Citizen science projects reveal drivers of Mallard decline 13th April 2023
In a recent study published in Ornithological Applications, survey data from citizen science projects were used in an integrated population model to assess what drives the decline of the Mallard breeding population in the Netherlands. The newly collected data on duckling survival proved decisive in explaining the negative population trend.
Text by Erik Kleyheeg, Sovon
Photo by Claus Lind Christensen, Waterfowlers' Network
The Netherlands hosts the highest densities of Mallard in Europe, but its breeding population has shown a steady decline, which accelerated in the early 2000s. In search of causes of the decline, a previous study in 2015 collated demographic data based on breeding bird monitoring, ringing and nest surveys, but was unable to pinpoint the driving factors. Whilst hypothesized as a likely relevant factor, duckling survival was identified as a major knowledge gap. Hence, in 2016 the general public was mobilized in a new citizen science project to report Mallard ducklings, enabling researchers to estimate duckling survival across the country.
High adult survival, low duckling survival
Based on repeated observations of over a thousand Mallard broods, the survival rates of ducklings were found to vary annually between 16 and 27% in recent years, which is low compared to historical observations but comparable to rates reported for other regions with declining populations. By contrast, ring recovery data revealed relatively high and stable adult survival in the Netherlands (mean 71%).
To further parameterize the population model, all other relevant vital rates were also quantified for the Dutch Mallard population. Mean nest success was estimated at 38% with high egg hatch rates (96%) but relatively small clutch sizes (8.2 eggs on average). Most of these data were generated by meadow bird nest surveys in agricultural grasslands, where volunteers also regularly stumble upon Mallard nests.
Integrated population model
To reveal the relative contribution of all vital rates to the negative population trend of the Mallard, an integrated population model was developed. This type of models enable joint analysis of all different survey data for a holistic estimation of vital rates, and is often used specifically to deal with limited and interdependent data. The model revealed that the population growth rate of the Mallard in the Netherlands was driven primarily by variation in duckling survival rates, strongly suggesting that low duckling survival has caused the recent decline.
This study is a good example of how a combination of advanced statistical methods and citizen science data, both coming with their own flaws and difficulties, can provide useful insights in population dynamics and contribute to species conservation. New projects will now focus on improving the reproductive success and further investigating the causes of the low duckling survival.
Centre for Avian Population Studies
The recently published Mallard population study was carried out by Sovon, Radboud University and the Dutch ringing centre. Together with other partners, these organisations collaborate under the Centre for Avian Population Studies (CAPS). Other recent studies by CAPS have focussed on a variety of species, including Bewick’s Swan, European Turtle Dove and Eurasian Oystercatcher. Publications can be found on the CAPS website: www.avianpopulationstudies.org.
The article referred to in this article can be found here: Integrated population modeling identifies low duckling survival as a key driver of decline in a European population of the Mallard.