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Recent Trends in Sierra Bird Populations and Ranges

In this chapter we review changes in populations and ranges of birds of the Sierra over almost four decades. We used data from Breeding Bird Surveys (BBS) and Christmas Bird Counts (CBC) and supplemented those data with observations cited in North American Birds, numerous publications (see the bibliography), and the personal experience of ourselves and many regional experts. We also compare our analyses of BBS data with those of Sauer et al. (2011). Those authors used a different definition of Sierra boundaries, a different time frame, and more sophisticated statistical methodology (see Appendix 3). We have also, where possible, attempted to put these trends into larger state- or continent-wide perspective.

There are 44 BBS routes and 25 CBC circles within the Sierra region (see Map 5). Most of these have been run throughout much of the past 35 years, providing a relatively robust, long-term dataset to explore. BBS routes are run during the spring and summer breeding season, and CBCs are run from late December into early January. Since many of the species counted during these surveys are migratory, the breeding season and wintering season birds may often be of different subspecies or from separate populations. Therefore, one should not necessarily expect that a given species should reflect similar trends from BBS and CBC data. BBS routes are limited to roads, so much of the Sierra (in particular the higher elevations) is not surveyed by these routes. CBC circles are almost completely limited to lower elevations that are accessible in the winter. Although both these sources have their limitations, when the data show significant and consistent trends over a number of circles or routes, one can be fairly confident they reflect real trends. We limited our analyses to species that are relatively common and widespread throughout the Sierra. Appendix 3 describes the methods we used to detect trends for each species.

POSITIVE TRENDS

Species that showed statistically significant positive population trends from either BBS or CBC data are summarized in Table 2. Of the 117 species we analyzed, more showed positive trends than negative ones. Population increases for several of the species can be attributed to direct or indirect effects of human activities. The California Department of Fish and Game first introduced Wild Turkeys into the state in 1908. However, those and all subsequent introductions (mainly using captive-bred birds) over the next 50 years proved unsuccessful. It was only when wild-captured birds from Texas began to be released from 1959 to 1988 that California populations began to become self-sustaining. Data from BBS and CBC surveys reflect the success of those efforts, with huge increases starting in the mid-1980s and continuing up to the present.

MAP 5 Locations of Breeding Bird Survey (BBS) routes and Christmas Bird Count (CBC) circles used for trend analyses

Large and growing, resident flocks of Canada Geese may be the result of a combination of factors including historical California Department of Fish and Game introductions (i.e., prior to the early 1970s), releases from private waterfowl breeders, and native wintering birds failing to migrate and producing nonmigratory offspring. Dramatic recoveries in populations of Bald Eagle and Osprey are likely due in large part to banning of DDT in North America in the 1970s. Eagles also benefitted from protection under the state and federal Endangered Species Acts. Increasing human population in the Sierra and the bird feeders which usually accompany that increase could be contributing to increases in Anna’s Hummingbirds and Lesser Goldfinches. These same factors have allowed House Finches to expand their year-round range into higher-elevation portions of the Sierra. Nest box programs are likely benefiting species like Wood Ducks, Western Bluebirds, and Tree Swallows. Planting of exotic, winter-fruiting trees in newly developing areas of the Sierra could help explain the increasing winter populations of American Robins, Cedar Waxwings, and Western Bluebirds, although increasing average winter temperatures could also be a factor.

We found increases in seven species associated with chaparral/early successional habitats (Bewick’s Wren, Blue-gray Gnatcatcher, MacGillivray’s Warbler, Spotted and California Towhees, Rufous-crowned Sparrow, and Lazuli Bunting) or forest-chaparral edges (Dusky Flycatcher) (Figure 13). The increase in large-scale, stand-replacing fires in the Sierra in recent years may have expanded these types of habitats to the benefit of these species.

FIGURE 13 Charts showing trends for selected species based on Christmas Bird Count (CBC) data (Spotted Towhee and Red-shouldered Hawk) or Breeding Bird Survey data (Dusky Flycatcher and Olive-sided Flycatcher). All trend lines are based on linear regression. Note that for CBC data, Count Year 76 is winter 1975–76 and so on.

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. As a result of this preference, the distribution of Chestnut-backed Chickadees in the Sierra is quite patchy.

RANGE CONTRACTIONS

While the news is mostly good, with many more Sierra bird species increasing in numbers and range than declining, we do have several species that have seen their ranges shrink substantially. For the most part, we have very poor understanding of the causes behind these changes. Two of the more perplexing examples of Sierra breeding range contractions are those of Ruby-crowned Kinglet and Swainson’s Thrush. Based on their status in the early 20th century, Grinnell and Miller (1944) considered both species to be common on the West Side of the Sierra, from the northern edge of the range south to Tulare County. Breeding populations of both species are now highly localized and very sparsely distributed in the region. BBS routes rarely record either species, even in places where the species was considered to be historically common.

In the past few years the Grinnell Resurvey Project has systematically surveyed areas covered by Joseph Grinnell and his associates and found both species missing from nearly all the locations where they were previously present (Moritz 2007). No convincing hypothesis has surfaced to explain these range contractions. CBC data suggest that the wintering population of Ruby-crowned Kinglets in California is stable. BBS data show declines for the kinglet in the Sierra and Cascade Range but not in the Rockies. Swainson’s Thrushes breeding in coastal California, while showing declines based on BBS data, still occupy most of their historical range in that region.

Greater Roadrunners, once relatively common and widespread in the western foothills of the Sierra, are now patchily distributed and rarely encountered in much of this part of their range. Historically, throughout most of California, conversion from rangeland to more intense forms of agriculture and increasing human development has coincided with the disappearance of this species. At least into the 1930s, Willow Flycatchers were still considered common throughout the Sierra in any area with suitable riparian/willow habitat. Today the species is the scarcest of all our Sierra flycatchers and persists in only few areas. Habitat loss from reservoir construction and water diversions and habitat degradation from overgrazing, combined with the incursion of Brown-headed Cowbirds into the Sierra, have all been blamed for their decline.

With Sierra temperatures predicted to continue rising, vegetation zones are likely to move upslope with changing climate. This could push species using the highest elevations, like American Pipits and Gray-crowned Rosy-Finches, toward extirpation from the Sierra. Because their breeding areas are difficult to reach, we have little data on breeding season trends for either species, but winter numbers of American Pipits in California have shown significant declines over the past 30 years.