by Steve Westerman

Wharfedale Naturalists Society

HURRAH - my annual opportunity to catch up with cuckoos on Ilkley Moor has finally arrived. As I described in a Nature Notes from this time last year, they leave their winter home in sub-Saharan Africa to visit us during the late spring and early- to mid-summer.

To recap, cuckoos are ‘obligate brood parasites’. In other words, their only means of producing offspring involves laying eggs in the nests of other species. Once the egg is in situ, the success of this strategy requires that it is incubated rather than rejected by the host bird and, subsequently, the chick is raised. All of this leads to what has been described as an evolutionary ‘arms race’. Over generations, Cuckoos evolve in ways that facilitate accessing nests and increase acceptance of eggs and support of young (e.g., eggs that better match those of the host). Meanwhile, host birds develop characteristics/behaviours that reduce the probability of their nests being infiltrated and that make them better able to detect intrusion.

Cuckoos target a number of host species. In the case of the Ilkley Moor cuckoos I think it is the fairly abundant Meadow Pipits. Other common hosts across the UK include Dunnocks and Reed Warblers. To make this diversity of targets possible, different ‘races’ – or ‘gentes’ of Cuckoos have evolved – with each ‘gens’ specialising in a particular host species.

One of the defensive tactics employed by host birds is to ‘mob’ cuckoos – to harass them, encouraging them to move on, while alerting other birds to their presence. There are several intriguing aspects to this. For example, birds that are not usually parasitised by Cuckoos will also sometimes mob them. The photograph shows a Cuckoo being harassed by a chaffinch. It is worth noting that this was a sustained behaviour, not just momentary. Given that chaffinches are not normally targeted by Cuckoos, and that there must be some risk associated with this activity, what is the associated advantage that makes it worthwhile?

One suggestion is that such behaviour is a ‘hangover’ from an old ‘arms race’ that the chaffinch won - i.e., chaffinches used to be targeted by a Cuckoo ‘gens’ that died out because it was not sufficiently successful. Alternatively, it may be that the chaffinch is responding to the ‘bird of prey’ appearance of the cuckoo (barred breast and yellow eye perhaps mimicking a Sparrowhawk). Whatever the underlying reason, small birds can be very persistent in harassing Cuckoos and sometimes pursue them for a distance when they fly off to look for a safer perch.