Tricolored Blackbird Itinerant Breeding in California

Abstract

To evaluate the abundance of Tricolored Blackbirds (Agelaius tricolor), the schedule of breeding throughout the Central Valley of California was determined in four years (1992-1994, 1997). By the end of April in 1994, all observed Tricolored Blackbirds were in the immediate vicinity of active breeding colonies. Only one colony of 600 birds was established in the Sacramento Valley north of Sacramento County. During late May and early June, more than 170,000 individuals settled in the Sacramento Valley, while attendance at colonies in the San Joaquin Valley was declining. Most breeding birds colonizing the Sacramento Valley in May and June probably already had completed nesting efforts elsewhere. This suggests that Tricolored Blackbirds are itinerant breeders. Surveys, conducted after initial settlement and before substantial movements from one breeding area to another occur, have the potential to estimate overall numbers. Inclusion of late season breeding colonies in estimates of overall abundance would result in substantial overestimates of the global population. Local and regional declines in the number of breeding Tricolored Blackbirds of an order of magnitude or more resemble population collapses but probably are attributable to itinerant breeding.