When
flocks of a million or more starlings and blackbirds make
pests of themselves by spending the night close to human
habitation, the phenomenon of communal roosting comes to
public attention. These gigantic swarms, however, hardly
approach the record for communally roosting birds. The
Red-billed Quelea (a weaver, related to the House Sparrow)
is so abundant that it may be considered the avian
equivalent of a locust plague when invading African grain
fields en masse. Quelea roosts may number tens of millions
of individuals; poisons, explosives, and even flame-throwers
have been used in attempts to control local outbreaks. The
record-holding communal rooster, however, was North
American; the now-extinct Passenger Pigeon roosted (and
nested) in gigantic colonies containing billions of
individuals and covering square miles. All birds roost -- that is,
have a period of inactivity analogous to sleep in human
beings. Some birds do it alone; others with mobs of
compatriots. Some change their roosting habits with the
season: male Red-winged Blackbirds usually roost alone on
their territories when breeding, but crowd together at night
during the rest of the year. Birds that roost communally do
so in a wide variety of situations. Small groups of
nuthatches or creepers spend the night together in tree
cavities. Some vultures roost on cliffs, and others on the
tops of cacti; many seabirds roost on islands, and swallows
may roost on telephone lines. Starlings choose an enormous
diversity of roost sites -- many kinds of woodlands,
cattails and other reeds, and numerous kinds of buildings,
to name a few. The question of why some
birds roost communally and others roost solitarily is
related to the question of why there are both communal and
solitary nesters. One possibility is that older, more
experienced birds are better able to find food; hence
younger birds roost with them in order to follow their
elders to better foraging grounds. The older birds accept
this social parasitism because they tend to be dominant, and
are able to appropriate more central and therefore safer
positions in the roosting crowd. As long as the costs of
increased competition are outweighed by the benefits of
increased safety from predators for the older birds, and the
benefits of locating rich food supplies for the young
outweigh reduced nighttime safety for them, roosting should
be communal. In fact, some studies have reported that older
Red-winged Blackbirds and Brown-headed Cowbirds are
concentrated in the centers of their roosts. In a Mexican mixed-species
roost of egrets, herons, and other species, Snowy Egrets and
Great Egrets displaced other species from the higher (and
presumably safer) positions in the trees; other species have
been found to get more food if they forage near Snowy
Egrets, and other birds tend to be attracted more to dummies
of Snowy Egrets placed in foraging sites than to dummies of
other species. No studies have yet been done to determine
whether the egrets are actually followed to foraging sites
from the mixed-species roosts, however. These observations,
and those of the blackbirds and cowbirds, are consistent
with the notion that older and younger birds (or species
with divergent capacities to locate food) join communal
roosts for different reasons. Further support for this
notion comes from observations of swallows in Denmark. Older
birds were more successful in finding food than younger
ones, and also displaced the youngsters from the safest
roosting positions. Evidence was also found that the young
swallows were somehow able to evaluate the feeding success
of adults and preferentially follow the well-fed ones when
they left the roost the next day. One further advantage can
accrue to birds that roost together at night -- they may be
able to huddle together to keep warm. For nuthatches that
jam together in cavities this might be the main advantage,
as they significantly reduce their heat loss. Even in large
roosts in which huddling does not occur, more central
positions may often be thermally advantageous as a result of
denser vegetation (relative to the periphery of the roost)
and a greater mass of bird bodies per unit area. Both of
these factors may act to reduce the loss of heat from
individual birds, primarily by reducing the cooling effects
of wind. That reduction, however, would rarely be enough to
compensate for the energy lost flying the extra distance to
the roosting site. In addition, there is some evidence that
birds in the lower positions in colonial roosts lose heat
because the rain of droppings from higher birds reduces the
insulating properties of their plumage. It thus seems
unlikely that thermoregulation is a prime reason for
communal roosting in most species. SEE: Coloniality;
Mixed-Species
Flocking;
Temperature
Regulation and Behavior;
Commensal
Feeding;
Flock
Defense. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.