Action: Use false brood parasite eggs to discourage brood parasitism
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A replicated, controlled experiment in the USA found lower parasitism rates for red-winged blackbird Agelaius phoeniceus nests with false or real brown-headed cowbird Molothrus ater eggs added to them.
Brood parasites choose to lay eggs in nests that have not previously been parasitized, to minimise intra-specific competition. Therefore, placing false parasite eggs in nests may protect nests by discouraging parasites from laying.
Supporting evidence from individual studies
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).