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US Fish & Wildlife Service FieldNotes

SAN LUIS NWRC: Tri-colored Blackbirds Benefit from Habitat Provided by California National Wildlife Refuges

Region 8, April 12, 2012
Tri-colored blackbird flock at Merced NWR
Tri-colored blackbird flock at Merced NWR - Photo Credit: n/a

By Madeline Yancey, San Luis NWR

Data collected during the recent banding of tri-colored blackbirds (Agelaius tricolor) by Dr. Robert J. Meese, Department of Environmental Science & Policy, University of California, Davis at the Merced National Wildlife Refuge (NWR) in California’s Central Valley, provided proof that national wildlife refuges are important to the conservation of tri-colored blackbirds (TCBB).

The tri-colored blackbird is a highly colonial species that forms enormous breeding colonies sometimes containing 100,000 or more individual birds and as many nests. These birds historically nested and foraged in the habitat provided by the expansive wetland and perennial grassland landscapes of the Central Valley. It is estimated, however, that today more than 95% of those habitats have been lost to urban and agricultural development.

But, the TCBB is a very adaptive species and the loss of so much of its “natural” habitat has led it to exploit alternate resources. Large dairies, for example, often provide all the habitats needed by TCBB – a source of water, vegetation for nesting, and foraging areas, sometimes all available on a single dairy. This frequently puts the TCBB in direct conflict with humans and its colonial nesting behavior makes its survival vulnerable to any single destructive event.

Merced NWR is the fourth refuge on which Meese has trapped and banded TCBB. The others are Bitter Creek (about 200 miles southeast), Kern (about 150 miles southeast), and Delevan (about 200 miles northwest). During the most recent round of banding at Merced, Meese, assisted by refuge personnel, captured and banded 3,667 TCBB. Among them were 175 previously-banded, or recaptured, birds. Twelve birds trapped and banded at Merced in 2011 were later recaptured at Delevan. One of those was recaptured multiple times at different tracts of the Delevan refuge. The importance of the recaptured birds, according to Meese, is that they provide information that can be used to document bird movements and shed light on the links between patches of suitable TCBB habitat. This data shows specifically that the species is utilizing more than one refuge and more than one area of individual refuges in a single season.

Even more significant among the recaptured birds were 124 birds that were “unique” recaptures, or individuals that were recaptured for the first time. Recaptured birds provide data that helps to illustrate both the short- and long-distance movement patterns exhibited by the species, according to Meese; and highlight the importance of California’s National Wildlife Refuges to TCBB conservation.

Merced NWR specifically employs several management strategies intended to make refuge habitat more attractive to breeding TCBB. In 2006, a colony of about 70,000 TCBB nested in a dairy farmer’s wheat field adjacent to the refuge. Since the timing of the wheat crop harvest coincided with the presence of tens of thousands of young birds in the nests, harvest would have resulted in wiping out the colony’s entire reproductive efforts for the season. However, the refuge was able to purchase the crop (with self-generated revenues) allowing the farmer to leave the field intact, letting the colony complete its nesting cycle and fledge an estimated 40,000 young birds. Another strategy attempted, with limited success, was to bait TCBB colonies away from surrounding dairy farmers’ feed stocks by purchasing rolled corn and placing it near established colonies on refuge land.

Refuge managers also provide TCBB nesting habitat directly by maintaining a rotation of summer water supporting varying ages of cattail stands. “The TCBB seem to prefer two-year-old stands of cattails over water,” says Merced NWR’s assistant manager, Rich Albers. “We also provide nesting and foraging habitat by allowing fallow (refuge) farm fields to grow thick, tall stands of milk thistle, black mustard, and other weeds.” Albers says the colonies display a preference for this mixed exotic plant community because the stems are robust enough to support the birds’ nests and the thistles seem to provide protection against potential predators.
This creation of “substitute habitats” also serves to attract birds away from agricultural fields, the harvest of which can threaten a colonies’ very survival.

The nesting habitat provided on the Merced NWR is an example of the diversity of habitats utilized by TCBBs, as indicated by Meese’s data. At other refuges, TCBB use stinging nettles in a creek bottom surrounded by annual and perennial grass uplands, or wetland vegetation composed of bulrush and cattails. Merced and other California NWRs are providing a diverse array of necessary nesting habitats attractive to TCBBs, surrounded by foraging habitats that are increasingly dominated by agriculture, says Meese.

Contact Info: Jack Sparks, 209-826-3508, jack_sparks@fws.gov