Which is more important in terms of influencing songbird nesting success, controlling predators or getting the habitat right?
Strong views are expressed in support of the two alternatives, but our long-running research into songbird nesting success at Loddington provides hard evidence to contribute to this debate. In particular, the combination of predator control (1993-2001) and habitat management (1993 onwards) at Loddington enabled us to investigate both the principal drivers of nest survival and the interactions between them. The results for blackbird, the species with the largest sample size, and the first to be fully analysed, were published in the ornithological journal, Ibis, back in 2008.
We considered nest survival separately for the two main stages of the nest cycle: incubation of eggs and provisioning of young, as these may be influenced by different factors. Nest success was lower when there was no predator control than when there was (see Figure 1). During incubation, density of corvid territories had a negative influence on nesting success, as did the proportion of cereal crop around the nest. This may be because the abundance of earthworms as food for adults is lower in arable land than in woodland or pasture. In previous research at Loddington, song thrush nests were more likely to fledge young if a large proportion of the area around them was pasture.
During provisioning of nestlings, control of nest predators had a positive influence and nest exposure had a negative influence on nest survival. There was an interaction between these two factors so that when predators were controlled, the negative effect of exposure was less extreme. This implies that predator control could allow birds to nest successfully in a wider range of locations, or earlier in the nesting season, than is otherwise possible, and that the provision of suitable nesting habitat is particularly important where there is no control of nest predators.
Figure 1: Annual variation in estimates of daily survival probability of blackbird nests during incubation between 1992 and 2007
Daily survival probability is a measure of nest success, and represents the probability of a nest surviving one day within a period of the nest cycle, in this case during incubation.
Constant-effort mist-netting during the summer in the two years before and the two years after stopping predator control revealed that the proportion of young birds caught was significantly higher during the predator control phase than when there was no predator control (see Figure 2). Past studies have used this proportion as an indicator of productivity, and so when it is higher it suggests birds are managing to produce more independent young, whether through improvements in nest success, post-fledging survival or other breeding parameters. This is most likely to be related to on-site productivity rather than immigration of young from outside the farm and provides further evidence for a positive effect of predator control on productivity.
Figure 2: Proportion of young blackbirds caught in the two years before and two years after cessation of predator control at Loddington
Transect data reveal an increase in blackbird breeding numbers through the predator control phase of the project, and a decline since cessation of predator control, relative to the regional trend (see Figure 3). The proportion of farmland territories did not change significantly across the two phases of the project, suggesting that there was no increased occupation of farmland as the population increased, or abandonment of farmland as it declined.
These long-term data from Loddington provide convincing evidence for benefits to nesting success of blackbirds resulting from the implementation of a game management system, particularly from predator removal, but further analysis is required to investigate whether this improves abundance. What this study highlights is that both habitat and predator control can interact to influence nest survival and that neither can be considered in isolation.
Figure 3: Breeding abundance of blackbirds at Loddington (from annual transects) relative to the regional trend
The regional population trend is based on the population index from British Trust for Ornithology (BTO) Breeding Bird Survey (BBS) data for the East Midlands, calculated from transect counts in between 122 and 264 randomly-chosen one kilometre grid squares in the region annually.