By Western Ecological Research Center (WERC), USGS
California’s breeding populations of dabbling ducks have declined and the factors contributing to these declines have not been identified.
However, duckling survival to fledging is a key factor influencing waterfowl population growth. After hatching, young ducklings travel from upland nest sites to wetlands that are flooded in the spring and summer. When ducklings travel from these upland vegetated fields to the brood wetlands, they are especially vulnerable to being killed by terrestrial and avian predators. A recent study by USGS scientists quantified the survival rates of mallard and gadwall ducklings in Suisun Marsh, with a particular focus on the type of predators that were eating ducklings and the factors that influenced the likelihood of a duckling surviving to when they are able to fly– about 56 days after hatching. To do so, this study radio-tagged and tracked 284 mallard and gadwall duckling broods in Suisun Marsh during the 2016–2019 breeding seasons.
This study demonstrated that duckling survival rates were relatively low in Suisun Marsh compared with other regions of the United States. Mallard duckling survival to fledging was < 3% during two drier years and < 17% during two wetter years. Gadwall survival to fledging was 9-11% and did not differ among years. Duckling survival rates were affected by the distances that ducklings had to travel between upland nesting habitats and nearby flooded wetlands, as well as the overall availability and salinity concentrations of the flooded wetland habitats. Predation accounted for 91% of duckling mortalities and was attributed to mammals (27.6%), birds (22.0%), snakes (4.4%), and unknown predators (46.0%). Notably, transmitters from radio-tagged ducklings were recovered from multiple nests of great-horned owls and red-tailed hawks that were in large, non-native trees (e.g., eucalyptus) that bordered upland nesting habitats.
Duckling survival was positively correlated with the proximity and amount of wetland habitat near the upland nest site. As an example, duckling broods that hatched ≤ 140m from flooded wetlands had a ≥ 75% chance of surviving the move from the nest to water, but duckling broods that hatched ≥ 970m from a wetland had ≤ 50% chance of surviving the initial move from the nest to a flooded wetland. Additionally, daily survival rates generally increased with the amount of flooded wetland habitat within 0.5 km (mallard) and 1.0 km (gadwall) of the nest where the duckling hatched. For ducklings that survived the initial move to wetlands, survival rates were negatively correlated with salinity concentrations in flooded wetlands. Mallard survival within the first week after hatching decreased by 9% (wetter year) to 31% (drier year) across the observed range of wetland salinities (0.5 ppt versus 12 ppt). Gadwall survival to one week of age decreased by 7% when ducklings were in wetlands with salinities of 12 ppt versus 0.5 ppt.
The findings suggest that maintaining a network of summer-flooded wetlands with lower salinity concentrations that are within 1 km of upland nesting habitats could minimize the distance that hens and ducklings travel from the nest site to reach wetlands and improve duckling survival rates and population recruitment.
Management Implications:
- Duckling survival rates were low and predators were the primary cause of duckling deaths.
- Predation rates were higher and survival rates were lower during years with less available wetland habitat.
- Greater consistency of flooded wetland habitats during the summer brood-rearing period may reduce predation pressure and improve duckling survival rates.
- Maintaining a network of low salinity wetlands within 1 km of upland nesting sites may improve duckling survival rates by reducing the distance that ducklings need to travel from their nest to nearby flooded wetland habitats.
