eWestern Birds

The Quarterly Journal of Western Field Ornithologists

Vol. 54, No. 3
August 2023
Western Field Ornithologists

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Tricolored Blackbirds’ Reliance on Insects from Dairies
David M. Goodward and Rudy A. Diaz

ABSTRACT: The Tricolored Blackbird has been in a severe population decline due to habitat loss and other factors. It is now heavily dependent on agricultural landscapes such as hay fields for foraging and grain fields for nesting. Dairies are known to provide a concentrated food source throughout the year in the form of grain gleaned from cattle feed. For nesting, however, Tricolored Blackbirds require large quantities of insects for their young. A colony of Tricolored Blackbirds in the San Jacinto Valley, Riverside County, in southern California, fed their young large numbers of house flies from nearby dairy-cattle yards, as well as drone fly larvae from dairy-effluent ponds. At this colony, 67% of the nestlings’ food came from the dairy and another 13% from the adjacent hay fields. Most other insects came from irrigated weedy growth adjacent to wetlands. Such an intense reliance on dairies is risky for the blackbirds as urbanization is displacing the dairy industry in the San Jacinto Valley and many other areas of California. Another potential risk to the blackbirds is the use of pesticides at dairies for fly control. Further research is recommended to determine if this utilization of flies from dairies is widespread in California. Research is also needed to quantify the decrease of insects in drought-stricken landscapes and how this may affect the Tricolored Blackbird’s reproductive success.

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