Saturday, July 24, 2010
Good question, Parker in New York City.
The answer is the Cape May Warbler (see female in photo, above), which usually lays six or seven eggs. Most other wood-warbler species typically lay four or five eggs in most instances. In extraordinary cases (that are seldom to rare), the Cape May deposits as many as nine eggs. In equally rare situations, merely four are laid.
Why does the Cape May lay eggs than other wood-warbler species? The answer may relate to the “boom and bust” cycle of its dominant food resource: spruce budworms (a larval form of a moth). When large population outbreaks of the spruce budworm occur, it behooves the Cape May to lay more eggs. Conversely, it makes sense that this species lays fewer eggs in years when spruce budworms are less plentiful.
Of course, this kind of phenomenon is not unique to Cape May Warbler. Many other bird species, in addition to other animals, act similarly in response to the presence or absence of food resources.
For more details about clutch sizes of North American breeding wood-warbler, see The Birder’s Handbook (Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye)
Other life cycle information about USA-breeding wood-warbler species appears in a “Warbler Tips ID Chart” at my Web site: www.warblerwatch.com (After arriving at this site’s home page, click on the button title “Warbler Tips ID Chart.”