F. Lynn Carpenter

Territory Area, Flower Density, and Time Budgeting in Hummingbirds: An Experimental and Theoretical Analysis

While migrating southward each summer along the Sierra Nevada Mountain Range in California, nectar-feeding rufous hummingbirds (Selasphorus rufus) establish feeding territories within isolated meadows. We investigated the effect of variable food density upon territory area and time budgeting by controlled removals and additions of flowers of Castilleja linariaefolia within the territories of immature birds. The experiments were replicated three times with generally consistent results. Within one day following a 50% decrease in flower density: (1) territory area usually about doubled, resulting in (2) the number of defended flowers remaining similar to the premanipulation value; (3) foraging time increased significantly from about 21% to about 26% of the day as the result of (4) a significant increase in foraging bout duration, with (5) bout frequency remaining unchanged; (6) daily sitting time decreased significantly from about 76% to about 71% of the day; and (7) daily defense time did not change significantly at about 3% of the day. These patterns reversed within 1 day following subsequent experimental increases in flower density. Recent evidence suggests that migrant hummingbirds are energy maximizers that gain weight as rapidly as possible while on their territories. By modifying a simple model of feeding-territory area for such foragers (Hixon 1980) to incorporate previously described aspects of hummingbird foraging, a priori predictions of changes in territory area and time budgeting were generated. The direction of these predictions were independently corroborated by the experimental results. Refining a single factor of the model using our field data produced quantitative a posteriori predictions that very closely approached the experimental results in magnitude as well as direction. These findings tentatively suggest that, in response to short-term variations in food availability, migrant hummingbirds are capable of adjusting territory size and time budgeting in a manner consistent with maximizing daily net energy gain.

Territorial responses to energy manipulations in the Anna hummingbird

SummaryTerritorial activity in the Anna hummingbird (Calypte anna) was measured while energy availability on the territory was varied. On days when energy availability was unlimited, residents defended highly exclusive territories primarily by energetically expensive defense behaviors. As energy availability decreased, exclusiveness declined gradually, relative use of energetically inexpensive defense increased, and owners spent less time on the territory.Territorial behavior also varied with short term depressions in energy availability: A lower percentage of intruders was chased and departures of an owner from its territory were more frequent shortly after feeding.When resource dispersion was increased without changing substantially total rewards per territory, chasing by owners increased.

Plant‐Pollinator Interactions in Hawaii: Pollination Energetics of Metrosideros Collina (Myrtaceae)

The most abundant tree species in much of the undisturbed Hawaiian forest was the subject of a 2—yr study on plant—pollinator interactions and energetics. The purposes of the study were (1) to determine the roles of insects and of some endemic Hawaiian birds in the pollination of the tree Metrosideros collina, (2) to test the hypothesis that maximal outbreeding and seed set occur at intermediate levels of nectar availability, (3) to understand the adaptive significance of profuse flowering in this species, and (4) to determine the degree of specialization in the pollination ecology of this species. Endemic Hawaiian birds (Drepanididae) are essential for high levels of fruit—set and outbreeding in M. collina. Fruit—set was much higher in red—flowered individuals when birds were allowed to use inflorescences than when only insects used them. This is apparently caused by partial self—incompatibility, such that maximal fruit—set occurs only with outbreeding, the primary agents of which are the birds. The pred...

Bird hematocrits: effects of high altitude and strength of flight.

Abstract 1. 1. Hematocrits of three bird species living at high altitudes were measured and compared to hematocrits found in the literature for eleven other bird species. 2. 2. There was no statistical difference between the hematocrits of highland and lowland natives but the difference between the hematocrits of strong and weak fliers is significant. 3. 3. Birds and mammals appear to respond similarly both to high oxygen demand and to high altitude.

A New Function for Torpor: Fat Conservation in a Wild Migrant Hummingbird

Laboratory studies of torpor in small endotherms suggest hat body temperature is lowered periodically only when the animal is energetically stressed. Almost no data exist on the use and importance of torpor outside laboratory situations. We have monitored the daily energy state of hummingbirds in the field. A rare observation of a torpid individual whose energy state and ecological situation were well-documented showed that migrant hummingbirds may use torpor when they are very fat and not presently energetically stressed. In this case, torpor may be a mechanism to conserve the energy stored for later use on migration.

