Action Synopsis: Bird Conservation About Actions

Remove brood parasite eggs from target species’ nests

How is the evidence assessed?
  • Effectiveness
  • Certainty
  • Harms

Study locations

Key messages

  • A controlled before-and-after study on Puerto Rico found lower rates of parasitism of yellow-shouldered blackbird Agelaius xanthomus nests when shiny cowbird Molothrus bonariensis eggs were removed from nests.
  • A replicated, controlled study from 1997-1999 in grassy fields in New York State, USA found that song sparrow Melospiza melodia nests that had cowbird eggs removed from them had lower success than nests which were parasitised and that did not have eggs removed.


About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. A controlled before-and-after study in mangrove forests on Puerto Rico in 2000, 2001 and 2003 (López-Ortiz et al. 2006) found that a significantly lower proportion of yellow-shouldered blackbird Agelaius xanthomus nests were parasitised by shiny cowbirds Molothrus bonariensis, compared to yellow warbler Dendroica petechia nests, following the removal of cowbird eggs and chicks from artificial nests used by blackbirds from 1991, and the control of adult cowbirds from 1983 (3% of 927 blackbird nests vs. 37% of 165 warbler nests). Prior to cowbird control, parasitism rates had been higher for blackbirds (91-95% of 202 blackbird nests in 1975-83 vs. 63% of warbler nests). Parasitism rates in areas without cowbird control were lower for blackbirds (44% of 32 nests) and higher for warblers (85% of 13 nests). The authors suggest that removing eggs and nestlings reduces the proportion of cowbirds that imprint on specific hosts, reducing future parasitism. The effect of adult cowbird removal is discussed in ‘Remove adult brood parasites’.

    Study and other actions tested
  2. A replicated, controlled study from 1997-1999 in grassy fields in New York State, USA (Hauber 2009) found that song sparrow Melospiza melodia nests paratised by brown-headed cowbirds Molothrus ater had lower productivity when cowbird eggs were removed, compared to paratised nests when cowbird eggs were not removed (median of 0% of eggs from nests with eggs removed produced nestlings vs. 75% of eggs from nests where cowbird eggs were not removed). There were no differences in the number of song sparrow nestlings surviving to five days old between paratised nests, non-paratised nests and paratised nests with cowbird eggs removed.

    Study and other actions tested
Please cite as:

Williams, D.R., Child, M.F., Dicks, L.V., Ockendon, N., Pople, R.G., Showler, D.A., Walsh, J.C., zu Ermgassen, E.K.H.J. & Sutherland, W.J. (2020) Bird Conservation. Pages 137-281 in: W.J. Sutherland, L.V. Dicks, S.O. Petrovan & R.K. Smith (eds) What Works in Conservation 2020. Open Book Publishers, Cambridge, UK.


Where has this evidence come from?

List of journals searched by synopsis

All the journals searched for all synopses

Bird Conservation

This Action forms part of the Action Synopsis:

Bird Conservation
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What Works in Conservation

What Works in Conservation provides expert assessments of the effectiveness of actions, based on summarised evidence, in synopses. Subjects covered so far include amphibians, birds, mammals, forests, peatland and control of freshwater invasive species. More are in progress.

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