Integrated population model for the Mallard in the Netherlands

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.