Since 2004, a relatively stable population trend on one sub-site of Porton Down (Battery Hill) has contrasted with a strongly declining population trend on a second sub-site (Easton Down) for Stone Curlews. To explain the patterns of declines, the potentially limiting effects of disturbance were investigated and compared to trends in settlement patterns, coupled with new detailed observations of nest behaviour and nest survival rates, using nest cameras, carried out between 2010 and 2012.

Statistically significant effects were detected for two behavioural variables, ‘Time off the nest’ and ‘Total alarm rate’. Both variables were negatively correlated with nest proximity to internal access roads. No significant effect was detected in relation to the public road (Winterslow Road) although the relationship was weakly negative. No effect was detected for daily nest survival rates relative to their proximity to potential disturbance agents. 

Analysis of pair densities revealed a negative effect of the internal access roads, woodlands and denser scrub (though interactions with shrubs warrant closer investigation). However, on Easton Down, birds nested closer to these features over time, while abandoning seemingly more suitable ground. Also on Battery Hill, birds nested closer to the public (Winterslow) road over time, implying habituation (supported by video data). There were no clear effects of the testing bund on nesting pairs but there was a strong negative effect of low organic soil, consistent with the population decline on Easton Down. Possibly, the abandonment of soils of lower organic content or more suitable swards elsewhere, have led to birds nesting closer than expected to roads and woodlands. 

The lower density of Stone Curlew breeding attempts on Easton Down was not consistent with disturbance from roads or buildings since the birds’ abandoned seemingly better placed areas relative to these features. Also, the abandonment of central Easton Down had begun before nest survival rates began to decline after 2006. Stone Curlew pair nest attempts tend to be within relatively close proximity to one another thus, the implication is that settlement decisions made early in the season (preceding nest-predation events) are paramount for determining settlement patterns. For already settled birds, the proximate cause of nest mortality was nocturnal predation (mainly by badgers). Neither spatial nor temporal variation in the activity patterns of predators could be measured in this study, but such an investigation would help determine the mechanism behind increasing predation rates. 

Contrasting settlement patterns between Battery Hill and Easton Down suggest that interactions between sward condition, soil type and declining key invertebrate populations, may explain the observed changes in settlement patterns (c.f Environmental Change Network monitoring). Analysis of such interactions alongside the monitoring of settlement patterns and nest success during the current phase of rabbit decline (rabbits may exacerbate effects on sward structure in very dry conditions) would be timely and instructive. 

Recommendations are made that complex interactions between light shrub cover (where nests were located in recent years), soil moisture, swards and invertebrate biomass would benefit from further investigation. The potential for shrub cover to buffer against excessive drying or support higher invertebrate biomass would be an important finding in the light of scrub clearance programmes: areas where very few birds have nested in recent years. More frequent recording of nest occupancy would improve the precision of daily nest survival rates, and provide a more accurate and standardised performance monitoring of nest outcomes across the site and between years, to assist management decisions.