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Providing evidence to improve practice

Individual study: Adding artificial eggs to red-winged blackbird Agelaius phoeniceus may reduce parasitism rates by brown-headed cowbirds Molthrus alter in wetland sites in the USA

Published source details

Ortega C.P., Ortega J.C. & Cruz A. (1994) Use of Artificial Brown-Headed Cowbird Eggs as a Potential Management Tool in Deterring Parasitism. The Journal of Wildlife Management, 58, 488-492


This study is summarised as evidence for the intervention(s) shown on the right. The icon shows which synopsis it is relevant to.

Use false brood parasite eggs to discourage brood parasitism Bird Conservation

A replicated, controlled experiment in 1985 and 1991 on seven wetland sites in Colorado, USA (Ortega et al. 1994) found that the proportion of red-winged blackbird Agelaius phoeniceus nests parasitized by brown-headed cowbirds Molothrus ater in 1985 was significantly lower for nests that had artificial or real cowbird eggs placed in them, than for control nests (5% of 57 and 32% of 54 nests parasitized respectively). In 1991, the rate of parasitism was again lower for nests with artificial eggs added but was not significantly different (6% of 40 and 16% of 25 of nests parasitized respectively). The authors suggest this may be due to the small sample size in 1991. Artificial egg size (26.1 x 17.2 mm, resembling blackbird eggs vs. 20.1 x 16.1 mm, resembling cowbird eggs) or the use of artificial or real cowbird eggs did not affect parasitism rates. Adding eggs did not alter clutch size or hatching success of blackbirds (average clutch size of 3.8 eggs/clutch for 97 experimental nests vs. 3.7 eggs/clutch for 79 controls; average hatching success of nests that hatched at least one egg: 3.2 eggs/nest for 48 experimental nests vs. 3.1 eggs/nest for 42 controls).