Goshawk invasions at Hawk Ridge: Indicator of changes in the boreal forest?

Background/Question/Methods

    For over 200 years, fur returns in Canada have shown a remarkably regular 10-year cycle in the numbers of snowshoe hares and their specialist predator, the Canada lynx.  For over 100 years, regular invasions of goshawks have been observed in southern Canada and the northern United States, generally following the periodic collapse of hare numbers in the boreal forest.  For the past 50 years, careful observations of the fall migration of hawks have provided more detailed information about invading goshawks.  We are interested in determining what these observations show about what is happening in the boreal forest, both during each 10-year cycle and in the longer term.

     In 1951 a fall hawk count was started at Hawk Ridge, in Duluth, Minnesota.  In 1972, the count was extended to cover most of the period of the hawk migration and a banding operation was begun.  We used the numbers of goshawks counted and their age distribution to help understand the nature of the cycles and an apparent change in their magnitude.

Results/Conclusions

     Since 1972, over 37000 goshawks have been counted at Hawk Ridge, and over 8000 have been trapped and banded.  Very large goshawk invasions were observed in the early 1970s and early 1980s; a smaller, but still substantial invasion was observed in the early 1990s.  Elevated numbers, but no sharp peak, were observed in the early 2000s, but the goshawk numbers have been low since then.  Earlier observations showed a substantial peak in the early 1960s, so there appear to have been regular goshawk invasions at Hawk Ridge for at least five decades, but the magnitude of the invasions seems to have been decreasing for the last 30 years.

     In non-invasion years, a majority of migrating goshawk are juveniles, while adults predominate in invasion years.  During the large invasions of the 1970s and 1980s there were very few juveniles, suggesting reproductive failure, but the numbers of juveniles have been higher during later invasions.  In later invasions, there have been fewer adults, suggesting either that there are fewer adults to migrate, or that the need to migrate is less.  The changing numbers of migrating goshawks and their age distribution suggest that there have been changes in the boreal forest community since the 1980s.