Action: Foster eggs or chicks of songbirds with wild non-conspecifics (cross-fostering)
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- A replicated study from the USA found that the survival of cross-fostered yellow warbler Dendroica petechia chicks was lower than previously-published rates for the species, although incubation and nestling periods were very similar.
- A replicated and controlled study from Norway found that the success of cross-fostering small songbirds varied depending on the species of chick and foster birds. However, only great tits P. major raised by blue tits P. caeruleus had lower pairing success than control birds, whilst blue tits raised by coal tits P. ater had higher recruitment than controls, or those raised by great tits.
If the wild populations of a species are very small then it may not be possible to foster offspring to conspecifics. However, it may be possible to foster chicks and eggs to a similar, but more abundant species, if one is present. This can increase the reproductive output of the wild population of the endangered species, or even allow for the reintroduction of a population into parts of its former range.
Supporting evidence from individual studies
A replicated study in 1978 and 1980 in a parkland site in Michigan, USA (Brewer & Morris 1984), transferred yellow warbler Dendroica petechia eggs and nestlings (two to six days old) to chipping sparrow Spizella passerina nests and found that four of six clutches transferred in 1978 produced fledglings that were fed by foster parents 16-26 days after hatching. In 1980, 34 fledglings were produced from a total of 64 eggs and 13 nestlings that were transferred into 26 nests. Eleven reached an age of at least 24 days and one male was seen returning to the study area in 1981 and showing normal yellow warbler behaviour. Incubation and nestling periods in cross-fostered chicks were identical to previously published results for yellow warblers, but survival of cross-fostered chicks was lower than previously published results. In 1978, nestlings and eggs were also transferred to field sparrow S. pusilla, and house wren Troglodtyes aedon, but the results were not provided. This study was used to investigate the possibility of cross-fostering Kirtland’s warblers D. kirtlandii, which was endangered at the time of the study.
A replicated and controlled study in woodlands in southern Norway in 1998-2000 (Slagsvold et al. 2002) found that cross-fostering did not affect the recruitment of great tits Parus major (12% of birds raised by blue tits P. caeruleus observed the following year, n = 155 chicks vs. 13% of control birds, n = 196) or pied flycatchers Ficedula hypoleuca (4% recruitment for birds raised by great or blue tits, n = 573 vs. 6% for control birds, n = 935). However, blue tits raised by coal tits P. ater had higher recruitment than controls, or those raised by great tits (18% recruitment for birds raised by coal tits, n = 38 chicks vs. 10% for birds raised by great tits, n = 242 and 7% for control birds, n = 175). Cross-fostering reduced pairing success in great tits (27% pairing success for cross-fostered chicks, n = 11 vs. 95% for controls, n = 20) but not in blue tits (100% pairing success for both cross-fostered and control chicks, n = 17 and 11 respectively) or flycatchers (95% pairing success for cross-fostered chicks, n = 19 vs. 95% for controls, n = 39). All fostering occurred during incubation, with eggs moved between nests before they hatched.
- Brewer R. & Morris K.R. (1984) Cross-Fostering as a Management Tool for the Kirtland's Warbler. The Journal of Wildlife Management, 48, 1041-1045
- Slagsvold T., Hansen B.T., Johannessen L.E. & Lifield J.T. (0) Mate choice and imprinting in birds studied by cross-fostering in the wild. Proceedings of the Royal Society of London. Series B: Biological Sciences, 269, 1449-1455.