For the first time, I’m seeing the prairie: not from the scratched, altitude-proofed windows of a plane, but against the calm wind of the Midwest. Mosquitoes cling to my leg like corn on the cob. The grass ripples in the breeze, then exhales. Across the sky, tiny white clouds travel across the blue sky like the hands on a clock.
My appreciation is dimmed by a dozen people. I’m not the only one drawn to the promise of prairie birds on a cool summer in June. At five in the morning, I rushed from my hotel room in Billings, Montana, where the 2014 Western Field Ornithologists conference is being held, and scoured the lobby for a sign reading “Snowy Mountain Grasslands.” Upon its discovery, I crammed into a packed van that drove past abandoned barns and hawk’s nests in a quest for a view of the prairie.
The world is silent. A Grasshopper Sparrow in mid-song is unaware that, through the dusky glass of a van, it is being watched. Inside the van, I am unaware that outside, in the June air, lie the ingredients for a symphony. The sun is the conductor. As it motions its rays over sleeping hills, the fluting of the meadowlarks begins in a pianissimo. Each note sound like how sweet peas taste: organic and sugary. The tinkling triangle of Horned Larks arise in contrast. They are quiet but insistent. And then crescendo! An Upland Sandpiper is the raunchy bachelor of the grassland. Its mellow wolf-whistle halts the careful instrumental schedule. Fluting longspurs reclaim the closing. And then back in the van it is; to the next stop and the next orchestra.
Here in Montana lies the last of North America’s raw prairie sounds. For most grasslands in the U.S., the end was imminent when the first colonists arrived. Montana is not without scars. Tilling, overgrazing, and development leave their nasty pockmarks on the landscape. But the grassland was designed to take blows; in fact, it depends on them, requiring fire and bison-grazing to trim it back from disrepair. When human presence reduced both, the grassland was at risk for undermaintenance. Now cattle and proper rotational grazing prove that a conclusion can be reached between nature and man.
Grass conquers all and gives all. For the breeding birds of Montana, the land is a machine that captures photons and creates usable matter. It is their Creator. For thousands of years, organisms scrambled to find their specific place in its arching web. In a fluid society where each bird has its own strategy for utilizing the resources given to them by the grass, many a species has evolved to fit a small niche, of which it is master. These are the obligate species that rely on the native grassland and have coevolved with it for thousands of years. Other species are facultative generalists who come and go as they please. Unlike the specialists, they are polygamous with the land, choosing quantity of habitat over quality.
Like humans, there are extroverts and introverts in the grassland. On the prairie, the movement of birds is like the molecules in a glass of water, with evolution governing instead of physics. The molecules are tugged by hundreds of factors. Risk of predatory attack is an ion, pulling together flocks of hundreds where lack of ground cover leaves birds vulnerable to watchful eyes. Crowds are not always effective: lots of mouths means extra energy expended, in searching for a food source large enough to support the entire flock. The larger the crowds, the scarcer these reserves are. Where there is a surplus of food, however, being too territorial wastes energy in defending against a endless slaught of thieves attracted to the promise of food. The dynamics behind flocking rely on these opposing factors as they push and pull in an infinite tug-of-war.
These are not simply theories. Where the ground is sparsely vegetated, massive flocks of Horned Larks, Lapland Longspurs, and Chestnut-collared Longspurs gather to feed. As the habitat moves from bare ground to mid-length grass so do the spatial relations between birds. Savannah Sparrows arrange their spacing based on the amount of predatory disturbance in a habitat; the higher the disturbance, the more likely a sparrow is to be found in the proximity of another. In the wet and long-grassed parts of the prairie the secretive Baird's Sparrow forages alone.
The masters of the grassland, its top trophic predators, are just as subject to the pulls of millenia as the songbirds. Over the grasslands of Montana, three buteos make themselves at home. Few characteristics distinguish them: the Ferruginous Hawk is the biggest of them all, and its feathered tarsi separate it from the familiar Red-tailed Hawk.The most graceful of them all, the Swainson’s Hawk, migrates in swathes of thousands from South America to the North American grasslands, where it coexists with the other two buteos. On a clear summer day it’s possible to see all three in one sky, the white wing patches of the Ferruginous Hawk distinguishing it from the long wings of the Swainson’s Hawk and the Red-tailed Hawk’s dark patagials. All three breed here in Montana. It’s a coexistence that defies the resources of the prairie. Raptors require trees, and on the grassland, they come in short supply. Large birds also require sizable prey. In a thousand other versions of the grassland, the three species would exist in separate domains.
What watching the hawks on a clear summer day does not reveal are the subtleties in the life cycle of each species. Resource partitioning, or specialization in response to interspecific competition, is obvious in some families. The beaks of shorebirds -- the upturned beaks of godwits, short stubby beaks of Sanderlings, and the long, curved appendage of the curlew --- all reflect their unique foraging methods and food sources. On the prairie, resource partitioning is more subtle than the comical shapes of shorebird bills. The Red-tailed Hawk and Ferruginous Hawk pick the same prey and nesting times. However, the nest sites of the Red-tailed Hawk are typically higher-elevation than either Swainson’s or Ferruginous Hawks, which prefer willows. Swainson’s Hawks fledge three weeks later and hunt smaller prey than either Ferruginous or Red-tailed Hawks. Each hawk overlaps in some life traits with the other two species, but differs enough that the distance between nests is insignificant to increasing species competition. Where there are enough resources, their existence is drawn on a delicate three-way Venn diagram.
Where evolution can take energy-saving shortcuts, it will. The drive to reduce interspecific competition is the not only pressure shaping the buteos of the prairie. For Swainson’s Hawks, compromises are taken to reduce competition with its own kind -- intraspecific competition. Shoulders are rubbed, not always between individuals, but age-groups: immatures and adults. Only adult Swainson’s Hawks breed. The non-breeding first-years, then, pose a wasteful drain to the small mammals adults hunt to give to growing chicks. So adult and first-year reach an unspoken solution -- abundant in the summer prairie, hopping and flying in easily breached swarms -- grasshoppers. For non-breeding birds, the opportunity to pick idly at nutrient-loaded insects outweighs the effort of constant hunting. For adults, the energy required to transport a single grasshopper to nestlings makes it a grossly inefficient endeavour. While adults hunt mammals, grasshoppers are left to the inexperienced combing of adolescents. The colloquial name "grasshopper hawk," belies a complex strategy that makes the most of limited resources.
If there’s one lesson to take away from the prairie, it’s that nature is only violent when it gets an individual or species ahead. Sometimes teamwork is the key to success. Other times, it is solitude. Here in Montana, I have the opportunity to appreciate the prairie both ways. As the many eyes in our van spot raptors I would have otherwise missed, I am reminded that for flocking birds, the same phenomenon occurs. Interactions within birds and within humans, then, are not so different. As always, we learn a little about ourselves by studying birds.