DOI: 10.5061/dryad.c2fqz61bg
title: Integrated population model for the Mallard in the Netherlands

description: Europe’s highest densities of breeding Mallards (Anas platyrhynchos) are
 found in the Netherlands, but the breeding population there has declined
 by ~30% since the 1990s. The exact cause of this decline has remained
 unclear. Here, we used an integrated population model to jointly
 analyze Mallard population survey, nest survey, duckling survival and
 band-recovery data. We used this approach to holistically
 estimate all relevant vital rates, including duckling survival rates for
 years for which no explicit data were available. Mean vital rate
 estimates were high for nest success (0.38 ±0.01) and egg hatch rate (0.96
 ±0.001), but relatively low for clutch size (8.2 ±0.05) compared to
 populations in other regions. Estimates for duckling survival rate for the
 three years for which explicit data were available were low (0.16-0.27)
 compared to historical observations, but were comparable to rates reported
 for other regions with declining populations. Finally, mean survival rate
 was low for ducklings (0.18 ±0.02), but high and stable for adults (0.71
 ±0.03). Population growth rate was only affected by variation in
 duckling survival, but since this is a predominantly latent state
 variable, this result should be interpreted with caution. However, it does
 strongly indicate that none of the other vital rates, all of which were
 supported by data, was able to sufficiently explain the population
 decline. Together with a comparison with historic vital rates,
 these findings point to a reduced duckling survival rate as the likely
 cause of the decline. Candidate drivers of reduced duckling survival are
 increased predation pressure and reduced food availability, but this
 requires future study. Integrated population modeling can provide valuable
 insights into population dynamics even when empirical data for a key
 parameter are partly missing.
description: We built an Integrated
 Population Model (IPM) and fitted parameters with four datasets: (1)
 population survey data, (2) nest survey data, (3) duckling survival data,
 and (4) annual band-recovery data. Our study area covers the entire
 country of the Netherlands, as all data were collected as part of national
 monitoring programs or citizen science projects. We limited our analyses
 to the years 2003-2020 due to a limited sampling effort in the years
 before 2003 for most datasets. <strong>Population survey
 data.</strong> Breeding bird populations have been monitored in
 the Netherlands as part of a national Breeding bird Monitoring Program
 (BMP) starting in 1984 (Boele et al. 2014). BMP is based on intensive
 territory mapping in fixed study plots carried out by well-trained
 volunteers and professionals who follow a standardized protocol. Territory
 mapping is based on a large, and annually constant, number of field visits
 (5-10 between March and July depending on species) in which all birds with
 territorial behavior (e.g., pair bond, display, nests) are recorded on
 maps. Species-specific interpretation criteria are used to determine the
 number and exact locations of “territories” at the end of the season. The
 counts of 1990 constitute the index baseline. Index values for the
 following years are then calculated as the relative difference between
 that year’s counts and those of the reference year (Sovon
 2019). <strong>Nest survey data.</strong>
 Nest data for Dutch
 Mallards were retrieved from the National Nest Record Scheme (Bijlsma et
 al. 2020). Mallard nests are mainly reported as ‘bycatch’ during nest
 surveys of grassland-breeding waders. To calculate clutch size and egg
 hatch rate, incomplete clutches were excluded by using only successfully
 hatched nests. In total, there were 4415 observations of clutch size with a range of
 149-402 nests observed annually. For egg hatch rate per successful clutch,
 we calculated for each year the total number of eggs laid and the number
 of eggs hatched in nests where at least one egg had hatched. In total 2383
 nest observations were available, with the number of observations ranging
 between 54 and 282 nests per year.
 <strong>Duckling
 survival data.</strong> We used data for Mallard females with broods
 collected through a citizen science project to estimate annual duckling
 survival rates. Contributors to this project were asked to report brood
 size and brood age on a mobile application specifically developed for this
 purpose. From 2018 onward, contributors were asked to make repeated
 observations of the same broods to allow for the calculation of duckling
 survival. We therefore used data for the years 2018-2020, as only for
 these years were there enough repeated observations of broods to estimate
 duckling survival. The final dataset included 2825 observations of 1212
 broods distributed over the three years. 
 <strong>Adult
 band-recovery data.</strong> Annual survival of post-fledgling birds was
 estimated using data for Mallards that were banded between 2003 and 2020
 and were later recovered and reported dead. In the Netherlands, Mallards
 are banded mostly in traditional duck decoys (Karelse 1994). Since the
 year 2000, 20133 Mallards have been banded, of which 1233 were recovered
 dead (Buijs and Thomson, 2001, van Noordwijk et al. 2003). We only
 included Mallards that were banded in the breeding season (March-July) to
 exclude winter migrants that breed in other countries. This dataset
 included 2615 Mallards, of which 203 were recovered
 dead.