Action: Foster eggs or chicks of gannets and boobies with wild conspecifics
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A small controlled study in Australia found that Australasian gannet chicks Morus serrator were lighter, and hatching and fledging success lower in nests which had an additional egg or chick added. However, overall productivity was (non-significantly) higher in experimental nests.
Natural variations in reproductive output can be detrimental when populations are very small, for example if pairs fail to produce fertile eggs or some pairs repeatedly fail to raise offspring successfully. One way to minimise this problem is to foster eggs and chicks between nests. Eggs and chicks from nests with more offspring than they are likely to be able to raise can be moved to those with infertile eggs. Alternatively, if a pair produces fertile eggs or healthy chicks but consistently fails to raise chicks then it may be possible to transfer offspring to a more successful pair.
In other circumstances it may be possible to foster chicks with other species (‘cross-fostering’). Studies describing this intervention are discussed in the following section ‘Foster eggs or chicks with wild non-conspecifics (cross-fostering).’
Supporting evidence from individual studies
A small controlled study at a marine reserve in Queensland, Australia, in the breeding seasons of 1997-8 and 1998-9 (Bunce 2001) found that Australasian gannet chicks Morus serrator were significantly lighter, and hatching and fledging success significantly lower in nests where a second egg or chick was added to the nest (‘experimental nests’), compared to control nests (maximum weight of approximately 2500 g for experimental nests in 1997-8, n = 4 vs. approximately 3250 g for controls, n = 8; data not provided for 1998-9; 1997-9: hatching success of35% for experimental nests vs. 70% for controls; fledging success of 63% for experimental nests vs. 90% for control). Over both years, the number of chicks fledged by experimental nests was higher than control nests, but this was not significant (1.3 chicks/nest for experimental nests, n = 8 vs. 0.9 chicks/nest for controls, n = 8). This study also investigated the impact of supplementary feeding on gannet chicks (see ‘Provide supplementary food to increase reproductive success’).