Causes of change

Recent tracking work suggests that reduced survival on migration could be a primary driver of population decline in Cuckoos. However, this may not be the only driver and a number of other hypotheses have been proposed.

Further information on causes of change

Recent tracking work from nine tagging locations across the UK has identified that Cuckoos nesting in the UK use two distinct routes to reach the same wintering grounds, and identified a strong correlation between population trends in each area and the proportion of Cuckoos following each migration route. This suggests that recent problems on the western migration route through Spain may have contributed to the population decline (Hewson et al. 2016).

Decreased food supplies on the breeding grounds has also been suggested as a possible cause (Glue 2006, Denerley 2014), following the rapid declines of many British moth species (Conrad et al. 2006), important prey items in Cuckoo diet. Denerley et al. (2018) found that Cuckoo presence in Devon was related to that of macro-moth prey species, and suggested that the trends in prey species could explain the differing UK trends and also an increasing association with semi-natural sites such as heathland and wetland rather than lowland farm habitats. Based on both molecular DNA sequencing of faeces and the study of recent photos from crowd sourcing, Mills et al. (2020) concluded that Cuckoo diet was largely similar to that previously documented prior to the decline; they also surmise that this could explain the regional patterns of decline.

Given that the Cuckoo is a migrant, and the fact that many long-distance migrants have been found to be declining (Sanderson et al. 2006, Hewson & Noble 2009), factors operating on wintering grounds have also been suggested as a possible primary driver of Cuckoo declines (Glue 2006, Payevsky 2006, Newson et al. 2009). However, as trends differ across the UK, the fact that the tracking work ( Hewson et al. 2016) found that all Cuckoos used the same wintering grounds suggests that over-winter factors can be discounted.

Cuckoo abundance may be related to their breeding success, which might in turn be determined by the abundance of breeding success of host species. Evidence from BBS data show strong variation in Cuckoo population trends between habitats, which may reflect regional differences in the main hosts and differing trends in Cuckoo breeding success among those host species (Newson et al. 2009). Douglas et al. (2010b) found a strong positive correlation between change in Cuckoo numbers and numbers of Meadow Pipit in the previous year, also based on BBS data, but this only accounted for 1% of the decline in Cuckoo populations so this is unlikely to be a primary driver. A study from the 1980s using Nest Record Scheme data also found changes in usage of some key host species (Brooke & Davies 1987) but the authors also thought that this was unlikely to be the main cause of population decline. There has perhaps been a disproportionate emphasis on the role of brood parasitism aspects in Cuckoo decline.

Another hypothesis for the decline of Cuckoos relates to phenological mismatch in the timing of host and Cuckoo breeding. There is evidence relating to climate-induced changes in phenology. The extent to which this may be driving population declines is unclear, although modelling suggests that climate change may have had a negative impact on the long-term trend for this species (Pearce-Higgins & Crick 2019). Newson et al. (2016) found that Cuckoo had not changed its arrival date between the 1960s and the 2000s (the date advance slightly by c.3 days but this change was not significant). Douglas et al. (2010b) used BBS data and found that in recent decades, earlier breeding Dunnock nests have become less available to Cuckoos, whilst those of Reed Warblers more so. However, they suggested that changes in host phenology are likely to have had only a minimal impact on Cuckoo population trend (Douglas et al. 2010b). In Europe, other recent studies have suggested that climate change might disrupt the association between the life cycles of the Cuckoo and its short-distance migrant hosts and they state that this mismatch may contribute to the decline in Cuckoo (Saino et al. 2009, Moller et al. 2011). Thus, evidence at European scale at least is equivocal.

Information about conservation actions

The research suggests that reduced survival on the western migration route through Spain may be the primary driver of the population decline of the Cuckoo in the south and east of the UK (Hewson et al. 2016), and therefore conservation action outside the UK may be required to reverse the declines. However, there is also evidence suggesting that declines in prey species, in particular larger moths, may also have contributed to the declines (Denerley et al. 2018), and therefore actions to improve habitat to benefit invertebrates may also benefit this species. At a local level, these may include habitat actions such as the provision of beetle banks, set-aside, planting buffer strips around arable fields and restoring or creating semi-natural grassland.

However, Cuckoo territories are comparatively large and, if prey availability is contributing to the declines, it is likely that these will only be reversed if policies ensure that habitat management occurs over a correspondingly large scale through a landscape scale approach.