This was an observation that would warm the cockles of the heart of any bird behaviourist: two male Common Mergansers were competing in their harassment of a female — and\u00a0doing so while all were planing.\u00a0<\/p>\n
About the harassment: This is the breeding season and the\u00a0males chased the female\u00a0back and forth across\u00a0the water before mating. The mating took place\u00a0behind a dock, but subsequent behaviour made it clear what had happened. I will let the pictures speak for themselves on the issue of harassment, and concentrate on the feat of planing.<\/p>\n
When moving across a water surface, most birds, mammals, and boats are in displacement mode: they are supported primarily by buoyancy. Their speed is effectively limited by (what is known as) their hull speed<\/a>. Some birds have the power to temporarily plane, just as can some recreational boats. (Among adult birds, those with the power to plane seem to be mainly divers.)<\/p>\n When planing, a\u00a0bird or boat is supported primarily by the rush of the water against its\u00a0tipped-up body. For a boat to plane, there also needs to be a sharp transition between the bottom of the hull and the transom to force a separation of the water flowing underneath. A bird has a rounded butt, which normally would prevent planing. However, when a bird wants to plane, it changes its shape by forcing its tail down into the water to create the necessary sharp transition.\u00a0<\/p>\n Male mergansers harass\u00a0a female while all plane. Bodies are tipped up and tails are pressed down. Planing is hard work. Sometimes when a bird tires, wings are used for supplemental power. No, the title does not contain\u00a0a spelling mistake. I really do mean plane, not plain.\u00a0 This was an observation that would warm the cockles of the heart of any bird behaviourist: two male Common Mergansers were competing in their … Continue reading
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