Saddleback Journal of Biology - Saddleback College

Spring 2010 Biology 3B Paper
Panagou, E. Z., Skandamis, P. N., & Nychas, G. J.
(2005). Use of gradient plates to study combined
effects of temperature, pH, and NaCl concentration
on growth of Monascus ruber van Tieghem, an
Ascomycetes fungus isolated from green table olives.
Applied Environment Microbiology, 71, 392-3
Effect of Artificial Concentrated Feeding Area Resource Depression on the Territoriality of
the Anna’s Hummingbird (Calypte anna)
Linda Mahoney and Kathleen Kuechler
Department of Biological Sciences
Saddleback College, Mission Viejo, Ca 92692
Territorial behaviors such as chases and gorget displays are often used by Anna’s hummingbirds to defend their
feeding territory. The intensity of such a display is determined by the quantity of food resources in a territory,
which in turn dictates the amount of energy that can be expended to defend the territory from intruders. This
study compared the frequency of high-intensity territorial displays when resource availability was either
abundant or depressed in a resident population of Anna’s hummingbirds whose main food supply was a spatially
concentrated locale of artificial feeders. It was hypothesized that the hummingbirds would exhibit a greater
frequency of high-intensity territorial displays when they received abundant resources, as opposed to resource
depression, due to increased energy uptake, allowing them to exert more energy defending their feeding territory.
It was found that Anna’s hummingbirds exhibited high-intensity territoriality at a frequency of
0.13270.0129(s.e.m) when receiving high-resource food and 0.11420.02893(s.e.m) when receiving lowresource
food. There was no statistically significant difference in the frequency of high-intensity territoriality
under the two conditions (p=0.2883,one-tailed t-test). These results were most likely attributed to elements of
Carpenter and MacMillan’s 1976 Threshold model as well as Myers et al argument of competition density.
Introduction
One consequence of bird evolution is the development
of a social organization structure that uses territoriality
as one of the primary mechanisms for interspecies and
intraspecies interactions (Brown, 1969). These
interactive dynamics determine individual fitness, with
the crux of an individual’s fitness resting on the
regulation of its energy budget (Carpenter et al.,
1989). In order to achieve maximum fitness level,
individuals must balance their expenditure of energy
with their ability to acquire energy. Territorializing
areas with sufficient amounts of nourishment ensures
that individuals obtain the energy they need to
maximize fitness. However, these behaviors most
often occur when the fitness benefits outweigh the
energy costs (Brown, 1964).
Male hummingbirds vigorously territorialize
high-resource feeding areas with the expectation that a
female will enter their territory seeking a stable
nesting site (Sibley, 2001). Female hummingbirds
additionally exhibit feeding territorial behaviors, but
primarily in the defense of resource obtainment in
their nesting site. (Sibley, 2001) Territoriality displays
can either be an energetically low-cost or high-cost
expenditure, whereby the hummingbird exerts either a
minimal or maximal amount of energy to perform its
intended behavior. According to Brown’s 1969 and
Ewald and Carpenter’s 1978 studies, territorial
exhibits such as attacking or long chases are
considered high energy-cost expenditures as the
defending bird literally chases an invading bird away
from its territory. Short chases, or those in which the
defender need not exit its territory before successfully
driving an intruder away, threats or gorget displays
and vocalizations are all considered low-cost
expenditures.
In order to ensure enough energy resources
are available to meet their energy needs, individuals
defend abundant food sources with a greater frequency
of high-cost displays than areas where resource
availability has been depressed (Eberhard and Ewald,
1994; Ewald and Orians, 1983; Carpenter, 1987). As
the quantity of food sources decreases, hummingbirds
invest less energy into their territoriality displays
because an energetic constraint has been imposed
(Ewald and Orians, 1983; Ewald and Carpenter, 1978).
According to previous studies, whether a
hummingbird will use high-cost or low-cost
territoriality behaviors is predicated upon the
availability of abundant food sources (Ewald and
Carpenter, 1978; Powers 1987; Ewald and Orians,
1983).
The studies previously discussed focus on
hummingbirds obtaining nourishment from both
natural and artificial sources, such as bird feeders, in
which resources are ostensibly scattered in a random
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Saddleback Journal of Biology
Spring 2010