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 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 [sic] ) 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

Notes:

• Many other bird species are also rapidly declining - in the UK, for example, almost a third of the native bird species are now on the endangered 'Red List'. See: Birds of Conservation Concern 5 , British Birds, 114, December 2021, 723–747

• Worldwide, of the approx. 11,000 bird species, nearly half currently have declining numbers. The reasons are unclear, but are probably a combination of factors including loss of habitat, pesticide use, climate change & etc..

Data from the IUCN Red List show that 49% of bird species worldwide (5,412) have declining populations, while 38% (4,234) are stable, just 6% (659) are increasing and 6% (693) have unknown trends."

Source :BirdLife.org, 2022.

Also see Sparrow declineplugin-autotooltip__plain plugin-autotooltip_bigSparrow decline

Large declines of urban and suburban house sparrow (Passer domesticus) populations have been recorded in many towns and cities across Europe. In London, sparrow numbers fell by 60% between 1994 and 2004. The cause(s) of the decline is still not resolved.