David C. Paton

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.

The importance of manna, honeydew and lerp in the diets of honeyeaters

Honeyeaters mainly collected manna, honeydew or lerp off the foliage and bark of eucalypts and not insects as has been previously reported. These carbohydrates were more abundant than insects and other invertebrates on the foliage or bark, offered energy rewards similar to those from nectar and were widespread, occurring in many habitats. Manna, honeydew and lerp have chemical compositions similar to nectar and were used as substitutes by many honeyeaters. Honeyeaters shifted feeding sites and showed seasonal movements with changes in the distribution and abundance of these resources and nectar. Manna, honeydew and lerp are important in the ecology of many honeyeaters.

Effects of landscape pattern on bird species distribution in the Mt. Lofty Ranges, South Australia

We assessed how well landscape metrics at 2, 5, and 10 km scales could explain the distribution of woodland bird species in the Mount Lofty Ranges, South Australia. We considered 31 species that have isolated or partially isolated populations in the region and used the Akaike Information Criterion to select a set of candidate logistic regression models. The 2 km distance was the most appropriate scale for a plurality of the species. While the total amount of area of native vegetation around a site was the most important determining factor, the effect of landscape configuration was also important for many species. Most species responded positively to area-independent fragmentation, but the responses to mean patch isolation and mean patch shape were more variable. Considering a set of candidate models for which there is reasonable support (Akaike weights > 0.10), 12 species responded negatively to landscapes with highly linear and isolated patches. No clear patterns emerged in terms of taxonomy or functional group as to how species respond to landscape configuration. Most of the species had models with relatively good discrimination (12 species had ROC values > 0.70), indicating that landscape pattern alone can explain their distributions reasonably well. For six species there were no models that had strong weight of evidence, based on the AIC and ROC criteria. This analysis shows the utility of the Akaike Information Criterion approach to model selection in landscape ecology. Our results indicate that landscape planners in the Mount Lofty Ranges must consider the spatial configuration of vegetation.

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.

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.

Impact of Off-road Vehicles on the Nesting Success of Hooded Plovers Charadrius rubricollis in the Coorong Region of South Australia

Hooded Plovers Charadrius rubricollis breed on ocean beaches in southern Australia. Off-road vehicles have access to many South Australian beaches and may affect Hooded Plovers in two ways: (1) by interfering with breeding behaviour, and/or (2) by running over nests. In the Coorong region, Hooded Plovers usually nested on the ocean beach, above high tide mark and close to the base of the frontal dunes. Most nests were within 6 m of the base of the foredune. Measurements of the density of vehicle tracks across the beach indicated that over 20% of the beach was covered by tracks, with the maximum density of tracks occurring 6 m from the base of the foredune. Tie budgets indicated that Hooded Plovers incubated eggs for 87% of the day. Passing vehicles did not cause a reduction in nest attentiveness. The potential rates at which nests were runover was measured by deploying painted pigeon eggs in artificial nests. On average 6% of these nests were runover per day. This rate is equivalent to 81% of the nests on beaches being runover during the incubation period. Thus, the use of off-road vehicles on ocean beaches potentially reduces the reproductive output of Hooded Plovers.

Bird Responses at Inherent and Induced Edges in the Murray Mallee, South Australia. 1. Differences in Abundance and Diversity

Summary: We quantified differences in the abundance and diversity of bird species at inherent (naturally occurring) and induced (human-created) edges in the Murray Mallee, South Australia, to explore the effects of anthropogenic landscape modification. Bird species were classified into edgeresponse categories based on numerical differences in abundance between the edge and interior of habitat patches. ‘Open-country’ species (e.g. Australian Magpie and Little Raven) increased in abundance near induced edges, but were rarely recorded > 200 m into patch interiors or at inherent edges. The Australian Ringneck, Red Wattlebird, Spiny-cheeked Honeyeater, Singing Honeyeater and Whiteeared Honeyeater increased in abundance near each inherent edge and were classified as ‘edge-users’. However, their responses at induced edges varied between sites. The Yellowplumed Honeyeater, Spotted Pardalote, White-browed Babbler, Chestnut Quail-thrush and Southern Scrub-robin decreased in abundance near one or more induced edges and were classified as ‘edge-avoiders’ at these sites. The Yellow-plumed Honeyeater, Spotted Pardalote, Chestnut Quail-thrush and Southern Scrub-robin are considered mallee habitat specialists in eastern Australia. These species may be particularly affected by anthropogenic modification of mallee vegetation.

