Biology Reference
In-Depth Information
If a shrine to rarity exists, the jack pine woods near Grayling, Michigan, is nature's
Lourdes. Each spring, thousands of birders journey to this small town in the state's Lower
Peninsula, about five hours' drive north from Detroit, to encounter the rarest breeding song-
bird in North America. Territorial males return in early May, having winged in from their
wintering grounds in the Bahamas. The females linger on the islands a bit longer, fattening on
fruit and insects, before beginning the perilous 2,500-kilometer migration back to their only
significant breeding site. From the exotic Caribbean islands to the stunted jack pine woods
on the outskirts of Grayling, such is the seasonal arc of a most uncommon species.
The day after our birding excursion, a group of young marchers fidgeted under the bright
morning sun, eager to start the celebration. It was the third Saturday in May, time for the an-
nual Kirtland's Warbler Wildlife Festival. A roving reporter asked a young girl in line, “Do
you know why the Kirtland's warbler is so rare?” In response, she could have pinpointed part
of the answer by quoting a verse from Dr. Seuss, the one they teach in science class about the
Nutches, “who live in small caves, known as Niches.” Unfortunately for the Nutches, there
simply weren't enough niches to go around.
The singing male Kirtland's we witnessed in the pine grove had to compete for his niche,
limited breeding space. Kirtland's warblers have historically nested only in stands of small-
stature jack pine maintained by natural fires. This extreme preference about where they will
or will not make their nest gives them, and a large number of other species, a special place
in the Kingdom of Rarities. There are species that have limited ranges, such as the golden-
fronted bowerbirds, and those that live at low densities, such as the jaguars. The Kirtland's
warbler, singing boldly from its perch in a fire-prone stand of young jack pines, meets those
conditions and raises the ante. Beyond having a narrow range and low numbers, it repres-
ents another dimension of rarity: extreme habitat specialization. By this I mean a species that
breeds only in a particular habitat, feeds only there, or requires some other feature there to
thrive. That could be a cool, dry, or wet sanctuary to maintain an ideal body temperature or
the proximity of a river or stream from which to drink several times per day.
In the grand scheme of nature, many species are quite particular in their habitats. Arctic
lemmings and other species that reside in the tundra could not live anywhere else, for ex-
ample. Parrot fish are literally a fish out of water beyond their preferred tropical coral reefs.
Even the ubiquitous prairie dog could not survive anywhere except a grassland. All these
habitat specialists are, fortunately, adapted to living in an ecosystem that is widespread in the
world, at least today. But others, including the Kirtland's warbler, are specialists on highly
restricted habitats. I came to see how such extreme specialists among species persist and re-
cover, but I also wanted to explore in what circumstances such a narrow habitat bandwidth
becomes a liability. All of the patterns now present could be in flux: as climate change sci-
entists are telling us, the distributions of preferred habitats may change dramatically over the
coming century. Those species that are specialists on widespread habitats and are common
