Spatial consistency in drivers of population dynamics of a declining migratory bird

Abstract

Many migratory species are in decline across their geographical ranges. Single-population studies can provide important insights into drivers at a local scale, but effective conservation requires multi-population perspectives. This is challenging because relevant data are often hard to consolidate, and state-of-the-art analytical tools are typically tailored to specific datasets. We capitalized on a recent data harmonization initiative (SPI-Birds) and linked it to a generalized modelling framework to identify the demographic and environmental drivers of large-scale population decline in migratory pied flycatchers (Ficedula hypoleuca) breeding across Britain. We implemented a generalized integrated population model (IPM) to estimate age-specific vital rates, including their dependency on environmental conditions, and total and breeding population size of pied flycatchers using long-term (34-64 years) monitoring data from seven locations representative of the British breeding range. We then quantified the relative contributions of different vital rates and population structure to changes in short- and long-term population growth rate using transient life table response experiments (LTREs). Substantial covariation in population sizes across breeding locations suggested that change was the result of large-scale drivers. This was supported by LTRE analyses, which attributed past changes in short-term population growth rates and long-term population trends primarily to variation in annual survival and dispersal dynamics, which largely act during migration and/or nonbreeding season. Contributions of variation in local reproductive parameters were small in comparison, despite sensitivity to local temperature and rainfall within the breeding period. We show that both short- and long-term population changes of British breeding pied flycatchers are likely linked to factors acting during migration and in nonbreeding areas, where future research should be prioritized. We illustrate the potential of multi-population analyses for informing management at (inter)national scales and highlight the importance of data standardization, generalized and accessible analytical tools, and reproducible workflows to achieve them.

Keywords: LTRE; annual survival; comparative analysis; environmental effects; full annual cycle; integrated population model; multi-population; pied flycatcher.

FIGURE 1

(a) Geographical location of flycatcher study populations (coloured dots) relative to the British breeding distribution (grey, with darker shading indicating higher relative abundance, data from EBBA, Keller et al., 2020). (b) Overview of location names and sampling years. (c) Mean number of nestboxes monitored per year in each study site (black bars indicate mean ± SD).

FIGURE 2

Annual estimates of the total number of females (dashed line) and the number of females breeding in nestboxes (solid line) for all seven study populations. The difference between these two estimates is that the former includes nonbreeders, temporary emigrants and birds nesting in natural cavities as opposed to nestboxes. Lines represent the posterior median estimates, ribbons mark the 95% credible intervals (pale = total number of females, more coloured = number of females breeding in nestboxes).

FIGURE 3

Posterior medians (dots) and 95% credible intervals (lines) for estimated time‐average vital rates for the seven study populations. Open symbols = younger age class (juveniles for annual survival, yearlings otherwise). Filled symbols = adults (combined age class for nest success probability). For numerical summaries, see Table S1.2.

FIGURE 4

Effects of rainfall (top row) and temperature (bottom row) on nest success probability, nestling survival and juvenile annual survival (columns) of the seven study populations. For nest success probability and nestling survival, environmental covariates represented conditions during a 16‐day window posthatching. For juvenile annual survival, the rainfall covariate covered a 7‐day period postfledging. Environmental variables are plotted on a standardized scale for easier comparison across populations. An alternative representation of the relationships, including visualization of raw data, can be found in Figure S1.25.

FIGURE 5

Posterior distributions of the contributions of reproduction (breeding probability, nest success probability, nestling survival probability), survival (juvenile and adult annual survival) and immigration rates to variation in realized annual population growth rates. Contributions from local population structure were negligible and are omitted here (but see Figure S1.20).

FIGURE 6

Posterior medians of stacked contributions of vital rates representing reproduction (turquoise shades) and survival (pink shades) to year‐by‐year changes in annual population growth rate over time for each study population. The sum of all contributions approximates the total rate of change in population size from 1 year to the next. Contributions from local population structure and immigration are omitted here to facilitate comparison of reproduction versus survival contributions but see Figure S1.21 for the same figure including all types of contributions.

FIGURE 7

Posterior distributions of the contributions of reproduction (breeding probability, nest success probability, nestling survival probability), survival (juvenile and adult annual survival) and immigration rates to changes in long‐term population trends within the study period. The time periods compared for each population are shown in Figure S1.1. Contributions include both effects of direct changes in vital rates and indirect effects caused by perturbation of population structure due to vital rate changes.