By Steve Westerman, Wharfedale Naturalists Society

MOORHENS can be seen in a number of places locally. However, in recent weeks I have spent some time watching chicks on two RSPB reserves. This led me to consider some of the unusual breeding behaviours of this species.

The cuckoos that can be heard and seen on Ilkley Moor are obligate brood parasites – ie, their sole means of reproduction is to lay their eggs in the nests of another species.

However, there are birds, that will normally incubate and raise their own offspring, that also sometimes lay one or more eggs in the nest of another bird of the same species. This is referred to as intraspecific (or conspecific) brood parasitism (IBP).

Waterfowl are a group of birds in which this reproductive strategy is more prevalent than most, and moorhens are one of the species that engage in it.

Various fascinating and seemingly plausible hypotheses have been put forward to explain IBP. It may be a strategy to minimise risk; it may be a means of enhancing fecundity; it may be both. At this point in time, it seems nothing is certain. For example, it has been suggested that IBP is an ‘insurance policy’, spreading the risk of loss, ie, ‘not having all your eggs in one basket’. So, for example, if a nest is destroyed through predation, it means there is a chance that all is not lost from that breeding attempt, and the birds’ genes will still be passed on.

Countering this, it has been argued that overall genetic advantage for this behaviour does not change. However, this assumes equal risks/benefits for both nests.

This is not necessarily the case. It has been shown (in goldeneye) that relatively safer nests, and ones that have a higher chance of success, are chosen for parasitic egg laying. In this way chances of the survival of genes can be increased.

An alternative hypothesis is that eggs are laid in the nests of birds that are related and that tend to be older (perhaps these older ‘recipient’ birds are laying fewer eggs than they are able to rear).

This hypothesis has been referred to as ‘relying on grandma’. It may occur naturally through natal philopatry (birds returning to the area where they were born). In this circumstance IBP becomes a means of maximising gene survival for both birds (younger and older) and this would also help to explain why introduced eggs are not usually rejected.

Of course, hypotheses such as these aren’t mutually exclusive. In any event, the evidence seems somewhat equivocal, or not available, and the debate goes on.