Understanding the factors that
affect a bird species' nesting success can be crucial for planning effective
conservation efforts. However, many studies of nesting birds last only a few
years—and that means they can miss the effects of long-term variation and rare
events. A new study from The Auk: Ornithological Advances demonstrates this
with nearly four decades of data from Song Sparrows in British Columbia.
The University of British
Columbia's Merle Crombie and Peter Arcese used 39 years of data from an island
population of Song Sparrows to examine how the factors influencing their
nesting success changed over long periods of time. Over almost 3,000 nesting
attempts, 64% of which were successful, a number of patterns emerged. Some,
such as the fact that older female birds were less successful,
remained consistent over time. However, others, such as the effects of
rainfall, population density, and nest parasitism, interacted with each other
in complex ways that caused their importance to wax and wane over the decades,
and inbreeding only became a significant negative factor when it increased
sharply during the middle portion of the study. Unpredictable, rare
fluctuations such as this can have large effects that shorter-term studies
rarely capture.
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