Birds are some of the most difficult animals to study because the fly so far and so fast. It's no wonder that biologists are still making astounding discoveries about these light-weight wonders. They grace our landscapes with color, song and acrobatics...but they do far more than that. They help pollinate key plant species. They turn seeds into protein. They plant seeds along their journeys...and they give so many other animals pleasure and food.
Bird populations matter.
Biologists for the first time have documented a second breeding
season during the annual cycle of five songbird species that spend
summers in temperate North America and winters in tropical Central and
South America
.
It was known that these species, which migrate at
night when there are fewer predators and the stars can guide their
journey, breed during their stay in temperate regions of the United
States and Canada.
But it turns out that they squeeze in a second
breeding season during a stopover in western Mexico on their southward
migration, said Sievert Rohwer a University of Washington professor
emeritus of biology and curator emeritus of birds at the Burke Museum
of Natural History and Culture at the UW.
"It's pretty much
unheard of to have a nocturnal migrant with a second breeding season.
It's a pretty special observation," Rohwer said. "We saw these birds
breeding and we were completely surprised."
Migratory
double-breeding has been observed in two Old World bird species on
their northward migration, but this is the first documented observation
of "migratory double breeders" in the New World, and the first anywhere
for the southward migration, Rohwer said.
The scientists traveled
to the lowland thorn forests of coastal western Mexico to survey and
collect songbirds that had raised their young in the United States and
Canada and then immediately migrated to Mexico to molt, or shed and
replace their feathers.
Yellow-billed cuckoos, orchard orioles, hooded orioles,
yellow-breasted chats and Cassin's vireos
But during July and August in three
consecutive summers, 2005-2007, the researchers found individuals from
five species - yellow-billed cuckoos, orchard orioles, hooded orioles,
yellow-breasted chats and Cassin's vireos - that were breeding rather
than molting.
They found evidence that the birds had, in fact,
bred earlier that year. Females of all five species examined in July
had dry and featherless brood patches, indicating they had bred earlier
that summer. (To more efficiently transfer heat to eggs, the abdominal
brood patch becomes featherless and thickened with fluid when females
are incubating, but as the young mature it dries out and remains
featherless.). In the Mexican breeding ground, there was a complete
absence of young birds, indicating the females had not bred in the area
of the thorn forests.
Active nests were found for two species and
males of all five species were singing and defending territories or
guarding females, behaviors associated with breeding. In addition,
isotopic analysis of the birds' tissues showed that many had recently
arrived in west Mexico from temperate areas farther north.
The observation is much more than an oddity in bird behavior, Sievert
Rohwer said. He noted that orchard orioles might raise a first brood in
the Midwestern and south-central U.S. and a second on Mexico's western
coast, yet both sets of offspring find the same wintering area in
Central America.
The question is how both groups find the right place,
since they must travel in different directions.
Then there is the yellow-billed cuckoo, once commonly seen
throughout the western United States and as far north as the Seattle
area but now seldom seen along the West Coast. Disappearing habitat in
the U.S. is usually cited as the reason.
But Rohwer believes the
real problem could be the transformation of thorn forests of southern
Sonora and Sinaloa, states in northwestern Mexico, into irrigated
industrial farms. That loss of habitat, he said, could mean not enough
young are produced in the second breeding season to sustain the
populations previously seen on the U.S. West Coast.
"It turns out
that many of those migrants, both molt migrants and the newly
discovered migratory double breeders, are dependent on the low-altitude
thorn forests that become very productive during the monsoon," Rohwer
said.
The thorn forests lie in an arid and forbidding scrubland
that springs to life with the monsoon lasting from June through August.
The monsoon brings virtually all of the area's annual rainfall. The
small trees leaf out and insects become abundant, making an ideal
stopover for migrating songbirds.
However, with plenty of biting
insects, temperatures often at 100 degrees Fahrenheit and humidity
hovering near 100 percent, it is a difficult place for researchers to
work, so there has been little previous documentation of life in the
thorn forest. The new findings could spur more work there.
"For
western North America, the conservation implications are pretty
serious," Rohwer said. "Biologists know theoretically that they should
pay attention to these migration stopover sites, but they've been
largely ignored for their conservation implications."
