56
RICHARD S. MILLER
as they are in the Palearctic.” These observations seem to suggest, in
other words, that competitive exclusion may still be an active ingredient
in the more recent evolutionary events affecting this genus in North
America, and that P. atricapillus, with its broader geographic range
and presumably greater tolerance to habitat variation, is only able to
survive outside the included niches of its competitors. Thus, Snow
seems to feel that the habitat differences that are observed among
members of this genus in Europe, such as between the coal tit (Parus
ater) which searches for insects on the trunks and larger branches of
trees and the blue tit (P. caeruleus) which concentrates on twigs and
leaves (Snow, 1949), are due to ecological differentiation and no longer
involve competitive displacement.
A more precise study of the relationships between two species of this
genus in North America has been made by Dixon (1954). In the past
two decades Parus rufescens has become established as a breeding bird
in the district immediately east of San Francisco, but its spread is
impeded by competition with its larger cogener, Parus inornatus. In all
cases where the two species come into contact, their breeding territories
are mutually exclusive and, in the face of antagonism by the larger
form, the adjustments permitting co-occupancy of the area appear at this
stage to be made entirely by the smaller, less specialized species. These
include the use of vacated or suboptimal nesting territories, modification
of territorial behavior, and a more varied choice of food items. Thus,
the recent entry of P. rufescens into the range of its larger cogener has
been made possible by the availability of a niche space outside the more
specialized, included niche of P. inornatus.
The yellow-headed blackbird (Xanthocephlus xanthocephalus) is restricted in its nesting sites to emergent vegetation, e.g. bulrush (Scirpus),
cattail ( T y p h ) or Phragmites in fairly deep water (Nero, 1964). The
red-winged blackbird (Agelaius phoeniceus) occupies the same marshes
as yellow-headed blackbirds but nests in a greater variety of situations.
Most nests are located in cattails but redwings will also nest in low trees,
shrubs or any weeds that will support a nest (Orians, 1961). Male redwings arrive on the breeding grounds and begin to establish territories
somewhat earlier than yellow-headed blackbirds. I n the vicinity of
Saskatoon, Saskatchewan the first recorded arrival of male redwings in
spring was 7 to 20 days (average = 13 days) earlier than the first male
yellow-headed blackbird during 6 years from 1960 to 1965 (J. B. Gollop,
personal communication). I n 1964 and 1965 the author observed interspecific relationships between these two species on a small marsh near
Saskatoon. Male redwings were engaged in territorial display throughout
most of the marsh by the time the first yellow-headed blackbirds
arrived, but within a few days the yellow-headed blackbirds had
RICHARD S. MILLER
as they are in the Palearctic.” These observations seem to suggest, in
other words, that competitive exclusion may still be an active ingredient
in the more recent evolutionary events affecting this genus in North
America, and that P. atricapillus, with its broader geographic range
and presumably greater tolerance to habitat variation, is only able to
survive outside the included niches of its competitors. Thus, Snow
seems to feel that the habitat differences that are observed among
members of this genus in Europe, such as between the coal tit (Parus
ater) which searches for insects on the trunks and larger branches of
trees and the blue tit (P. caeruleus) which concentrates on twigs and
leaves (Snow, 1949), are due to ecological differentiation and no longer
involve competitive displacement.
A more precise study of the relationships between two species of this
genus in North America has been made by Dixon (1954). In the past
two decades Parus rufescens has become established as a breeding bird
in the district immediately east of San Francisco, but its spread is
impeded by competition with its larger cogener, Parus inornatus. In all
cases where the two species come into contact, their breeding territories
are mutually exclusive and, in the face of antagonism by the larger
form, the adjustments permitting co-occupancy of the area appear at this
stage to be made entirely by the smaller, less specialized species. These
include the use of vacated or suboptimal nesting territories, modification
of territorial behavior, and a more varied choice of food items. Thus,
the recent entry of P. rufescens into the range of its larger cogener has
been made possible by the availability of a niche space outside the more
specialized, included niche of P. inornatus.
The yellow-headed blackbird (Xanthocephlus xanthocephalus) is restricted in its nesting sites to emergent vegetation, e.g. bulrush (Scirpus),
cattail ( T y p h ) or Phragmites in fairly deep water (Nero, 1964). The
red-winged blackbird (Agelaius phoeniceus) occupies the same marshes
as yellow-headed blackbirds but nests in a greater variety of situations.
Most nests are located in cattails but redwings will also nest in low trees,
shrubs or any weeds that will support a nest (Orians, 1961). Male redwings arrive on the breeding grounds and begin to establish territories
somewhat earlier than yellow-headed blackbirds. I n the vicinity of
Saskatoon, Saskatchewan the first recorded arrival of male redwings in
spring was 7 to 20 days (average = 13 days) earlier than the first male
yellow-headed blackbird during 6 years from 1960 to 1965 (J. B. Gollop,
personal communication). I n 1964 and 1965 the author observed interspecific relationships between these two species on a small marsh near
Saskatoon. Male redwings were engaged in territorial display throughout
most of the marsh by the time the first yellow-headed blackbirds
arrived, but within a few days the yellow-headed blackbirds had