Why is the population of breeding Mallards in the Netherlands declining? Based on an analysis of four different datasets collected by citizen scientists, researchers from Sovon Vogelonderzoek, Radboud University and the Vogeltrekstation have come to the conclusion that a high duckling mortality is the main cause. If we want to protect the Mallard, we will have to do something about the survival of the ducklings, they emphasize in a publication in Ornithological Applications.

With an estimated number of breeding pairs between 180,000 and 280,000, the Netherlands is home to the largest population of Mallards in Europe. Yet the numbers have declined by about 30 percent since 1990, a development that is not visible in our neighboring countries. The question is what the background of this decrease is: is the mortality rate among adult ducks too high, are too few young being born, or is there something else going on?

To answer these questions, researchers from Sovon Vogelonderzoek, Radboud University and the Vogeltrekstation have developed a so-called integrated population model. In this mathematical method all variables are included that can play a role in the development of the population, such as numbers of breeding birds, clutch size, hatching success of the eggs and mortality of young and adult birds.

### Population model

For most variables in the population model, data is available over many years. The dynamics in numbers are retrieved from breeding bird monitoring schemes, clutch size and nest success are determined by nest research and survival of adult birds is derived from band-recovery data. To gain insight into the survival of the ducklings, volunteers did counts with the ‘kuikteller’ over a number of years. This mobile application allows them to follow duck families and keep track of how many ducklings are fledging.

All collected data are interdependent. For example, if the population decreases, there are fewer females laying eggs. If fewer eggs hatch, there will be fewer ducklings, and so on. The strength of these mutual relationships can be summed up in formulas. All these formulas together yield an integrated population model. The calculated relationships between the variables also make it possible to predict what will happen if a variable changes.

### Duckling survival too low

Duckling survival in particular appears to have declined in recent decades. Based on historical data, it seems that this survival has even halved compared to the 1950s. The Dutch survival rate is also low in comparison with neighboring countries where the populations of Mallards are stable. Calculations with the integrated population model confirm that duckling survival is a determining factor for the development of the Mallard population in the Netherlands. None of the other variables could explain the decline in numbers.