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Providing evidence to improve practice

Action: Remove brood parasite eggs from target species’ nests Bird Conservation

Key messages

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  • 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.


Supporting evidence from individual studies


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’.



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.


Referenced papers

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. (2019) Bird Conservation. Pages 141-290 in: W.J. Sutherland, L.V. Dicks, N. Ockendon, S.O. Petrovan & R.K. Smith (eds) What Works in Conservation 2019. Open Book Publishers, Cambridge, UK.