Variation in the Effect of Corvid Predation on Songbird Populations
Populations of farmland songbirds have declined rapidly in the UK in recent decades. Over the same period the populations of predators which take songbird eggs and chicks from the nest (nest predators), have increased. For example, the numbers of corvids on farmland have grown steeply since the 1950s. Although, meta-analyses have suggested that, in general, predators do not limit bird populations, in some cases the removal of corvid predators has led to improved breeding success of songbirds, and increased songbird breeding population numbers. In this thesis, I explored potential explanations for this variation in the impact of corvids on songbird populations. Firstly, I examined how variation in the ecology and behaviour of songbirds influenced their susceptibility to nest predation by corvids, and conversely how variation in the ecology and behaviour of corvids affected the extent to which they predated the nests of songbirds. Secondly, I investigated how variation in the habitat use of both songbirds and corvids, affected the likelihood of corvids encountering and predating songbird nests.
I showed that the breeding biology of songbird species significantly affected their risk of nest predation by corvids. Species that nested in open nests, placed low in the shrub, and with higher breeding season overlap with the breeding season of magpies, were particularly susceptible. It might be expected that population numbers of these vulnerable species would be more likely to decline in response to corvid population increase. I found that species which were susceptible to corvid nest predation did suffer higher egg and chick mortality, suggesting that higher corvid predation could be a driver of higher mortality in the nesting period. However, populations of songbird species with higher nesting mortality were not more likely to have declined over a time period when corvid numbers were increasing. Additional mortality during the nesting period does not appear to limit breeding population numbers of these songbird species. Mortality at other life history stages, such as overwinter, may be more likely to limit their populations.
Variation in the ecology and behaviour of corvids also affected the extent to which they predated songbird nests. Predation of artificial nests by magpies was greater inside magpie territories late in the breeding season, but nests inside specific magpie territories suffered particularly high rates of predation. Territory owners may have differed in their propensity to predate nests, either because of internal 9 differences between territorial magpies, or differences between the territories they owned.
Finally, the effect of corvid predation on songbird species was found to be affected by the habitat context. By examining the habitat selection of both predators and prey, I could ascertain how songbird prey utilised habitat variation to avoid corvid predation. Songbirds chose to nest in particularly dense, inaccessible hedgerows, away from the most frequently occupied magpie nest sites. Therefore, magpies may have indirectly had a negative effect on songbirds, even though magpies did not appear to select habitat based on songbird presence. If good quality habitat is limited, songbirds avoiding magpies may be forced into using sub-optimal sites.
Examining the different sources of variation in the corvid-songbird relationship also provided insight into factors causing variation in predator-prey relationships more broadly. I therefore concluded by analysing the implications of this thesis for management of predator-prey relationships in general, and for the corvid-songbird relationship in particular. In the case of the latter, I considered how the findings of this thesis could inform management strategies which might reduce the effect of corvid nest predation on songbird populations. Management could be targeted towards specific songbird species which are susceptible to predation or particular habitats where songbird nests are likely to be vulnerable. Conversely it may be possible to identify, and concentrate management on, particular corvid territories in which nest predation risk is high and/or particular corvid territory holders who are likely to predate nests. Using this targeted management could increase the productivity and breeding population numbers of farmland songbirds, a group of species in long-term decline.