Migrating hawks travel through Yolo county

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Shorter days and colder nights are not the only signs of the changing seasons in Yolo county. Crops are being harvested and birds are heading south. Migrating hawks can be seen perched on the telephone wires strung alongside those fields. From late August through early November, migrating hawks follow an unseen path down to their winter homes in the southern United States, Central America and South America. The hawks cut a path through Yolo county to follow the harvest activities. They aren’t looking for fresh alfalfa or newly shaken walnuts. They’re hunting mice and other agricultural pests. Hawks, which are a kind of raptor, are divided into three main groups: buteos, accipiters and falcons. Red-tailed hawks, which live in the region year-round and also migrate through Yolo county, are buteos. They can be identified by their eponymous tail and gliding flight pattern. Buteos are defined by their long wingspan, which allows the raptors to soar on air currents for long distances without flapping their wings. Other buteos, like the swainson’s hawk, are more noticeable during the migration season. These birds, which are listed as a threatened species in California, can be seen travelling in groups of 200-300. The Swainson’s hawk has the longest migration of the North American raptors. They travel from Canada and the United States all the way to Argentina. They have also been spotted travelling in mixed groups of adults and juveniles, which is uncommon for most migrating raptors. Accipiters like the Cooper’s hawk and the Sharp-shinned hawk also make their way through Yolo county on the path to the southern hemisphere. Unlike buteos, accipiters have shorter wings, which they use for quick, darting flight. They hunt smaller birds for prey. Some raptors are travelling vast distances to pass through Yolo county. Rough-legged hawks and Ferruginous hawks are high altitude dwelling arctic raptors, who head closer to the equator during the winter season. Shannon Skalos, a Ph. D. candidate at U.C. Davis, spoke with the Express about the region’s yearly influx of hawks. She explained that while each species has its own migration strategy, many use the region’s agricultural fields as a prey buffet. “Agricultural fields support a lot of raptors, in the summer and in the winter,” Skalos says. Farmers also benefit when the raptors eat the pests that gnaw on their crops and machinery. As the hawks are moving along their migratory path, researchers and ameture bird watchers track their populations and keep records of the numbers. These records can provide information about the migratory birds’ behavior. Skalos shared that while tracking birds at the Golden Gate Raptor Observatory in the Marin headlands, researchers discovered that there were more juvenile birds migrating along the coast. The adults birds flew inland, along the valley. The current theory for this is that the juveniles, being less familiar with the route, follow the coastline to get to their destination. Aside from learning birds’ behavior, researchers can use the travelling populations to predict the health of entire ecosystems. Raptors are apex predators, meaning that they are at the top of the food chain and have no natural predators. When researchers look at their population size, they can understand more about the organisms living lower on the food chain. If the population is large, the birds have had no trouble searching out prey. If the number is dwindling, then researchers can assume that some link along the food chain is broken. Maybe the water source dried up, which caused the native grasses to die off, which affected the grasshopper population, then the mouse population and eventually the Swainson’s hawk. Sometimes even a healthy prey population cannot protect the hawks from dying off. As apex predators, hawks suffer the effects of bioaccumulation. Bioaccumulation is the build up of a chemical in a living organism’s tissue. If a wild mouse consumes a small amount of rodenticide, it might survive. If a hawk eats 50 of those poisoned mice a day, every day, the dose of rodenticide in each of those mice builds up in the hawk’s body. Skalos says that the hawks she sees in the clinic are often suffering from the effects of bioaccumulation. Some are able to survive, but many aren’t. She suggests that people begin looking to hawks more as an ally in pest control, instead of turning to chemicals that eventually harm these raptors’ populations. For now, there are a few more weeks to spot these apex predators flying through Yolo county as they travel to their winter homes. ]]>

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