PATTERN AND PROCESS IN COMPETITION
63
numerous examples among competing species of vertebrates. Additional
evidence is found in the fact that character displacement in the direction
of greater difference in body size is pronounced in zones of overlap
between closely related and presumably competing species (Brown
and Wilson, 1956; Hutchinson, 1959). We may conclude that larger
animals tend generally to be more aggressive and more successful in
competition, although N. Tinbergen has pointed out in personal conversation that the universal tendency of animals to avoid aggressive
encounters may be as important, or more so, in maintaining dispersion
of individuals and species (see also Ripley, 1961).
Klopfer (1962) also emphasizes that smaller size may have adaptive
value in allowing the subordinate animd to escape the effects of competition and survive, as suggested by Kennerly (1959) to account for
size difference between Geomys bursarius and G . personatus. The smaller
titmouse, Parus caeruleus, can feed from the extremities of twigs that
cannot be reached by its larger competitor, P. major (Snow, 1949), and
(Klopfer, 1962) the shorter-billed downy woodpecker (D. pubescens)
can deal with smaller branches than the more heavily-weaponed hairy
woodpecker (D. villosus). In fact, in the examples given for species
pairs in included niches there was generally an inverse relation between
body size and niche size, suggesting that (1) larger body size may confer
competitive advantage but (2) animals with smaller body size are
adapted to a wider range of ecological conditions and are better able
to survive outside preferred habitats, often on smaller food particles.
Miller (1964a) postulated a correlation between body size, Competitive
ability or aggression, and territory size, assuming that a larger body.
size would enable an animal to sustain its aggressive drive or to display
over a greater area, thereby restricting the movements of its smallei
competitors to smaller and perhaps less favorable areas. Unfortunately,
territory is highly variable and difficult to measure. Territory size
depends on topography and local habitat conditions (Beer et al., 1966),
and accurate data relating territory to competitive ability are seldom
available. What little information does exist shows that in some cases
the animal with the larger territory is the superior competitor, while
in others the opposite relationship exists. The crimson-crowned bishop
(Euplectes hordeacea) has a relatively large territory which is not much
affected by the abundance of breeding males, whereas its cogener the
Zanzibar bishop (E. nigroventris) has a smaller territory which is highly
compressible, according to population density (Moreau and Moreau,
1938). E. nigroventris occupies less favorable habitats and its smallest
territories are often those with the most obvious digadvantages of lack of
food and orowding by other species. Colonies of the tri-colored blackbird
(Agehius tricolor) do not have territories (Orians, 1961) and are also
63
numerous examples among competing species of vertebrates. Additional
evidence is found in the fact that character displacement in the direction
of greater difference in body size is pronounced in zones of overlap
between closely related and presumably competing species (Brown
and Wilson, 1956; Hutchinson, 1959). We may conclude that larger
animals tend generally to be more aggressive and more successful in
competition, although N. Tinbergen has pointed out in personal conversation that the universal tendency of animals to avoid aggressive
encounters may be as important, or more so, in maintaining dispersion
of individuals and species (see also Ripley, 1961).
Klopfer (1962) also emphasizes that smaller size may have adaptive
value in allowing the subordinate animd to escape the effects of competition and survive, as suggested by Kennerly (1959) to account for
size difference between Geomys bursarius and G . personatus. The smaller
titmouse, Parus caeruleus, can feed from the extremities of twigs that
cannot be reached by its larger competitor, P. major (Snow, 1949), and
(Klopfer, 1962) the shorter-billed downy woodpecker (D. pubescens)
can deal with smaller branches than the more heavily-weaponed hairy
woodpecker (D. villosus). In fact, in the examples given for species
pairs in included niches there was generally an inverse relation between
body size and niche size, suggesting that (1) larger body size may confer
competitive advantage but (2) animals with smaller body size are
adapted to a wider range of ecological conditions and are better able
to survive outside preferred habitats, often on smaller food particles.
Miller (1964a) postulated a correlation between body size, Competitive
ability or aggression, and territory size, assuming that a larger body.
size would enable an animal to sustain its aggressive drive or to display
over a greater area, thereby restricting the movements of its smallei
competitors to smaller and perhaps less favorable areas. Unfortunately,
territory is highly variable and difficult to measure. Territory size
depends on topography and local habitat conditions (Beer et al., 1966),
and accurate data relating territory to competitive ability are seldom
available. What little information does exist shows that in some cases
the animal with the larger territory is the superior competitor, while
in others the opposite relationship exists. The crimson-crowned bishop
(Euplectes hordeacea) has a relatively large territory which is not much
affected by the abundance of breeding males, whereas its cogener the
Zanzibar bishop (E. nigroventris) has a smaller territory which is highly
compressible, according to population density (Moreau and Moreau,
1938). E. nigroventris occupies less favorable habitats and its smallest
territories are often those with the most obvious digadvantages of lack of
food and orowding by other species. Colonies of the tri-colored blackbird
(Agehius tricolor) do not have territories (Orians, 1961) and are also