Action: Alter artificial nest sites to discourage brood parasitism
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A replicated trial from Puerto Rico found that brood parasitism levels were extremely high across all nest box designs tested.
Supporting evidence from individual studies
A replicated trial in 1980 in Puerto Rico (Wiley et al. 1991) found that, shiny cowbirds Molothrus bonariensis parasitised yellow-shouldered blackbird Agelaius xanthomus nests in all 16 nest box types tested with 96% of 103 nests parasitised. The effect of cowbird control is discussed in ‘Threat: Invasive and other problematic species - Remove/control brood parasites’.