244
M.F. Willson et al.
and correlations of clutch sizes and number of broods per year with
nesting habits are not clear (data from Anonymous, 1985; Falla, Sibson,
& Turbott 1981; but see Yorn-Tov , Christie, & Iglesias, 1994). .Regional
comparisons offer the opportunity to test the ecological hypotheses about
the evolution of clutch size on a larger scale.
Some of the Chilean species may have small clutches for historical or
phylogenetic reasons (as opposed to ecological reasons). Some belong to
taxa -that characteristically produce small clutches everywhere (e.g.,
pigeons and hummingbirds). Clutch size of others might be constrained if
they were geologically recent immigrants to southern forests from the
lowland tropics, where small clutches are common. Such an hypothesis
leaves unexplained the relatively large clutch sizes of several Chilean
species, unless they immigrated from other source faunas or did so earlier.
The high level of endemism in the Chilean avifauna (VuiIleumier, 1985)
suggests.that there has been time for the evolution of significant morphological and behavioral differences in many taxa, and there is no apparent
reason why clutch size should be more constrained than morphology and
behavior.
The avifaunas of coastal rainforests in Chiloe and southeast Alaska
differed somewhat in guild structure.. Guild structure of Chilote forest
avifaunas was not more similar to Alaskan coastal forest than to Canadian
forest avifaunas in the same general region. Long-distance migrants are
more often among the common birds in Alaska (and to a slightly lesser
degree in Washington and Oregon; Carey, Hardt, Horton, & Biswell,
1991; Gilbert & .Allwine, 1991; Manuwal, 1991) than in Chiloe and
comprise a much larger proportion of the avifauna. Thus, biogeographical
differences and biological interactions (or other factors) appear to override
the climatic and topographic similarities in structuring the bird community
in these forests.
Guild Structure of North and South Temperate Forests
Broad regional comparisons documented the high richness of covered
nesters in south temperate forests. South temperate forests appeared to
have relatively few species and birds that forage principally by leaf gleaning, and northwestern forests supported relatively few species and individuals of aerial feeders . Big-tree users were proportionately more
speciose .and often more abundant in broadleaf forests than in conifer
forests,and covered nesters were more diverse in broadleaf forests.
Aerial feeders were proportionately more diverse and abundant in broadleaf forests. .In contrast,leaf-gleaners were relatively more diverse and
abundant in conifer forests, and understory birds were relatively more
abundant there.
We did not analyze another interesting guild-that of frugivores that
disperse the seeds of fleshy fruits. Fleshy fruits and frugivory are common
in both Chiloe and Alaska in late summer (our observations; Armesto,
M.F. Willson et al.
and correlations of clutch sizes and number of broods per year with
nesting habits are not clear (data from Anonymous, 1985; Falla, Sibson,
& Turbott 1981; but see Yorn-Tov , Christie, & Iglesias, 1994). .Regional
comparisons offer the opportunity to test the ecological hypotheses about
the evolution of clutch size on a larger scale.
Some of the Chilean species may have small clutches for historical or
phylogenetic reasons (as opposed to ecological reasons). Some belong to
taxa -that characteristically produce small clutches everywhere (e.g.,
pigeons and hummingbirds). Clutch size of others might be constrained if
they were geologically recent immigrants to southern forests from the
lowland tropics, where small clutches are common. Such an hypothesis
leaves unexplained the relatively large clutch sizes of several Chilean
species, unless they immigrated from other source faunas or did so earlier.
The high level of endemism in the Chilean avifauna (VuiIleumier, 1985)
suggests.that there has been time for the evolution of significant morphological and behavioral differences in many taxa, and there is no apparent
reason why clutch size should be more constrained than morphology and
behavior.
The avifaunas of coastal rainforests in Chiloe and southeast Alaska
differed somewhat in guild structure.. Guild structure of Chilote forest
avifaunas was not more similar to Alaskan coastal forest than to Canadian
forest avifaunas in the same general region. Long-distance migrants are
more often among the common birds in Alaska (and to a slightly lesser
degree in Washington and Oregon; Carey, Hardt, Horton, & Biswell,
1991; Gilbert & .Allwine, 1991; Manuwal, 1991) than in Chiloe and
comprise a much larger proportion of the avifauna. Thus, biogeographical
differences and biological interactions (or other factors) appear to override
the climatic and topographic similarities in structuring the bird community
in these forests.
Guild Structure of North and South Temperate Forests
Broad regional comparisons documented the high richness of covered
nesters in south temperate forests. South temperate forests appeared to
have relatively few species and birds that forage principally by leaf gleaning, and northwestern forests supported relatively few species and individuals of aerial feeders . Big-tree users were proportionately more
speciose .and often more abundant in broadleaf forests than in conifer
forests,and covered nesters were more diverse in broadleaf forests.
Aerial feeders were proportionately more diverse and abundant in broadleaf forests. .In contrast,leaf-gleaners were relatively more diverse and
abundant in conifer forests, and understory birds were relatively more
abundant there.
We did not analyze another interesting guild-that of frugivores that
disperse the seeds of fleshy fruits. Fleshy fruits and frugivory are common
in both Chiloe and Alaska in late summer (our observations; Armesto,