was an amazing Broad-winged year at the GGRO.  Most fall seasons see between 25 and 240 Broad-winged Hawks with numbers generally concentrated in the last half of September.  But during the 2012 season (and only through the end of November, since the count season is still ongoing) hawkwatchers have counted a record-shattering  755 Broad-winged Hawks!  This total included one day which recorded a total of 295 which is higher than the previous record season total of 248!  No one is completely sure what caused this boom of Broad-wings, but one interesting facet is that of the 755 birds seen this year, about 99% of them were hatch-year birds.  This indicates that the population of Broad-winged Hawks that breed in western Canada had a very good year this past spring and summer.

The western expansion of the Broad-winged Hawk breeding range roughly matches the westward movement of the Barred Owl (Strix varia).  It also roughly matches the eastern expansion of the Evening Grosbeak (Coccothraustes vespertinus) across the same geographic area of Canada, although the Evening Grosbeaks moved east earlier than the hawk or owl moved west.  All three of these species prosper in mixed deciduous-conifer forests, and that hints at a possible explanation.  These range expansions could be the result of increasing edge habitat that results from timber harvesting in areas of what would otherwise be wide swaths of coniferous forest.  They could also be due to the increased numbers of trees that are being planted in and around cities in the great plains of Canada and the USA as wind-breaks.  Such human-induced changes to the landscape will no doubt cause changes to the distributions of other organisms, and these three species may be examples.  More investigation into these changes in range are needed before any convincing explanation is reached.