Geography Reference
In-Depth Information
the annual productivity of 8,201 pairs of birds, mostly without sighting—or
disturbing—their nests. Like Swainson's thrushes and robins, each pair of to-
whees or Pacii c wrens l edged two young per year. Juncos, song sparrows,
and Bewick's wrens were a bit more successful, typically l edging three to four
young per year.
The collection of species that we could study in detail—what we call our
“focal species”—dif ered substantially in annual productivity within and around
subirdia. Swainson's thrushes and Pacii c wrens were most productive in for-
est reserves. Juncos were especially productive in developments. Robins,
song sparrows, Bewick's wrens, and spotted towhees were equally successful
in reserves and in neighborhoods that were undergoing construction. The
ability of towhees to capitalize on nesting near forest edges was also noted by
researchers in nearby Portland, Oregon.
Woodpeckers and birds that nest in the holes they drill into trees broad-
ened our perspective a bit more. Tina Blewett and Jorge Tomasevic studied
more than three hundred nests of native primary and secondary cavity nest-
ers. Primary cavity nesters are what we call woodpeckers. Secondary cavity
nesters include native species, such as chickadees, which nest in old woodpecker
burrows, and nonnative species, such as the European starling and house spar-
row, which nest in crevices, cracks, and holes in our houses and other built
structures. Compared with open-cup nesters, cavity nesters had extremely high
nesting success. Their relatively large and very noisy broods grew up in the
security of a solid-walled house and were only infrequently preyed on. Snakes
cannot climb up slick, metal poles to eat young violet-green swallows in
weatherproof and warm nests placed in streetlamp i xtures high above the
road. Even the most persistent raccoon cannot reach a brood of starlings that
grows in a cement utility pole. Jorge and Tina found that only one in i ve na-
tive primary and secondary cavity nests failed to l edge young. This result is
substantially better than the one-in-two average nest failure rate experienced
 
Search WWH ::




Custom Search