The trends for Brown-headed Cowbirds have been complex. Consistent with their historical spread throughout California in the past century, cowbirds followed human expansion and cattle grazing up into the Sierra, and our BBS data show a steady increase in numbers up to 1990. However, since then, the trend has been flat to negative. Analyses of BBS data by Sauer et al. (2011) showed a significant negative trend. CBC data show a slightly positive trend that is not statistically significant.
NEGATIVE TRENDS
Species that showed statistically significant declines based on either CBC or BBS data are summarized in Table 3. For most of the species on this list, a variety of different sources have documented long-term, widespread population declines throughout their North American ranges. Degradation of winter habitat in Central and South America has been implicated in the long-term declines seen in Olive-sided Flycatchers and Western Wood-Pewees. In addition, some forestry practices like postfire salvage logging may have contributed to the alarming negative population trend for Olive-sided Flycatchers (see Figure 13) in the West. Reduced quality of montane riparian habitats is a likely contributor to declines of Western Wood-Pewees, Yellow Warblers, and Wilson’s Warblers. Brood parasitism by Brown-headed Cowbirds is also implicated in Yellow Warbler declines. Killdeer, Horned Larks, and Lark Sparrows are among the 70 percent of grassland- associated birds that have shown highly significant population declines throughout North America.
The steady decline in American Kestrel numbers across the continent has been studied intensively but remains largely unexplained. Extensive development in the lower elevations of the West Side of the Sierra Nevada may be partly to blame for the negative population trends for Lark and Chipping Sparrows, as both species are particularly sensitive to habitat fragmentation. Habitat fragmentation in its low-elevation breeding range may also be responsible for the declines in Bullock’s Orioles. Reasons for significant declines in year-round Sierra residents such as Mountain Chickadees, Purple Finches, and Cassin’s Finches are uncertain as most of their habitats are relatively undisturbed.
The negative trend for Rough-legged Hawks in the Sierra is reflected throughout the western, eastern, and southern parts of this species’ winter range in North America. However, this decline is offset by much higher numbers wintering in the northern Great Plains, possibly as a result of reduced snowpack in that region in recent decades. Negative trends for Brewer’s Blackbird and Violet-green Swallow are perplexing but consistent with state-wide trends in California. Indeed, of all Sierra species generally thought to benefit from human development, Brewer’s Blackbird was the only one to show a significant decline. Also surprising was our finding that American Robins and Mourning Doves both show significant negative trends from BBS data but significant positive trends from CBC data, the only species to show this discrepancy.
As pointed out in the introduction to this section, the migratory nature of these species means that the winter and summer populations may be from different sources. In this case, milder winter conditions and/or the presence of winter-fruiting trees might explain the winter increases. American Robins were the only species to show significant contradictory results between our BBS analyses and those of Sauer et al. (2011), who reported a positive trend for robins. The difference in definitions of the Sierra region may explain this as the region they used included areas to the north and excluded areas to the south as well as lower elevations on both the East and West Sides. A possible shift in breeding range uphill and/or to the north could be responsible for this discrepancy.
TABLE 2 Species showing significant positive trends.
RANGE EXPANSIONS
A number of species have either expanded their range into or within the Sierra in recent decades. Since the first Great-tailed Grackles crossed the Colorado River in the 1960s, they have spread rapidly through California. While still largely confined to lower-elevation areas on both slopes, numbers and range within the Sierra increase each year. Many experts thought the grackle expansion was as rapid as any could be, but the spread of the Eurasian Collared-Dove has proceeded with truly breathtaking speed. Just since 2005, this species has gone from a rarity to a common year-round resident on both the East and West Sides, with breeding records now in every Sierra county.
As noted earlier, the recovery of populations of Osprey and Bald Eagle have allowed those species to spread their breeding range into the Sierra. In the first 25 years of the BBS, only 3 Ospreys and no Bald Eagles were recorded on any Sierra routes. Since the early 1990s, nearly 100 Ospreys and 15 Bald Eagles have been recorded, and these species are now consistently found on nearly a half-dozen BBS routes. It is less clear why Red-shouldered Hawks have increased so dramatically in California in recent decades, but that population growth has been accompanied by a spread into the Sierra with the species now much more numerous on the West Side and regular in areas on the East Side, where they were never observed before the 1990s (see Figure 13).
TABLE 2 (continued)
Warmer temperatures in the Sierra in the past several decades may have encouraged some species to move upslope. Hutton’s Vireos are now recorded on twice as many BBS routes than in the 1970s and 1980s, due mainly to the species now showing up on higher-elevation West Side routes. Anna’s Hummingbirds have also shown a breeding season upslope trend on both the East and West Sides, probably in response to widespread use of feeders. Mourning Doves, recorded on fewer than half of the Sierra CBCs prior to the 1990s, are now found on nearly 90 percent of the counts. We are also beginning to see Hooded Orioles creeping up into western foothills following development (and planted palms).
Common Ravens are now found regularly on lower-elevation West Side BBS routes and CBC circles where they were historically rare; they have also expanded their range into the high Sierra in dramatic fashion (see Common Ravens in the “Family and Species Accounts” chapter). Somewhat counterintuitive has been the apparent southward expansion of the breeding ranges of Buffleheads and Ring-necked Ducks in the Sierra. Hooded Mergansers, whose breeding range has extended southward into the Central Valley and whose wintering populations in California have increased in the past 20 years, are now found much more frequently on Sierra CBCs.
TABLE 3 Species showing significant negative trends.
Barred Owls have been expanding their range westward and southward throughout the Pacific Northwest and have become relatively common in northwestern California. This expansion into the range of the Spotted Owl, a species already in decline due to habitat loss, has added another threat to that species. Barred Owls compete with, hybridize with, and even prey upon Spotted Owls. This species has only recently begun to spread into the Sierra, but records as far south as the Grant Grove in Tulare County suggest that it is just a matter of time before we begin to see similar negative impacts on the Sierra Nevada subspecies of Spotted Owl.
Many readers may be surprised to learn that Chestnut-backed Chickadees are a relatively recent addition to the Sierra avifauna. In Joseph Grinnell’s (1904) assessment of this species’ range, he considered it entirely absent from the range. Before 1951, the only record of this chickadee in the Sierra was a single specimen collected in Butte County (1939). In 1951 flocks were noted in El Dorado and Calaveras Counties and Chestnut-backed Chickadees have since spread through the lower-mid-elevations of the West Side of the range as far south as Madera County south of Yosemite National Park. The subspecies responsible for this colonization (Parus rufescens rufescens) ranges from Mendocino County northward. There is no indication that birds from the central or southern California coast (which have much paler sides than this subspecies) have made the leap across the Central Valley. It appears that this colonization has had little or no effect on Mountain Chickadee numbers, probably because of the Chestnut-backed Chickadee’s preference for wetter, cooler areas with Douglas-fir, madrone, and other moisture-loving trees.