Inadequate reproductive success is a potential cause of Spotted Flycatcher (Muscicapa striata) population decline in England

Abstract

Most demographic studies of multi-brooded species require information on breeding success per nesting attempt and the number of nesting attempts made to derive a measure of whole-season productivity. However, measuring the number of nesting attempts is often challenging. We used individual-based re-nesting models to estimate whole-season productivity for the Spotted Flycatcher Muscicapa striata in England to investigate demographic drivers of long-term population decline. Between 2004 and 2006 we monitored 248 nests in two regions of England covering three broad nesting habitats: farmland, garden and woodland. We combined field-derived measures of nesting start date, clutch size, hatching and fledging rates, whole-nest survival and re-nesting intervals, with nest-stage durations taken from the literature, to estimate whole-season per-pair productivity for each habitat and region combination. We weighted these estimates by the proportion of Spotted Flycatchers counted in each region-habitat (derived from extensive monitoring data for 1994–2006 and 2007–2021) to derive region-specific productivity measures. Finally, we compared these derived empirical productivity measures with the level required to maintain population size given current knowledge of survival rates of UK-breeding Spotted Flycatchers. Whole-season productivity only reached the required threshold for population stability in gardens. Regional estimates of whole-season productivity were substantially lower than the level required for population stability in both regions and both time periods, and the number of nesting attempts needed to achieve stability was also implausibly high in both regions. Assuming the same rates of whole-season productivity for Spotted Flycatchers distributed across these habitats and regions based on Breeding Bird Survey data for 2007–2021, productivity declined further to levels below that needed to sustain the population in all three habitats, mainly as a consequence of lower proportions recorded in gardens in southwest England. Historical and recent population declines of Spotted Flycatchers in eastern and southwest England are likely to have been caused by low productivity, particularly low survival of nests at the egg stage. Our study illustrates the utility of re-nesting models as a tool for understanding the demographic drivers of population change in multi-brooded species.