Geography Reference
In-Depth Information
Male house sparrows are larger than females throughout their extensive
geographic range, probably because of sexual selection for large males during
battles to claim breeding territories. The degree to which males are larger
than females, however, also increases in northern cities. Exceptional size in
male northern sparrows likely rel ects the dual benei ts accrued during com-
petitive duels over territory and limited food, as well as the ability to better
survive severe winter storms.
House sparrows demonstrate a superb ability to track new environmental
demands by adapting quickly to new climates. Ironically, despite this great
evolutionary potential at places where they have been introduced, house spar-
rows are declining rapidly in parts of North America, Australia, and espe-
cially their native Europe where they have gone from pest to conservation
concern in a few decades. Across the whole of the United Kingdom and much
of northwestern Europe, sparrow populations declined by more than 50 per-
cent from the early 1970s to the late 1990s. Loss of habitat within cities and
suburbs is one important reason for these declines. Al uent homeowners in
particular have removed rough grass, nettles, and native weeds that produce
the seeds and insects sparrows eat, paved portions of their front gardens to
increase parking space, and removed old roof tiles and rotten fascia boards
and soi ts the birds use for nesting. Sparrows are now mainly found in poor
neighborhoods where residents cannot af ord such improvements. As spar-
rows are forced into smaller green areas, they are also more vulnerable to local
pollution, competition with pigeons and starlings, and predation by sparrow-
hawks, tawny owls, and cats. Sparrowhawks, as their name implies, may be
especially important. The rise in the abundance and distribution of hawks
after environmental pollutants such as DDT were banned closely paralleled
the decline in sparrows. These myriad challenges are overwhelming the spar-
row's demonstrated ability to adapt and putting the burden of conservation
on the urban poor. Nest boxes, bird feeders, increased greening of al uent
 
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