Biphasic Mass Gain in Migrant Hummingbirds: Body Composition Changes, Torpor, and Ecological Significance

Body mass of migrant Rufous Hummingbirds (Selasphorus rufus) on refueling stopovers increased on average from 3.2 to 4.6 g over a period ranging from several days to 3 wk. In birds arriving with body masses below °3.5 g, the initial period of mass gain was very slow. This slow gain was not explained by energy costs associated with territory establishment or learning to secure food, since it occurred even in years when nectar resources were superabundant and territoriality was nearly nonexistent. Data on body composition indicate that mass gain up to °3.5 g was due to deposition of nonlipid body components, which we hypothesize to be proteins involved in rebuilding muscle catabolized during the last stage of the recent migratory flight. Following the initial phase of slow mass gain, an accelerating rise in body mass consisted entirely of lipid gain. On average, overnight mass loss decreased prior to migration, suggesting that nocturnal torpor facilitated lipid deposition. The slow phase of mass gain is potentially important constraint on migrating hummingbirds, because if they deplete their fat stores and allow their body mass to fall 3.5 g, they incur a substantial cost in terms of greatly increased time spent on the subsequent stopover.

Intercropping legume trees with native timber trees rapidly restores cover to eroded tropical pasture without fertilization

As tropical deforestation progresses, increasing areas of land are being degraded through erosion, overgrazing and other processes, leading subsequently to soil infertility and loss of agricultural productivity. Reforestation is a potential way to rehabilitate some of these lands. We planted a commercially valuable native timber species on a degraded pasture in southwestern Costa Rica to test methods of forest establishment that would enhance tree growth without inorganic fertilizer and provide diverse economic and ecological benefits. In addition to an unmanipulated control and fertilized plots, we mixed the timber tree (Terminalia amazonia) with two species of nitrogen-fixing trees, with cover crops, or with edible beans. In the beans treatment, height growth of T. amazonia at the end of 4 years was significantly lower than in any of the other treatments, and bean production was poor. In the first year fertilized trees grew significantly faster than in all the other treatments. At 2 years, fertilized trees were still significantly taller than in six of seven other treatments. The cover crops did not establish and one of the two legume trees suffered high mortality. Fastest height growth occurred when T. amazonia was interplanted with legume trees, especially Inga edulis. Plots with I. edulis interplanted with T. amazonia closed canopy first, saved effort in hand-weeding, and provided large amounts of litter biomass as well as edible fruit pods. Block effects were large and were probably related to unmanipulated factors with which growth in plots correlated, in particular, degree of erosion and plot topography. Our results showed that fertilizer is wasted investment on this site because T. amazonia grows well with minimal input even on eroded land. Importantly, timber trees in species-diverse plantations can perform at least as well as in monocultures. The more diverse systems provide ecological benefits including nitrogen fixation and multiple products for humans.

Interference asymmetries among age-sex classes of rufous hummingbirds during migratory stopovers

SummaryThree age-sex classes of rufous hummingbirds (Selasphorus rufus) overlap temporally and defend feeding territories during migratory stopovers in the Sierra Nevada of California. We demonstrate that these classes differ in their ability to secure and maintain high-quality feeding territories for refueling, and that these differences result in differences in resource use. Data on acquisition of territories, territory characteristics, and responses of territory owners to intruders suggest that several mechanisms are involved in determining dominance, involving sex- and age-related differences in wing disc loading, coloration, and experience. We discuss the implications of these results for understanding intraspecific variation in migration strategies.

Torpor in an Andean Hummingbird: Its Ecological Significance

Field studies on an Andean hummingbird showed that nocturnal torpor occurs more frequently and lasts longer in the winter. Energy depletion does not seem to cause this yearly torpor cycle, and a photoperiodically controlled rhythm that enables the birds to automatically conserve energy in early evening for possible metabolic expenditures required later in the winter night is suggested.

Hooks for mammal pollination

Two species of Banksia (family Proteaceae) studied in Australia were shown to be pollinated by small, non-flying mammals rather than by birds as previously thought, and to possess several adaptations appropriate for mammal-rather than bird-pollination: odor, troughs that channel excess nectar to the ground for attraction, open inflorescence structure for nectar accessibility, hooked wiry styles for effective pollen transfer, crepuscular and nocturnal nectar and pollen presentation, and copious nectar. This apparently is the first documentation with quantified data of pollination by non-flying mammals, although many other probable examples exist.