Research Spotlight: New Study Explores Nocturnal Nest Breaks and Predation Events for Dabbling Ducks

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Ducks must incubate eggs for about 24-26 days in order to successfully hatch ducklings.  Mallard and gadwall hens typically take periodic breaks from incubating eggs (nest breaks) in the early morning and late afternoon to feed and take care of their own physiological needs. Nest breaks can also take place at night, but little has been known about these nocturnal nest breaks until recently. In addition to these voluntary nest breaks, hens may also leave their nests at night when approached by a predator such as a raccoon or skunk. The new study describes the frequency, timing, and duration of nocturnal nest breaks for dabbling ducks and explores how voluntary nest breaks at night differ from predation events.

USGS scientists placed temperature dataloggers inside duck nests amongst the eggs to identify nest breaks based on the drop in nest temperature when the hen left the nest. Small video cameras were also placed at a subset of nests, helping the researchers to determine when nest breaks were initiated by the hen voluntarily or driven by the presence of a predator. Duck hens typically cover their eggs with down feathers when they leave voluntarily for a nest break but leave their eggs uncovered when they are flushed off the nest by a predator.

Together, these tools revealed that mallard and gadwall hens nesting in Suisun Marsh initiated nocturnal nest breaks regularly (14% of all breaks were at night). Most nocturnal nest breaks (75%) were initiated by the hen (eggs were covered before leaving for a nest break) rather than being flushed off the nest by a predator (eggs left uncovered).

USGS scientists observed that when predators appeared at nests on video, hens left their eggs uncovered 94% of the time. Nest temperature decreased more rapidly when eggs were left uncovered than when the eggs were covered with down feathers and other nesting material before the hen went on her nest break. When hens flushed in response to a predator, more than half of the nests (56%) had evidence of egg depredation at the subsequent nest monitoring visit. Timing also differed between the two types of breaks: hens stayed away from the nest for longer than when they left voluntarily, and predator-initiated nest breaks were more evenly distributed throughout the night whereas the nest breaks initiated by the hen occurred closer to dawn. The results provide insight into nocturnal duck behavior and nest predation, which play an important role in nest survival, and will inform waterfowl management for Suisun Marsh and the Pacific Flyway.

Management Implications

• Duck hens regularly leave their nests at night (14% of nest breaks) but are often flushed out of the nest by predators
• When predators flush a hen from the nest, hens leave eggs uncovered and stay away from their nests for longer than they do when they leave voluntarily, putting eggs at risk 
• Predators can negatively impact nesting ducks even if the predators do not eat all the eggs in the nest

This research spotlight refers to the following paper and data release:

Croston, Rebecca, Peterson, S.H., Hartman, C.A., Herzog, M.P., Feldheim, C.L., Casazza, M.L., Ackerman, J.T (2021). Nocturnal incubation recess and flushing behavior by duck hens. Ecology and Evolution, 00:1– 10. https://doi.org/10.1002/ece3.7561

Peterson, S.E., Croston, R.L., and Ackerman, J.T., 2021, Nocturnal incubation recess and flushing behavior by duck hens nesting in Grizzly Island Wildlife Area 2015-2018: U.S. Geological Survey data release, https://doi.org/10.5066/P9XG4KSK.

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