SHORT COMMUNICATIONS 
157 
European passerines have delayed autumnal 
migration, although other species have advanced 
departure dates (Jenni and Kery 2003). It is 
possible the migratory songbird prey of Sharp- 
shinned Hawks in boreal forests north of HRBO 
(Bildstein and Meyer 2000) could delay their 
migration if their food was available for longer 
summers ( cf Penuelas and Filella 2001), which in 
turn could cause a later migration of hawks that 
track the movement of passerines. Raptorial 
species, such as accipiters, use powered flight 
for migration and must hunt regularly while on 
migration (Ydenberg et al. 2007, Goodrich and 
Smith 2008). However, we know of no data 
indicating temporal changes in autumnal songbird 
migrations north of or at HRBO (G. J. Neimi, 
pers. comm.). 
A possible explanation for the shift in timing 
may be migratory short-stopping, whereby Sharp- 
shinned Hawks north of HRBO may be moving 
less, and perhaps less per day, in response to 
increased prey availability (possibly at bird 
feeders) north of HRBO. This could result in 
hawks taking longer to pass through HRBO. This 
phenomenon was suggested as one explanation for 
declining, inter-year autumnal numbers of Sharp- 
shinned Hawks observed at Hawk Mountain, 
Pennsylvania and Cape May Point, New Jersey 
(Viverette et al. 1996). However, there has been 
no significant inter-year variation in counts of 
Sharp-shinned Hawks at HRBO. Further, the 
decline in observations of Sharp-shinned Hawks 
at Cape May particularly involved juveniles 
(Viverette et al. 1996), and the shift in later 
timing of migration at HRBO is likely by adults. 
There is no pattern of agreement in trends of 
counts of migrating Sharp-shinned Hawks at 
different watch sites (including HRBO) in eastern 
North America. This suggests there is consider¬ 
able spatial structure in a regional population or 
that migration geography varies with sub-region 
(Farmer et al. 2008). 
Our results augment findings demonstrating 
recent shifts in phenological events for birds and 
other animals (Root et al. 2003, Crick 2004, 
Miller-Rushing et al. 2008), and we examined 
factors possibly altering the timing of Sharp- 
shinned Hawk autumnal migration in northcentral 
North America. The potential proximate causes, 
such as the timing of autumnal songbird migration 
north of HRBO, and the consequences of the later 
timing of migration of hawks should be investi¬ 
gated. Changes in environmental conditions could 
influence the survivorship of maladjusted individ¬ 
uals given potential decoupling between migra¬ 
tion schedules of Sharp-shinned Hawks and their 
songbird prey (Both et al. 2006, Heller and 
Zavaleta 2008). 
ACKNOWLEDGMENTS 
We thank the many individuals who counted migrating 
birds of prey at HRBO. Partial funding for this study came 
from the Personnel Development Committee at the 
University of Wisconsin at Stevens Point. This manuscript 
was improved by the comments of E. A. Anderson, John 
Bielefeldt, T. L. Booms, M. A. Bozek, C. E. Braun, W. E. 
Stout, and two anonymous reviewers. 
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