Take a visit to the mixed conifer forests of the Santa Cruz Mountains in Northern California and you will be greeted by a noisy and flashy bird. While these forests support a wide diversity of native birds, the screeching calls and rapid flights between trees make the Steller’s Jay (Cyanocitta stelleri) one of the more noticeable forest residents.
Not just common in Northern California forests, the Steller’s jay range extends from the southern parts of Alaska through Canada and the United States, reaching as far south as Nicaragua in Central America. In California, Steller’s jays are mostly found in montane coniferous and hardwood forests at low to medium elevations. These crested jays are also found as edge species near agricultural and recreation areas adjacent to forests.
Why the Steller’s jay is not the only crested jay west of the rockies
The Steller’s jay is still frequently described as being the “only crested jay west of the Rockies.” Historically, this was true as the western range of the Blue jay once ended at the eastern foothills of the Rocky Mountains. Since the westward and northwestward expansion of both the wintering and breeding range of blues jays (Cyanocitta cristata) started to emerge in the 1960s, this geographic fact has not been true of the Steller’s jay for decades.
Starting in the 1960s the Blue jay population started to expand westward. A study published in American Midland Naturalist in 1972 looked at Audubon Christmas Bird Counts between 1962 and 1971. The authors, Bock and Lepthien, found that the number of wintering blue jays increased 25% in their western range. By 1978, blue jays had expanded their breeding range into eastern Montana, Wyoming, and New Mexico. To the north, wintering blue jays have expanded to the coast of Washington and British Columbia.
Why did blue jays expand to the western United States and Canada?
Researchers have found two drivers of this range expansion of the blue jay. Both of these reasons for blue jay range expansion are attribute to human influences.
Putting up bird feeders during the winter became popular in the 1960s
The first reason is “city-hopping”, which means the blue jays followed the increasing availability of bird feeders that proliferated during the 1960s. These feeders proved to be a reliable source of winter food supplied by humans and allowed blue jays to move into areas that were formerly inhospitable.
Oaks and other blue jay friendly trees matured in western U.S. cities
A literature review published in Bird-Banding in 1978 also found that the western range of blue jays was also increasing as a result of the maturation of trees in western cities. One example given by Smith (1978) is the maturation of live oak trees around residences in Midland, Texas. Smith explains that in the 1950s, the presence of blue jays in this city was incidental but has become established as a regular winter resident as the oak trees have matured.
Overlapping range of blue jays and Steller’s jays
While there are some areas where the western range of blue jays now overlaps with the eastern range of Steller’s jays, researchers have found that these birds occupy different niches and therefore the two jay species, for the most part, don’t compete for the same food and habitat.
References
Bock, C. E., & Lepthien, L. W. (1976). Changing winter distribution and abundance of the Blue Jay, 1962-1971. American Midland Naturalist, 232-236.
Smith, K. G. (1978). Range extension of the Blue Jay into western North America. Bird-Banding, 49(3), 208-214.
Data used for map
U.S. Geological Survey (USGS) – Gap Analysis Project (GAP), 2018, Steller’s Jay (Cyanocitta stelleri) bSTJAx_CONUS_2001v1 Range Map: U.S. Geological Survey data release, https://doi.org/10.5066/F78C9V9P.
U.S. Geological Survey (USGS) – Gap Analysis Project (GAP), 2018, Blue Jay (Cyanocitta cristata) bBLJAx_CONUS_2001v1 Range Map: U.S. Geological Survey data release, https://doi.org/10.5066/F79G5KZQ.
Fonte : National Geographic
