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.

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.

Population survey data. 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).

Nest survey data. 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.

Duckling survival data. 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.

Adult band-recovery data. 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.