11. Avian Communities in Temperate Rainforests
235
Chilote forest-bird community (or <25% if Columba araueana is included
as a long-distance migrant; we lack information on the migratory patterns
of Chilote populations of C. araueana). Several additional species in both
southeast Alaska and Chiloe make local seasonal movements.
A second major difference between North American and Chilean rainforests is found in the clutch sizes of many species of birds. Many species
of breeding birds in Chiloe usually lay clutches of two to three eggs, and
only a few regularly lay at least four eggs (Johnson, 1967). In contrast,
many North American birds commonly lay clutches of at least four eggs;
most of the data derive from studies below the 49th parallel (Ehrlich,
Dobkin, & Wheye, 1988; Harrison, 1979), and clutches at high northern
latitudes are often even larger than those at lower latitudes (Lack , 1968;
Morton, 1974). The geographic differences in clutch size are found in
birds of various taxonomic and biogeographic affiliations, habitats, and
nesting habits . Even closely related species commonly , but not invariably,
exhibit the difference in clutch size: Turdus migratorius (3-4) versus T.
falklandii (2-3); Colaptes auratus (5-8) versus C. pitius (4-6); Troglodytes aedon (6-7) and T. troglodytes (5-6) versus T. aedon (4-7);
Taehycineta bieolor (5-7) versus T. leueopyga (4-6); Zonotriehia
leueophrys (3-5) versus Z. eapensis (3-4); Pieoides pubeseens (4-5)
versus P. lignarius (3-5) . In addition, several genera not found in our
censuses also show this pattern (e.g ., Mimus , Agelaius, Sturnella, and
Cistothorus) .
Guild Structure
Big-tree users comprised equivalent proportions of the avifauna, by
species (refer to part A in Table 11.2) and by abundance of individuals
(part B, Table 11.2), in Chiloe and Alaska conifer stands, and a significantly lower proportion in Alaska deciduous stands. Species using covered
nests were most frequent in Chiloe, and least frequent in Alaska deciduous forest (part A, Table 11.2). The relative abundance of coverednesting species was lowest in Alaska deciduous forest and slightly higher
in Chiloe than Alaska coniferous forest (part B, Table 11.2). Thus,
although the diversity of covered nesters in Chiloe was much higher than
in Alaska coniferous forest, their relative abundance was less different,
reflecting the commonness of Troglodytes troglodytes in conifer forests
and the relative rarity of some Chilean covered -nesting species.
Understory birds comprised a greater proportion of the avifauna, by
both species richness (part A , Table 11.2) and abundances (part B, Table
11.2) in Alaska deciduous stands, followed by Alaska conifer stands.
Chilote forests held the fewest species and individuals of this guild (parts
A and B, Table 11.2). Leaf-gleaners were least speciose and least
abundant in Chiloe (parts A and B, Table 11.2). In contrast, aerial
feeders were most diverse and most abundant in Chiloe .
235
Chilote forest-bird community (or <25% if Columba araueana is included
as a long-distance migrant; we lack information on the migratory patterns
of Chilote populations of C. araueana). Several additional species in both
southeast Alaska and Chiloe make local seasonal movements.
A second major difference between North American and Chilean rainforests is found in the clutch sizes of many species of birds. Many species
of breeding birds in Chiloe usually lay clutches of two to three eggs, and
only a few regularly lay at least four eggs (Johnson, 1967). In contrast,
many North American birds commonly lay clutches of at least four eggs;
most of the data derive from studies below the 49th parallel (Ehrlich,
Dobkin, & Wheye, 1988; Harrison, 1979), and clutches at high northern
latitudes are often even larger than those at lower latitudes (Lack , 1968;
Morton, 1974). The geographic differences in clutch size are found in
birds of various taxonomic and biogeographic affiliations, habitats, and
nesting habits . Even closely related species commonly , but not invariably,
exhibit the difference in clutch size: Turdus migratorius (3-4) versus T.
falklandii (2-3); Colaptes auratus (5-8) versus C. pitius (4-6); Troglodytes aedon (6-7) and T. troglodytes (5-6) versus T. aedon (4-7);
Taehycineta bieolor (5-7) versus T. leueopyga (4-6); Zonotriehia
leueophrys (3-5) versus Z. eapensis (3-4); Pieoides pubeseens (4-5)
versus P. lignarius (3-5) . In addition, several genera not found in our
censuses also show this pattern (e.g ., Mimus , Agelaius, Sturnella, and
Cistothorus) .
Guild Structure
Big-tree users comprised equivalent proportions of the avifauna, by
species (refer to part A in Table 11.2) and by abundance of individuals
(part B, Table 11.2), in Chiloe and Alaska conifer stands, and a significantly lower proportion in Alaska deciduous stands. Species using covered
nests were most frequent in Chiloe, and least frequent in Alaska deciduous forest (part A, Table 11.2). The relative abundance of coverednesting species was lowest in Alaska deciduous forest and slightly higher
in Chiloe than Alaska coniferous forest (part B, Table 11.2). Thus,
although the diversity of covered nesters in Chiloe was much higher than
in Alaska coniferous forest, their relative abundance was less different,
reflecting the commonness of Troglodytes troglodytes in conifer forests
and the relative rarity of some Chilean covered -nesting species.
Understory birds comprised a greater proportion of the avifauna, by
both species richness (part A , Table 11.2) and abundances (part B, Table
11.2) in Alaska deciduous stands, followed by Alaska conifer stands.
Chilote forests held the fewest species and individuals of this guild (parts
A and B, Table 11.2). Leaf-gleaners were least speciose and least
abundant in Chiloe (parts A and B, Table 11.2). In contrast, aerial
feeders were most diverse and most abundant in Chiloe .