08 December 2009

Clutch size variation within species groups

a) BIG BODIES PROPORTIONAL TO CLUTCH SIZE--In grouse, as you move from small to large species, you move from clutch sizes of 13.7 to 11.4 to 11.3 to 9.0. Similarly in hawks, you move from 4.1 to 4.1 to 3.2 to 2.4. In wrens, you move from 6.8 to 6.5 to 6.0 to 6.0 (the numbers represent average clutch sizes for various size classes of birds).

b) HOLE NESTERS HAVE MORE EGGS THAN NONHOLE NESTERS IN SAME FAMILY. In British passerines, open cup nesters average clutches of 5.1 while hole-nesters average 6.9 eggs. WHY? A cup may be too conspicuous--all eggs in 1 basket?

c) HABITAT--forest birds have smaller clutches than birds that nest in meadows. There is likely more food in meadows.

d) SOCIAL STRUCTURE-- no pattern! This is kind of surprising. One reason may be that, with no males, females can still fend for young.

Wood Thrush

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