By Lee Anna Sherman
Nothing looks more vulnerable than a meadowlark hatchling: a scrap of fluff, a fraction of an ounce, blind, immobile except for its gaping mouth. As if that’s not enough fragility, the baby bird’s bowl-shaped nest sits on the ground — the same ground where herds of 800-pound cattle may graze.
But the threat implied by this scenario isn’t as dire as it seems. So says Pat Kennedy, an Oregon State University ornithologist who studies western meadowlarks (Sturnella neglecta) and other ground-nesting songbirds on the Zumwalt Prairie in Oregon’s northeast corner, where The Nature Conservancy is partnering with OSU to research the effects of cattle management on grasslands. While grassland birds across North America have declined steeply in recent years, the hooves of cows are not the only, or even the main, culprit, according to Kennedy.
“These birds have evolved along with things that walk around — elk, deer, bison — for millennia,” says Kennedy. “They hide their nests in thick vegetation or low-traffic spots. If their eggs get mushed, they double-clutch. Trampling is not the biggest problem for grassland birds.”
Bigger problems than crushed nests, she says, are predation and starvation. When cattle tromp and chomp on vegetation, nests may become more visible to badgers, bobcats and several species of hawks in the genus Buteo (red-tailed, ferruginous and Swainson’s) that Kennedy studies along with songbirds, including the horned lark, savannah sparrow, vesper sparrow and grasshopper sparrow. Also, insects may be scarcer where cattle graze heavily. “Songbirds are insectivores,” Kennedy says. “They like bugs, especially bugs that live in tall grass. Cattle reduce those bugs.” Fewer bees, spiders and butterflies mean fewer meals for a chick’s gaping mouth.
But these kinds of cattle-related problems crop up mainly at heavy stocking rates — that is, where per-acre cow density is high and the animals graze for long periods on the same pasture. In one recent study, Kennedy reports, “High stocking rates had significant effects on all food web components.” On the other hand, low to moderate rates showed far fewer negative effects.
Teasing out the tangled variables in prairie ecosystems — not just the hooved browsers and the winged nesters but the native pollinators and leafhoppers, the bunchgrasses and riparian plants, the raptors and invasive weeds — has dominated the career of Kennedy and several colleagues for a couple of decades. “Grazing can be compatible with birds, just as logging can be sustainable,” she concludes. “It’s all about finding where the thresholds occur and then managing around them.”
Oregon State ornithologists are carrying out studies from the Oregon coast to the Willamette Valley, Cascade Range, Columbia Plateau and the Zumwalt Prairie. See Avian Nations in the fall 2014 issue of Terra.
CATEGORIES: Healthy Planet