Habitat use by breeding and molting red-breasted mergansers in the Gulf of St. Lawrence

The choice of habitat plays an important role on the population dynamics of ducks (Anatinae). However, little is known of the patterns of habitat use by nearctic Red-breasted Mergansers (Mergus serrator). From 2002 to 2006, I studied habitat use and selection by breeding and molting Red-breasted Mergansers in the Gulf of St. Lawrence, Canada, by measuring macro- and microhabitat and obtaining radio telemetry, time-activity, and stomach content information. My objectives were to examine 1) nest-site selection, 2) brood habitat selection, and 3) habitat use by flightless male mergansers. Concealment was the most important feature for nest placement by Red-breasted Mergansers on Tern Islands at Kouchibouguac National Park, New Brunswick. Over 95% of nests were in dense stands of marram grass (Ammophila breviligulata), and overhead concealment, vegetation density, and vegetation height at nests were greater than that at random sites. Microhabitat at successful nests was similar to that at unsuccessful nests. Radio-marked broods originating from Tern Islands moved large distances (>3 km) to rearing habitat and remained highly mobile throughout the rearing period (mean = 1.0 km/day). Broods selected shallow waters (<75 cm) over continental estuarine intertidal flats and in tidal streams, and avoided tidal rivers and marine habitats. Age class I broods (≤10 days post-nest exodus) selected continental estuarine intertidal, tidal stream, and saltmarsh habitats, whereas older broods often used estuarine subtidal wetlands. By the early brood-rearing period, most male Red-breasted Mergansers had migrated from Kouchibouguac National Park to molting sites. At Anticosti Island, Québec, I estimated that males were flightless for 30-33 days between late July and early September. Flightless males used clear, shallow waters (≤12 m) near shore (<850 m) in coastal bays with a sand-rock substrate and stands of submergent algae. Foraging flocks were generally restricted to intertidal and shallow subtidal areas (<4 m), whereas nonforaging flocks often used deeper subtidal sites. These observations were consistent with analyses of stomach contents, which indicated that males fed on small, inshore fish, particularly grubby (Myoxocephalus aenaeus) and sandlance (Ammodytes spp.). My thesis identifies factors influencing the distribution of breeding and molting Red-breasted Mergansers in the Gulf of St. Lawrence, and prioritizes future research that will provide additional insight into habitat selection and its implication on survival of this species.

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