The difference in the coloration of the Mallard ducklings is not gender related. The darker ones have the typical coloration of Mallard ducklings. Mallards are the ancestors of most, if not all, domestic ducks, and they do interbreed with them, so a wide range of coloration can show up in their offspring at any time.
After having spent a month or so incubating her eggs, the mallard hen begins to hear her ducklings vocalizing from inside their eggs, roughly 24 hours before they start to hatch. She responds with quiet calls, and begins turning the eggs frequently. Within 36 hours the ducklings crack open (“pip”) their eggs with the help of an egg tooth that is lost soon after they hatch. The down of the ducklings dries within 12 hours and often the morning after her young hatch, the hen leads them to water (not necessarily the closest water to the nest). She encourages them to follow her by quacking up to 200 times a minute as they travel over land to their watery destination. The ducklings can feed on their own, consuming mostly invertebrates and seeds. Once in the water, if the ducklings start to scatter, the mother can be heard repeatedly and softly quacking to her brood to gather them around her. She will continue to provide them with cover and warmth for the next couple of weeks, especially at night and during cold weather.
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