Mariana Mallard - Ecology

Ecology

The Mariana Mallard inhabited wetlands, mostly inland but occasionally also in coastal areas. On Guam, it was most abundant in the Talofofo River valley, on Tinian on Lake Hagoi and Lake Makpo (now Makpo Swamp) before it was drained, and on Saipan on the Garpan lagoon and on and around Lake Susupe. The birds were rather reclusive, preferring sheltered habitat with plenty of wetland/water plants – fern thickets (Acrostichum aureum) and reed beds (Scirpus, Cyperus and Phragmites (australis) karka), as described in detail by Tenorio et al. (1979) and Stemmermann (1981) –, where they also nested. Usually, pairs or small flocks were encountered, but in the key habitats larger groups of dozens and rarely up to 50-60 individuals could be found. Apart from possible inter-island movement, the birds were not migratory.

Feeding and reproduction are not well documented, but cannot expected to differ significantly from its immediate relatives. The Mariana Mallard fed on aquatic invertebrates, small vertebrates and plants, and although up-ending was not observed, they probably utilized it too.

Breeding was recorded from at least January to July, with a peak in June–July at the end of the dry season. One male specimen taken in October was also in breeding condition; thus, the birds may have bred nealy year-round at least on occasion. Unfortunately, the courtship behavior, which in the strongly sexually dimorphic mallard is focused more on presentation of visual cues than in the monomorphic Pacific Black Duck (although it is generally similar in both species), was never recorded. Clutches consisted of 7–12 pale grey-green oval eggs, measuring 61.6 x 38.9 mm on average. Incubation lasted around 28 days, males took no part in it and neither in caring for the ducklings. The precocial and nidifugous young fledged when about eight weeks old and became sexually mature the following year.