DOI: 10.5061/dryad.g1jwstqpj
title: Apparent breeding success drives long-term population dynamics of a migratory swan

description: The ability of a species to adapt to environmental change is ultimately
 reflected in its vital rates – i.e., survival and reproductive success of
 individuals. Together, vital rates determine trends in numbers, commonly
 monitored using counts of species abundance. Rapid changes in abundance
 can give rise to concern, leading to calls for research into the
 biological mechanisms underlying variations in demography. For the NW
 European population of Bewick’s swan (Cygnus columbianus bewickii), there
 have been major changes in the population trends recorded during nearly
 five decades of monitoring (1970-2016). The total number of birds
 increased to a maximum of c. 30,000 in 1995 and subsequently decreased to
 about 18,000 individuals in 2010. Such large fluctuation in population
 numbers is rare in long-lived species and understanding the drivers of
 this population change is crucial for species management and conservation.
 Using the integrated population model (IPM) framework, we analysed three
 demographic datasets in combination: population counts,
 capture-mark-resightings (CMR) and the proportion of juveniles in winter
 over a period of ~50 years. We found higher apparent breeding success in
 the years when the population had a positive growth rate compared to years
 with a negative growth rate. Moreover, no consistent trend in adult and
 yearling survival, and an increasing trend in juvenile survival was found.
 A transient life-table response experiment showed that apparent breeding
 success and adult survival contributed most to the variation in population
 trend. We explored possible explanatory variables for the different
 demographic rates and found a significant association between juvenile
 survival both with the water level in lakes during autumn migration, which
 affects food accessibility for the swans, and with summer temperatures.
 Such associations are important for understanding the dynamics of species
 with fluctuating population sizes, and thus for informing management and
 conservation decisions.
description: Encounter history -- data was collected over 47 years of
 mark-resighting research with both legrings and neckbands. In the csv, the
 years of the study (1969 - 2015) are indicated with they year index as the
 column header (1-47) and the individual ID as the rows. When an individual
 was resighted that year, a '1' is present in the matrix
 'encounter-history.csv', otherwise a '0'.
 Counts -- internationally coordinated counts of Bewick's
 swans take place every 5 years in January. The results of these counts are
 imputed to fill missing years based on a lineary imputation (see Methods
 section in Manuscript). Census years are indicated with 'TRUE'
 in the CensusYear column in the csv 'counts-jp.csv'. 
 Juvenile percentage (jp) -- every year, the proportion of
 juveniles on the total population are done in the Netherlands and the UK
 in December/January as a proxy for breeding success. The average of the
 estimates for both countries are presented in the csv
 'counts-jp.csv' in the column
 'JuvenilePercentage'. Output IPM -- in the
 csv 'ipm-output.csv' the estimates and their sd for adult
 survival (phi.a), yearling survival (phi.s), juvenile survival (phi.j),
 apparent breeding succes (kappa) and the population growth rate (lambdaW)
 are given. The Brooks-Rubin-Gelman diagnostic is given to check
 convergence of the MCMC chains (should be below 1.1; Brooks &
 Gelman 1998). Brooks, S. P. and Gelman, A. 1998. General methods for monitoring convergence of iterative simulations? J. Comput. Graph. Stat. 7: 434–455.