230
M.F. Willson et al.
(at least 10nets/site); capture rates for an additional conifer site near
Ketchikan were obtained from Canterbury. Most of the nets were 30mm
or 36mm mesh size, and 12m by 2m in net size. Mist-net data are used
here only to compare capture rates of birds in understory.
The broader comparisons , using data in the literature, included forests
with diverse species composition and a variety of stand size. South temperate data were derived from Cody (1970), Ralph (1985), and Vuilleumier (1972), with a latitudinal range of about 38°S-42° S (plus one site
at 30°S). Midwestern U.S. data for deciduous forest were obtained for
Wisconsin (Bond, 1957; using only stands >100ha, to ensure that some
large tracts were included) and Illinois (Willson, 1974; using each stand
only once, to avoid pseudoreplication), and for coniferous forest in
Minnesota and Michigan (from Willson, 1986); latitudinal range was
about 40
0 N - 4 9 ° N . In a general way, the midwestern U.S. data provide a
partial control for simple latitudinal effects such as gradients in species
diversity, but more importantly, they are a ready source of comparative
information that allowed us to attempt these broad contrasts. Data for
forests of the north and west in North America came from Spindler and
Kessel (1980; coniferous and deciduous), Flack (1976; deciduous),
Theberge (1976; coniferous and deciduous) , Kessler and Kogut (1985;
coniferous), with a latitudinal range of about 49°N-63°N. These studies
were used only for comparisons of total species present and of guild
structure among regions and forest types. Differences in census methods
among these studies mean that all comparisons must be taken as preliminary estimates.
Guild assignments for each species were based on our observations in
Alaska and Chile and on published information (e.g. , Harrison, 1975,
1979, for nest locations in North America; Holmes , Bonney, & Pacata,
1979, and Manuwal, 1991 , for foraging of some species; Flack, 1976, for
aspen forests; Johnson, 1967, for Chilean birds). Root (1967) defined a
guild as "a group of species that exploit the same class of environmental
resources in a similar way." The guilds used for our comparisons represent
broad categorizations of patterns of habitat use; fine tuning of categories
(e.g., distinguishing subcanopy from canopy foragers, or ground from
understory foragers, or insectivores from various types of omnivores in
Chile) is not yet possible. Nevertheless, our guilds capture the major
distinctions that emerged from more detailed and quantitative studies
elsewhere , in which vertical stratum and proximity to tree trunks provided
the major distinctions among guilds (Holmes, Bonney , & Pacala, 1979;
Holmes & Recher , 1986).
Although Holmes and colleagues dealt with foraging guilds, here we
include nest-sites as well, given that nesting biology has been shown to
have important consequences for community structure (Martin , 1988a,
1988b). The guilds include the following broad categories: 1) big-tree
users , nesting or foraging on trunks and large branches ; 2) users of
M.F. Willson et al.
(at least 10nets/site); capture rates for an additional conifer site near
Ketchikan were obtained from Canterbury. Most of the nets were 30mm
or 36mm mesh size, and 12m by 2m in net size. Mist-net data are used
here only to compare capture rates of birds in understory.
The broader comparisons , using data in the literature, included forests
with diverse species composition and a variety of stand size. South temperate data were derived from Cody (1970), Ralph (1985), and Vuilleumier (1972), with a latitudinal range of about 38°S-42° S (plus one site
at 30°S). Midwestern U.S. data for deciduous forest were obtained for
Wisconsin (Bond, 1957; using only stands >100ha, to ensure that some
large tracts were included) and Illinois (Willson, 1974; using each stand
only once, to avoid pseudoreplication), and for coniferous forest in
Minnesota and Michigan (from Willson, 1986); latitudinal range was
about 40
0 N - 4 9 ° N . In a general way, the midwestern U.S. data provide a
partial control for simple latitudinal effects such as gradients in species
diversity, but more importantly, they are a ready source of comparative
information that allowed us to attempt these broad contrasts. Data for
forests of the north and west in North America came from Spindler and
Kessel (1980; coniferous and deciduous), Flack (1976; deciduous),
Theberge (1976; coniferous and deciduous) , Kessler and Kogut (1985;
coniferous), with a latitudinal range of about 49°N-63°N. These studies
were used only for comparisons of total species present and of guild
structure among regions and forest types. Differences in census methods
among these studies mean that all comparisons must be taken as preliminary estimates.
Guild assignments for each species were based on our observations in
Alaska and Chile and on published information (e.g. , Harrison, 1975,
1979, for nest locations in North America; Holmes , Bonney, & Pacata,
1979, and Manuwal, 1991 , for foraging of some species; Flack, 1976, for
aspen forests; Johnson, 1967, for Chilean birds). Root (1967) defined a
guild as "a group of species that exploit the same class of environmental
resources in a similar way." The guilds used for our comparisons represent
broad categorizations of patterns of habitat use; fine tuning of categories
(e.g., distinguishing subcanopy from canopy foragers, or ground from
understory foragers, or insectivores from various types of omnivores in
Chile) is not yet possible. Nevertheless, our guilds capture the major
distinctions that emerged from more detailed and quantitative studies
elsewhere , in which vertical stratum and proximity to tree trunks provided
the major distinctions among guilds (Holmes, Bonney , & Pacala, 1979;
Holmes & Recher , 1986).
Although Holmes and colleagues dealt with foraging guilds, here we
include nest-sites as well, given that nesting biology has been shown to
have important consequences for community structure (Martin , 1988a,
1988b). The guilds include the following broad categories: 1) big-tree
users , nesting or foraging on trunks and large branches ; 2) users of