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It's nests (i.e, predation), not competition!

At least this is the conclusion of this very important new book:

Bennett, P. M., and I. P. F. Owens. 2002 Evolutionary ecology of
birds: life histories, mating systems, and extinction. Oxford University

This book is claimed and as far as I know, accepted as the
successor/update to David Lack's seminal works on avian life history.  It
breaks with Lack's "food limitation" hypothesis for explaining the
evolution of life history traits in birds, and instead finds that nesting
type--strategies that were a part of the initial ancient radiations--are
the best predictor of these traits.  This analysis is derived from their
own statistical studies, and of reviews of recent research literature
(particularly, T. E. Martin).  Here is the key sentence in the chapter
entitled, _Ecological basis of life-history diversity_: "Thus, both
Martin and ourselves identify variation in nesting-habit, and the
associated variation in pre-fledging mortality, as the key variables in
explaining differences in life-history strategies among birds." 

If I may translate it: Imperatives of nest attendance are the primary
selective force on a wide range of traits: body size, egg size, age of
first breeding, clutch size, altriciality/precociality, number of broods
per year, developmental rates, and incubation period to name the most