The tricolored blackbird forms the largest colonies of any North American land bird, often with breeding groups of tens of thousands of individuals. In the 19th century, some colonies contained more than a million birds — enough to make one observer exclaim over flocks darkening the sky “for some distance by their masses,” not unlike passenger pigeons. But because a small number of colonies may contain most of the population, human impacts can have devastating results. Over the past 70 years, destruction of the tricolor's marsh and grassland homes has reduced its populations to a small fraction of their former enormity.

While its big breeding colonies make the species seem abundant to casual observers, the blackbird's gregarious nesting behavior renders these colonies vulnerable to large-scale failures. In agricultural habitat the birds experience huge losses of reproductive effort to crop-harvesting; every year, thousands of nests in dairy silage fields — where grass is being fermented and preserved for fodder — are lost to mowing. In what little remains of California's native emergent-marsh habitat, tricolors are vulnerable to high levels of predation. The species has been in decline ever since widespread land conversion took hold in California.

The Center submitted state and federal listing petitions for the species in 2004, but continuing threats to tricolors were ignored for many years. California announced its refusal to protect the species, as did the U.S. Fish and Wildlife Service in 2006. But in 2014, when the bird's population reached the smallest number ever recorded, only 145,000 — and when comprehensive statewide surveys showed that an additional two-thirds of remaining tricolored blackbirds had been lost since 2008 — 2014 the Center again petitioned for an endangered listing under the California Endangered Species Act, on an emergency basis. And finally, in December 2015, the California Fish and Game Comission announced it was making the species a "candidate" for state protection — a definitive victory, since candidates for state protection enjoy actual safeguards until they receive a place on the state's endangered species list (unlike federal "candidate" species).