Breeding chronology, movements, and life history observations of tricolored blackbirds in the California Central Coast

The tricolored blackbird (Agelaius tricolor, hereafter, tricolor) has experienced population declines and is currently under review for listing under both the California and Federal endangered species acts. Tricolors form the largest breeding colonies of any extant North American land bird, and are mostly found within the California Central Valley. However, smaller numbers of tricolors breed in other parts of lowland California, including the California Central Coast where the species is little-studied. From 2012 to 2014, we studied radio-tagged tricolors at four breeding colonies in Monterey County. The relative cover of the tricolors' primary grassland foraging habitat within 5 km of colony sites was lower than reported elsewhere. Birds arrived at colony sites from early-April to mid-May and remained up to 90 days. Nest-building occurred between 5 April and 18 June, with asynchronous nest-building at individual colonies occurring over periods up to 35 days. The largest colonies each year ranged from 600 to 800 birds. Twenty-five percent of radio-tagged birds moved between the study colonies during the breeding season, suggesting itinerant breeding. After breeding, 15% of tagged birds were observed near colony sites while 21% dispersed up to 37 km north and northeast towards known over-wintering areas (northern San Joaquin Valley, San Francisco Bay-Delta, and coastal Marin County). We also recorded, apparently for the first time, 1) arboreal foraging in oak woodlands, 2) use of lichen (Ramalina or Usnea spp.) as nest material, and 3) likely predation by long-tailed weasels (Mustela frenata). Given the observed variability in nesting chronology, our findings suggest that surveys of prospective breeding colonies should occur throughout the entire breeding season to accurately assess presence or absence of nesting tricolors. Additionally, inter-colony movements of tricolors suggest that continued use of any one location may depend upon the suitability and productivity of nearby colonies. Management regimes for the species must therefore ensure cooperation among multiple land-management entities controlling nearby tricolor habitats.