Population trends of birds wintering in the Central Valley of California

Abstract

Since the 1970s, the Central Valley of California has seen a large investment in preservation and restoration of wetlands and riparian areas. At the same time, grasslands have been lost to vineyards, orchards, and residential development at an accelerating rate. We analyzed data from 17 Christmas Bird Count circles that were surveyed regularly between winter 1978–79 and winter 2013–14 to document population trends for birds wintering in this region. We selected 112 taxa (species or species groups) that were relatively abundant and widespread in the Central Valley during winter and used a hierarchical model to estimate annual rates of population change from the count data while accounting for varying survey effort. A much larger proportion of taxa showed positive (46%) than negative (18%) trends; about a third (36%) showed no detectable change. Central Valley habitats that showed the highest proportion of taxa with increasing vs. decreasing trends were riparian (59% vs. 9%; n = 32), wetlands (49% vs. 11%; n = 47), and open water (44% vs. 0%; n = 9), likely reflecting the conservation efforts in these habitats in recent decades. In contrast, a greater proportion of the taxa associated with grasslands and other open habitats (n = 25) showed decreases (48%) than increases (28%). As expected, species that adapt well to areas of human habitation showed stable or increasing trends. Examples of such species with strong positive trends include Anna's Hummingbird ( Calypte anna ), Black Phoebe ( Sayornis nigricans ) and recent Central Valley arrivals, Eurasian Collared-Dove ( Streptopelia decaocto ) and Great-tailed Grackle ( Quiscalus mexicanus ). Scavenging, opportunistic species such as Turkey Vulture ( Cathartes aura ) and Common Raven ( Corvus corax ) also showed strong positive trends. Trends in wintering populations were largely concordant with estimated trends available from breeding areas in California and western North America. Overall, these abundance data suggest that recent efforts to preserve and restore wetland and riparian habitats may be benefiting birds. However, a similar focus on conservation of the Central Valley's remaining grasslands may be needed to maintain populations of grassland-associated birds.