Since the early 1980s Cuckoo numbers [in the UK] 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. 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 arrival of Cuckoos, Dunnock and Pied Wagtail nests are likely to have become less available to Cuckoos, but the late breeding Reed Warbler 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 includes reduced prey (mainly caterpillers) availability during the breeding season or deterioration of conditions along migration routes or on over-wintering grounds in sub-Saharan Africa.
Source: British Trust for Ornithology
Also see Sparrow decline