Steep Decline in Tricolor Abundance

July 11, 2013

Figure 1

The most recent statewide surveys documented a rapid decline in the number of Tricolored Blackbirds, from about 400,000 in 2008 to about 250,000 3 years later. Annual monitoring and research into the relationship between reproductive success (RS) and insect abundance has shown a chronic low RS in dozens of colonies studied since 2007 and relatively high RS only in instances when colonies were surrounded by landscapes with unusually high insect abundance. Thus, the recent decline in the numbers of tricolors is believed to be due to chronic low RS (the number of birds recruiting into the population has been much smaller than the number dying) due to insufficient insect abundance. The decline in abundance is seen in the sizes of the largest colonies (Figure 1) from 2005 – 2013, where as recently as 2006 the average of the 5 largest colonies was nearly 72,000 birds, but in 2013 was less than 12,000 birds. And this decline in the sizes of the largest colonies comes at a time when researchers have documented a large number of colonies in new locations (Figure 2), nearly 100 since 2005. All else being equal, such a rapid increase in the number of known colony locations should have resulted in an increase in the population estimate, so the rapid decline in abundance is even more alarming. The next statewide survey, scheduled for April, 2014, will help to quantify the rate of decrease in the numbers of tricolors and provide a current statewide population estimate.


Figure 2