Each year we find a few Cuckoos parasitizing the Reed Warbler nests that we monitor at sites in the Brecks. These are invariably found at the egg stage, the Cuckoo egg similar in background colour and pattern to the Reed Warbler eggs but somewhat larger. The degree to which the Cuckoo achieves successful mimicry of the Reed Warbler eggs is important, since it needs to trick the host into thinking the egg is one of its own and not that of another species. If the Reed Warblers are suspicious that they might have been parasitized by a Cuckoo then they will abandon the nesting attempt and start again.
Cuckoo eggs have been recorded from some fifty different host species in Britain but those most often targeted are Reed Warbler, Meadow Pipit, Dunnock, Pied Wagtail and Robin (roughly in that order). I have also seen a Cuckoo egg in the nest of a Wren, though how successful Cuckoos are in using Wrens is debatable. Wrens, like other small birds that build an enclosed nest, seem particularly sensitive to any change in the size of the nest entrance and any damage caused by the Cuckoo during laying is likely to cause the Wrens to desert. Additionally, there is the size difference between the eggs of the two species and the enclosed nature of a nest which seems unlikely to cope with the volume of a growing Cuckoo chick.
Just last week I saw a Cuckoo egg in one of the Dunnock nests that we are monitoring. As you probably know, the eggs of a Dunnock are bright blue and the Cuckoo egg in this nest was brown with splodges of darker colour – the female presumably a Reed Warbler mimic. You would think that a Dunnock would recognise the difference between the two eggs as well, but it seems that this is not the case. It is thought that this is because the Dunnock is a relatively recent host and that it has not yet developed the ability to recognise and respond to the presence of a Cuckoo. That Reed Warblers can and do respond, suggests that they have been Cuckoo hosts for a much longer period of time. However, even the Reed Warblers are fooled if they don’t suspect that a Cuckoo has visited the nest and once the egg hatches the resultant chick is reared as their own.
Finding a nest with a Cuckoo egg in it delivers mixed emotions. The presence of the egg spells the end of the host’s nesting attempt but, with Cuckoo numbers in decline, it gives hope that another Cuckoo will be recruited into a flagging population.