Bubbola tricolore

Tassonomia

The tricolored blackbird does not have any officially recognized subspecies, although there is a population in southern California that may require genetic evaluation.

Nome scientifico

Agelaius tricolor

Autorità

(Audubon, 1837)

Ordine

Passeriformi

Famiglia

Icteridae

Genere

Agelaius

Areale e habitat

The tricolored blackbird nests in colonies, but scholars disagree on whether the costs outweigh the benefits of these breeding habits. Nonetheless, nesting in colonies makes the tricolor susceptible to environmental changes. Although the tricolor has been able to adapt to some of the landscape changes, habitat loss played a major role in the reduction of its population.<p>The tricolor originally could be found in the marshes of California, nesting in wetland cattails and bulrushes. Scholars have noted a reduction in freshwater marshes as breeding grounds for the tricolored blackbird. Largely, this is due to human activity, and it’s estimated that between the 1930s and 1980s alone, over 95% of wetlands were disappeared. Also in this period, the observed tricolor population saw an 89% reduction, while the average colony size saw a 63% reduction. Altogether, the tricolor population dropped from several million to only a few hundred thousand during the twentieth century. Nevertheless, the tricolor was able to adapt in response to these severe landscape reductions. It began to use both native and non-native vegetation as well as agricultural fields as their breeding and foraging grounds.<p>In the 1930s, over 93% of the tricolor’s colonies were nested in freshwater marshes, but by the early twenty-first century, only 35% of colonies could be found in wetlands. In 2008, over a fourth of colonies were using non-native vegetation to nest, most notably Himalayan blackberry brambles. Although population decline was a consequence of agricultural intensification over the twentieth century, the birds were able to use these environmental changes to their advantage. The 2008 tricolored blackbird census found that nearly half of the total population nested in colonies inside of the grain fields of dairy farms.<p>As the twentieth century progressed, the tricolor began to increase in concentration within certain colonies. Specifically, in 2000, 59% birds were housed in one of California’s ten largest colonies, and this number increased to 81% by 2011. Mega-colonies of the tricolor have begun to form in the San Joaquin Valley’s numerous agricultural fields. Although the tricolor population consistently decreased during the twentieth century and into the twenty-first century, the population nested in San Joaquin Valley grew exponentially during this time. The number of tricolors in California’s San Joaquin Valley in 1994 was 230% of what it was in 1937. By 2008, over 86% of California’s population were located here.<p>Conservation efforts by the National Audubon Society in collaboration with the U.S. Fish and Wildlife Service have seen an impact on population numbers. The California population of tricolors saw an increase in the 2017 state survey from previous years. Nonetheless, most of this population gain was within the San Joaquin Valley or in San Benito County. Other regions of California saw a reduction in population in the 2017 census, including Sacramento Valley, with observations down by about 33% from the prior census in 2014. There has been a steady decline of the tricolors observed in this region over the twentieth and twenty-first centuries, with a greater proportion of birds moving to the San Joaquin Valley. There has also been an overall population decline in Southern California as well, following the dairy industry’s move out of Southern California in the 1980s, in favor of the San Joaquin Valley. The other California region that saw a population increase in 2017 was the Central Coast. Historically, the Central Coast and Central Valley housed millions of tricolors before the population decline associated with human agricultural activity. Still, the population increase seen in 2017—from 627 to 17,576 birds—likely was not due to a growing number of tricolors coming to the Central Coast. Instead, this drastic population increase likely was a result of better survey efforts. Three large new colonies were observed in 2017, two of which were in previously unknown or un-surveyed locations. Expanded knowledge of the tricolor’s breeding habits led researchers to increase their survey efforts in locations they suspected housed bird colonies.<p>Although most regions of California either remained constant or saw a decrease in the tricolor population in 2017, there was an increase in the state overall. The 2017 census, however, did prove that human conservation efforts can be successful. In Southern California, the population remained relatively constant from 2014 to 2017, but the distribution in this region changed. Riverside County saw a significant increase in population, and 57% of the tricolors observed in Southern California were in one single colony. The San Jacinto Wildlife Area in Riverside County had been the site of successful conservation efforts to provide the tricolor with nesting and foraging habitats.

Migrazione

Resident species

Conservazione

Tendenza della popolazione

declining