At nearly 750,000 acres (303,525 ha), and elevations that range from 2,000 feet (610 m) to over 13,000 feet (3,962 m), Yosemite National Park provides habitats for many bird species. Over 150 species regularly occur in the park, with around 80 additional species that have been seen in Yosemite only a few times (see species list ). Of the regularly- occurring species, approximately 80% are known or suspected to breed in the park. Most of these bird species migrate to lower elevations or latitudes in the late summer and fall. For example, of the 84 species known to nest in Yosemite Valley, 54% are rare or absent in winter. Fewer yet remain at higher elevations.
Noticeable population declines have been detected in numerous bird species in the Sierra Nevada, including Yosemite. Possible causes for these declines include grazing, logging, fire suppression, development, recreational use, pesticides, habitat destruction on wintering grounds, and large-scale climate changes. Nest parasitism by brown-headed cowbirds ( Molothrus ater ), a non-native species, has also been identified as a significant factor in the declines of certain species. Cowbirds lay their eggs in the nests of other birds, usually songbirds. When the cowbird eggs hatch before those of their host species, the larger, more vigorous cowbird young eject the eggs or young of the host species or compete the host's young for food.
One example of population decline is the willow flycatchers ( Empidonax trailii ). These birds are known to breed in only three locations in Yosemite, and there are only and estimated 200 breeding pairs in all of California. Factors such a destruction of their willow-studded meadow habitat, grazing, and brown-headed cowbirds are suspected causes. While some meadow habitats have been affected in the park (especially in Yosemite Valley) and cowbirds are present in the park, the decline of willow flycatchers in Yosemite is thought to be largely a reflection of losses across the whole of the Sierra Nevada. Much suitable habitat remains in Yosemite, but the factors that have decimated this species across the state have ultimately affected the number in the park. Great gray owls occupy a special place among Yosemite's birds.
To help protect birds in Yosemite, there have been studies and monitoring efforts to collect essential data. This has included monitoring of peregrine falcons ( Falco peregrinus ), a species whose numbers were decimated in the 1940s through 1960s from the effects of the pesticide DDT, and have only now staged a recovery after the banning of this pesticide in 1972. Great gray owls ( Strix nebulosa ) and their habitats have been surveyed, and several studies have mapped the distribution of California spotted owls ( Strix occidentalis occidentalis ) in the park. For the last 10 years, the Institute for Bird Populations (IBP) has operated MAPS (Monitoring Avian Productivity and Survivorship) bird-banding stations in Yosemite, yielding valuable data on long-term population trends. IBP also conducted a parkwide survey of bird species in 1999-2000 that provided extensive information about species-habitat relationships in Yosemite. This work also provided an assessment of meadows, with recommendations for some to be designated Important Bird Areas. Research on northern goshawks ( Accipiter gentilis ) has shown the importance of old-growth forests to this species, and how low-intensity fires can have a beneficial effect on the goshawks.
Great gray owls are of special interest in Yosemite because here they reach the furthest southern extent of their global range, and they are isolated by hundreds of miles from the next closest population in far northern California.
Besides brown-headed cowbirds, there are two other non-native bird species in Yosemite that are of concern. White-tailed ptarmigan ( Lagopus leucurus ) were introduced as a game bird into the east side of the Sierra in the 1960s and have since expanded their range into Yosemite, occupying wide areas of alpine habitat in the park. Along the park's western boundary, wild turkeys ( Meleagris gallopavo ) have invaded. This species, also, was also introduced in California for hunting, but may be having adverse effects on native plants and animals.