A Novel Locality for the Observation of Thousands of Passerine Birds during Spring Migration in Los Angeles County, California

Avian migration is a spectacular phenomenon, representing the annual movements of billions of birds globally. Because the greatest diversity and numbers of birds migrate at night, opportunities to observe active migration are rare. At a number of localities in North America, however, observers can quantify movements of many typically nocturnal migrants during daylight where they continue after dawn. Such locations have provided much information about species-specific phenology, status, and orientation during migration. Localities where morning flights of land birds can be observed are unevenly distributed, however, and are little reported along the Pacific coast. Here we describe a novel location for the observation of spectacular morning flights of nocturnal migrants during spring migration at Bear Divide, in the western San Gabriel Mountains, Los Angeles County, California. In two years of informal surveys at the site, we have recorded at least one morning with an estimated ~13,500 individual birds passing. Our preliminary analyses suggest that the peak of a species’ migration at Bear Divide is correlated with the latitude of a species’ breeding, being later in the spring as that latitude increases. Our data from Bear Divide provide an independent perspective on migration as quantified by local radar. Further work at this locality may help inform our knowledge of migration phenology and population trends.

Age and Sex Influence Natal and Breeding Dispersal of Purple Martins

Dispersal patterns deepen our understanding of population dynamics. Dispersal by all age and sex classes enhances a species’ ability to respond to environmental changes, such as in habitat availability, artificial nest sites, and climate. The migration dynamics of the eastern subspecies of the Purple Martin (Progne subis subis) are well known, but we know less about its patterns of annual dispersal. We compared the frequency, distance, and direction of dispersal by each age/sex cohort of martins in central Alberta, at the northwestern limit of their breeding range. We used two datasets: (1) adult martins banded in central Alberta as nestlings and encountered during the summers of 2017 and 2018, and (2) records of encounters of banded martins in Canada from 1935 to 2016 from the Canadian Wildlife Service’s Bird Banding Office. In Alberta, 36% of birds dispersed from natal sites (by an average distance of 24 km), most commonly to the northeast. Across Canada, 29% of birds dispersed (by an average distance of 183 km), most commonly to the east and northeast. In Alberta, martins at least two years old dispersed less frequently than yearlings since some older martins returned to their natal site after first breeding elsewhere. Dispersal distances of after-second-year martins, which represent natal plus breeding dispersal, were greater than those of second-year birds, which represent natal dispersal alone. Thus some martins continue to disperse after their second year and do not maintain complete fidelity to a breeding site, which is different from our current understanding.

Fall Waterfowl Use of Bridgeport Reservoir, Mono County, California

Aerial surveys from 2003 to 2019 documented the abundance of waterfowl at Bridgeport Reservoir in Mono County, California, from September through mid-November. Waterfowl totals at Bridgeport Reservoir averaged 33,106 ± 4050 (standard error) in the fall. Annual peak counts averaged 10,474 ± 1349, ranging from a low of 2583 in 2014 to the highest single-day count of 23,150 in 2005. Bridgeport Reservoir is a man-made water body in the intermountain West that waterfowl use primarily a mid-migration stopover site, with peak numbers occurring in September. The dominant waterfowl species, the Northern Shoveler (Spatula clypeata), Gadwall (Mareca strepera), Mallard (Anas platyrhynchos), Northern Pintail (A. acuta), and Green-winged Teal (A. crecca), showed both unimodal and bimodal migration chronologies. Regional drought, as indicated by the Palmer drought severity index, combined with a downward trend in waterfowl numbers explained 61.4% of annual variation in fall waterfowl totals. These data may allow future assessment of change in waterfowl abundance at Bridgeport Reservoir in the context of local or regional conditions, and as influenced by climate change.

Using Song Dialects to Assess the Migration Strategy of the Golden-crowned Sparrow

Most techniques used to study migration of wild birds require capture for banding or for attachment and/or recovery of tags or transmitters. We took advantage of the fact that the Golden-crowned Sparrow (Zonotrichia atricapilla) sings in winter, combined with published data on the distribution of its distinct song dialects in the breeding range, to assess its migration strategy and migratory connectivity by means of these dialects. Using recordings of the Golden-crowned Sparrow’s song across much of its winter range, we categorized these birds by song type to identify their likely origin in some subset of the breeding range. This method allows examination of migration without the need to capture birds. Our results fit best with a pattern of chain migration, with the northernmost breeders wintering in the northernmost part of the winter range, and the southerly breeders wintering farther south. The results suggest strong migratory connectivity between segments of the breeding and winter ranges, though our small sample size makes it difficult to draw firm conclusions on connectivity.