科学美国人60秒:Grazing Cattle Trim the Menu for Birds

When birds face the destruction of their habitat, some species don’t make it while others survive. But what happens at the very beginning of the process, just as a bird's habitat starts to change?

Research in Argentina's Monte Desert has provided some answers. Protects parts of the desert have lots of plant diversity: trees, tall shrubs, short shrubs, grasses and flowering plants. With so many options, most seed-eating birds choose to feast on large grass seeds. The birds can get all the energy and nutrients they need with minimal effort.

But when cattle show up to graze the desert's natural landscape, birds face changes in food availability. Some birds are happy to change their diets in response. Others, not so much. And it's the ones set in their ways that are at the highest risk. Understanding how birds react to grazing can help conservationists figure out how to help those species most in jeopardy

Ecologists from the Argentine Arid Zones Research Institute compared soil samples from the desert's Ñacuñán biosphere reserve to samples from two neighboring cattle ranches. They discovered that grass seeds—the birds' favorites—were just one quarter as likely to be found in the ranches compared with the reserve.

Next, they captured birds and flushed their digestive tracts to see what they were eating. The Common Diuca-Finch [finch sounds] and the Rufous-Collared Sparrow [sparrow sounds] had adjusted their diets, opting to dine on their less preferred options at the ranches, even while they still focused on large grass seeds in the reserve.

Meanwhile, the Many-Colored Chaco Finch [chaco finch sounds] and the Ringed Warbling-Finch [warbling finch sounds] were apparently unable to switch their foraging tactics. Even at the ranches, they worked hard to find the few grass seeds available. If they burn more energy foraging than they get from the few seeds they find, they could starve. At best, their dietary rigidity could limit their ability to reproduce or to care for their young. The results are in the journal The Condor. [Luis Marone et al, Diet switching of seed-eating birds wintering in grazed habitats of the central Monte Desert, Argentina]

Studies like this can help predict which species are at higher risk in degraded habitats. And they can help ranchers protect these vulnerable species, even while allowing their livestock to graze. For example, the ranchers can plant species for their cattle that will also be more palatable and nutritious for local seed-eating birds. The cows won't care about the menu change—but the birds sure will.

—Jason G. Goldman