Tricolored Blackbird (Agelaius tricolor)

Multi‐scale demographic analysis reveals range contraction via pseudo‐source and sink population structure

“…The regional IPM allowed us to determine that a significant portion of the population in the south is lost due to emigration each year, in agreement with our multistate analysis. These results support the idea that the apparent stability of the population in the north is bolstered by a large number of immigrants, likely from the south as almost the entire known tricolored blackbird population is within California (Beedy et al 2020). This suggests that the decline in the south is in part driven by individuals emigrating north and that our inability to find a decline in the north is likely due to an influx of individuals from the southern region.…”

“…Tricolored blackbirds are found almost entirely in California and historically nested in the wetlands across the state; however since the 1980s, they have also nested in silage fields and in stands of some invasive plants (Holyoak et al 2014, Weintraub et al 2016, Robinson et al 2018a, Beedy et al 2020). Tricolored blackbirds have experienced a 63% decline in breeding abundance from 1935 to 1975 and a 34% decline since 2007 (Graves et al 2013, Robinson et al 2018).…”

“…Tricolored blackbirds have experienced a 63% decline in breeding abundance from 1935 to 1975 and a 34% decline since 2007 (Graves et al 2013, Robinson et al 2018). This decline leads to the species being listed as threatened under the California Endangered Species Act (ESA) in April 2018, and it is currently under consideration for listing under the United States ESA (Beedy et al 2020). As such, it is imperative that we understand population dynamics across their range to effectively identify and prioritize research needs and conservation actions.…”

“…We were concerned that assuming equal survival for males and females could bias our results if a true difference exists. In a typical tricolored blackbird colony, there are two females for each male, although this ratio could be as high a 4:1 (Beedy et al 2020). We estimated the annual growth rate ( r ) for each region during the 10 yr of the study and the overall decline across the entirety of the study from the posterior estimates of relative abundance.…”

“…We use an example based on research efforts focused on understanding the drivers of population decline for tricolored blackbird ( Agelaius tricolor ), a colonial passerine of high conservation concern in the western United States. Recent work estimated a range‐wide population decline of 34% since 2010 in California (Robinson et al 2018b), where ~99% of the global population is found (Beedy et al 2020). As the species is a colonial breeder with low site fidelity (Holyoak et al 2014, Airola et al 2016), permanent emigration is likely to influence population‐level inferences on population decline.…”