Survival and dispersal drive population dynamics of UK breeding Pied flycatchers

This blog post is provided by Malcolm Burgess and Chloé Nater and tells the #StoryBehindthePaper for the paper ‘Spatial consistency in drivers of population dynamics of a declining migratory bird‘, which was recently published in Journal of Animal Ecology. In their study, they used long-running nest monitoring and individual mark-recapture data collected mainly by citizen scientists and shared through the SPI-Birds Network and Database. They used these data in integrated models to determine the main drivers of short- and long-term population dynamics of the declining migratory pied flycatcher.

Many migratory birds are in decline, but the causes are often difficult to diagnose, in part because they can operate in any location across the annual cycle. If we are to halt and reverse population declines we need to know where and when to focus conservation actions. For pied flycatchers, as for many other migratory bird species, we have a good knowledge of reproductive ecology thanks to a multitude of studies conducted at breeding grounds throughout Europe. In the UK, for example, this is possible thanks to long-term monitoring of breeding attempts by dedicated citizen scientists who run nestbox projects across the species’ entire British breeding range, some over several decades.

Over the last 65 years, citizen scientists across Britain have monitored tens of thousands of pied flycatcher nests, often as part of the British Trust for Ornithology’s Nest Record Scheme, and ringed hundreds of thousands of individual flycatchers. Scientific studies have predominantly focused on single locations. One of the main reasons for this is that analyses at larger scales requires data from different monitoring programmes to be collated, standardised, and made available in a common format to facilitate comparisons between them. So, despite the same kind of data having been collected following similar protocols in several locations, effort is still needed to link them all up. For studies of pied flycatchers across the UK, this process was started with the foundation of PiedFly.Net and the resulting harmonised data are now made available in a standard format through the SPI-Birds Network and Database. Importantly, this ensures the data firstly are findable and accessible to others and, secondly, interoperable and reusable (so for example, the methods of collection are documented), i.e. according to the FAIR principles, maximising the scientific and conservation value of the data collected.

The availability of standardised data across a multitude of breeding sites in this way has opened new possibilities for uncovering the causes of decline in British-breeding pied flycatchers. These causes could be at the breeding sites in the UK, along the migration route, or in the non-breeding areas. At the breeding sites, reproduction (e.g. clutch size, breeding success) may be impacted by changes in habitat quality or availability, or from changes in prey availability in terms of either their amount, or their seasonal timing through a so-called trophic mismatch, with not enough prey available at the time chicks are in the nest. On the contrary, populations could also be impacted by conditions experienced during migration, or in the non-breeding areas in western Africa.

Our study’s aim was to investigate whether changes in population status, including a long-term decline since the 1990s, were related primarily to reproduction, indicating drivers acting during the breeding season, or to annual survival, indicating drivers acting on non-breeding stages.

For our analysis we used data from seven pied flycatcher populations representing the entire British breeding range including Scotland, Wales, and England, encompassing nearly 20,000 monitored nests and more than 113,000 individual captures of adults and young over a period of 64 years. With Rob Robinson from the BTO, we developed and fitted generalized Integrated Population Models (IPMs) to these data. These models combine data on different aspects of pied flycatcher life-cycle, allowing us to identify demographic and environmental drivers of short- and long-term changes in population dynamics. Among other approaches, we use transient Life Table Response Experiments (LTREs), a new statistical tool for quantifying the contributions of each life-cycle stage to the overall observed change in breeding numbers.

While we generally found negative effects of local rainfall on reproductive parameters (nest success and nestling survival) during the breeding period, there were clear similarities in population changes observed at the different breeding sites. Broadly speaking, all populations followed the same long-term patterns of increases and decreases. This was important,because it suggested that there are large-scale drivers of population dynamics that extend beyond specific breeding site effects. These large-scale drivers are likely linked to conditions that pied flycatchers face on migration and in non-breeding areas.

This same conclusion was also supported by the LTRE analyses, which attributed past changes in both short-term population growth rates and long-term population trends primarily to variation in annual survival and dispersal between sites. Variation in annual survival is expected to largely stem from factors acting during migration and the non-breeding season, but this part of the life cycle of pied flycatchers, like for many other migratory birds, is severely understudied. Knowledge is also limited regarding drivers and determinants of immigration by new breeding individuals, which was identified as a crucial factor, especially for the short-term changes seen in our study populations. This suggests the surplus of non-breeding pied flycatchers can potentially act as a buffer in years with lower survival and return rate of breeders. While increasing nest box provision at sites can increase the number of breeding pairs, it is not as simple as just adding more nest boxes, as periods of decline also occurred when nest box densities were high or increased. Taken together, our study highlights the importance of prioritizing research that focuses on the factors impacting survival and dispersal of migratory birds throughout migration and the non-breeding period.

This paper also has a second purpose, besides shedding light on the drivers of pied flycatcher population dynamics, which is to highlight how SPI-Birds data (and standardized data more generally) can be used to study demography and population dynamics at scales larger than that generally achievable by a single study, leading to a more complete understanding of why populations might change. Generalized and policy-relevant inference at national and international scales hinges not only on standardized data, but also on accessible, well-documented, and reproducible workflows for others to follow and adapt approaches like ours. To that end, we have made our entire analysis toolbox for running IPMs on SPI-Birds data openly accessible via GitHub, and published a detailed, and hopefully user-friendly manual with it. We hope that this will inspire more data owners to share their data via networks like SPI-Birds, and more researchers to conduct the multi-population, and range-wide, analyses necessary for more effective conservation, especially for migratory species. Ultimately, it is only by harmonizing data from different programs and sharing transparent workflows that the dedicated work of many field workers, citizen scientists, and researchers can continue to be used for conservation science in perpetuity.

Read the full paper here: Nater, C. R., Burgess, M. D., Coffey, P., Harris, B., Lander, F., Price, D., Reed, M., & Robinson, R. A. (2022). Spatial consistency in drivers of population dynamics of a declining migratory bird. Journal of Animal Ecology, 00, 1–1 5. doi: 10.1111/1365-2656.13834

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