Exploitative compensation by subordinate age-sex classes of migrant rufous hummingbirds

SummaryThe three age-sex classes of rufous hummingbirds (Selasphorus rufus) that directly interact on southward migratory stopovers in our California study system differ in territorial ability and resource use. Immature males are behaviorally dominant to adult and immature females and defend the richest territories. Here, we test the hypothesis that the territorially subordinate age-sex classes compensate exploitatively for their exclusion from rich resources. Our results show that females were able to accumulate energy stores at rates comparable to males despite their subordinate territorial status. Territorial females gained body mass at the same rate and in the same pattern as males, and resumed migration at the same body masses. Moreover, during periods when birds were nonterritorial and used dispersed resources, adult and immature females maintained or gained body mass, whereas immature males lost mass. We suggest that females may be energetically compensated by (1) lower costs of flight incurred during foraging and defense, resulting from their lower wing disc loading, and (2) greater success at robbing nectar from rich male territories, resulting from duller coloration (immature females), experience (adult females), and, possibly, hormonal differences. In the future, experiments will be necessary to distinguish the various hypotheses about the mechanisms involved in compensation.

Effects of nectar volume and concentration on sugar intake rates of Australian honeyeaters (Meliphagidae)

SummarySugar intake rates of captive Australian honeyeaters (Meliphagidae) feeding at artificial flowers varied across species, and as a function of nectar volume and concentration within each species. Red Wattlebirds (Anthochaera carunculata, 110 g), achieved higher intake rates than New Holland Honeyeaters (Phylidonyris novaehollandiae, 20 g), and both achieved higher rates than Eastern Spinebills (Acanthorhynchus tenuirostris, 10 g). These results reflect differences in bill and tongue dimensions as well as in body mass. Sugar intake rates for all three species increased with volume (5–50 μl) at any given concentration (10–60% mass/mass sucrose). For a given volume, sugar intake rates peaked at intermediate concentrations: 40–50% for the two larger species, and 30–40% for the smallest species. Published studies for other nectarivores foraging at unlimited volume feeders also show optimal nectar concentrations of 30–50%. However, biophysical theory predicts optima at 20–26% for small volumes, and plants presumed to be adapted for bird-pollination often have dilute nectar (20–30%). To explore this discrepancy further, we presented New Holland Honeyeaters with a range of sucrose concentrations (10–50%) using two presentation schemes. In the first we varied concentration but kept volume constant, thus varying gross sugar reward available in each concentration. This gave maximum sugar intake rates at 50%. In the second we varied both volume and concentration so that gross sugar rewards were equal for all solutions, decoupling high concentrations and large sugar rewards. This gave optima at 20%. We argue that variation among plants in nature more closely resembles the latter, “equal sugar presentation” scheme, and therefore, that dilute nectars may indeed represent adaptations for bird pollination.

Bird Responses at Inherent and Induced Edges in the Murray Mallee, South Australia. 2. Nest Predation as an Edge Effect

We assayed nest predation as an edge effect, using artificial ground nests, at inherent (naturally occurring) and induced (human-created) edges in the Murray Mallee, South Australia. Nests were constructed at distances between 0–120 m away from habitat edges. The relative predation rate on nests generally increased close to induced edges with a significant difference (P < 0.05) recorded for two out of five experiments. Predation rate at inherent edges was similar from the edge to the interior, and was lower than that recorded at induced edges. Our results suggest that increased predator numbers, activity or efficiency at locating nests occurred close to the induced edges at our study sites.