The Cuckoo is arguably the UK’s best-known summer visitor. Spending the winter months in Africa, Cuckoos arrive back in the UK during late April and early May, timing this arrival to match the breeding season of its host species here.

Since the early 1980s Cuckoo numbers have dropped by 65%. The reason for this decline is not known, but it has been suggested that declines in its hosts or climate-induced shifts in the timing of breeding of its hosts could have reduced the number of nests that are available for cuckoos to parasitize, resulting in Cuckoo declines. 

Using BBS data to understand the decline

The main hosts in the UK are the Dunnock, Meadow Pipit, Pied Wagtail and Reed Warbler. The latest research using BBS and Nest Record data examines whether changes in the abundance or timing of breeding of these four species is behind the large-scale decline of the Cuckoo.

Of the four host species, Meadow Pipit is the only species to have declined during the period examined (1994-2007). Whilst there was a relationship between declining Meadow Pipits and Cuckoos, this only accounts for about 1% of the observed Cuckoo decline.

At the same time, Dunnocks, Pied Wagtails and Reed Warblers have shifted their breeding forward by about 5-6 days. Considering the timing of the arrival of Cuckoos, it is likely that Dunnock and Pied Wagtail have become less available to Cuckoos, but the late-breeding Reed Warbler may be more available. 

However, the lack of a relationship between shifts in Dunnocks and Pied Wagtails and Cuckoo abundance in the following year suggests that shifts in these two species also do not explain Cuckoo declines. For Reed Warbler there was a positive relationship, so that earlier breeding of Reed Warbler may be benefiting Cuckoos.

Considering this study, and given the Cuckoo breeding ecology and migration strategy, the remaining plausible explanations for the decline of Cuckoos include reduced prey (mainly caterpillar) availability during the breeding season, or deterioration of conditions along migration routes, or on over-wintering grounds in sub-Saharan Africa.