We studied the nesting habits o f Hooded Mergansers in northeastern Ontario. Twelve females exhibited vigorous nest defence from early incubation onwards; seven incubating females spent 85.3% o f their day on the nest, taking an average o f 4.7 recesses each day, each with an average duration o f 60 minutes. They lost up to 16% body mass during incubation. Our results confirm earlier studies which suggested that interspecific nest parasitism is a common feature o f Hooded Mergansers nesting biology.