Biology Reference
In-Depth Information
Kirtland's Warblers are extraordinary specialists, nesting only on the ground beneath
the bottom branches of jack pine trees. Jack pine is fire-adapted. Its cones remain tightly
closed, often for many years, until exposed to the intense heat of fire, when the seeds are
releasedandgerminateontheburntground.Asjackpinetreesage,theylosetheirbottom
branches, making them unsuitable for Kirtland's Warblers to nest. Once the jack pines in
an area have lost their lower branches, that area will be abandoned by the warblers until
fire starts the cycle over again.
Kirtland's Warblers were never abundant, but declined dangerously in the midtwenti-
eth century until they numbered fewer than 200 pairs by the 1970s. Two factors caused
their decline. Fire suppression prevented new jack pines from replacing old ones, and
Brown-headed Cowbird populations mushroomed locally. By the late 1960s, in one
sampleof29nests,70percentwereparasitizedandonlytwofledglingwarblerswerepro-
duced.
Thanks to controlled burning and cowbird trapping, Kirtland's Warblers are currently
estimated at over 3,000 individuals.
The Wonderful Whooper
Whooping Cranes, which had dwindled to 15 or 16 individuals in 1941, numbered 328
in the wild (including natural and introduced populations) as of April 2009. Protection of
the wintering habitat at Aransas National Wildlife Refuge and intensive efforts at reintro-
duction are responsible for these increases.
Despite this success, Whooping Cranes are still extremely vulnerable. The natural
populationthatwintersintheestuaryalongtheTexasGulfCoastdependsonrichsupplies
ofbluecrabsfortheirwinterdiet.Whencrabsuppliesareabundant,thebirdshaveexcel-
lent winter survival and put on plenty of weight. During those years, they arrive on their
spring breeding grounds in top condition, and their breeding success in the north is high.
But blue crab numbers decline as the estuary's salinity increases. More Whooping
Cranes die during these winters, sometimes from emaciation, sometimes in collisions
with power lines and other structures as they fly greater distances in search of food. The
winter of 2008-09 was such a year; as of the end of April, they had lost 23 birds, more
than an 8 percent loss.
The salinity of the marsh is weather-related; when rainfall declines, water levels go
down in the Aransas River and other waterways emptying into the estuary. But these wa-
ter levels are also related to human consumption of water, not just for drinking and ag-
ricultural irrigation but also for swimming pools and lawns. As the human population
of Texas continues to grow, the future of the Whooping Crane remains in question. For-
tunately, a great many people and organizations remain focused and committed to pro-
tecting this beautiful and charismatic species.
