John P. McCarty

THE ENERGETIC COST OF BEGGING IN NESTLING PASSERINES

A critical assumption of many of the current discussions of parent-offspring conflict and sibling competition is that the begging behavior of nestling birds is costly, either in terms of energetics or increased risk of predation. I measured the energetic expenditures associated with the begging of nestling birds using closed-chamber respirometry and found this cost to be surprisingly low. Active metabolic rate (AMR) while begging was 1.05 times the resting metabolic rate (RMR) in European Starlings (Sturnus vulgaris) and 1.27 times the resting metabolic rate in Tree Swallows (Tachycineta bicolor). The cost of 1 s of begging was 0.001 J/g in European Starlings and 0.008 J/g in Tree Swallows; this cost increased with age for all nestlings. Results of measurements on five other species are consistent with these values. The ratio of AMR:RMR did not change with ambient temperature for either Tree Swallows or European Starlings, but data for all seven species pooled did show a significant decrease in energetic costs with temperature. The amount of time spent begging had a negligible effect on cost for both Tree Swallows and European Starlings. Likewise, the in- tensity of the begging display had no effect on the AMR:RMR ratio in either species alone, but was positively correlated for the seven species pooled. Compared to the energy require- ments for other avian behaviors, the cost of begging is low. Most discussions of the evolution of begging behavior in nestling birds have assumed that begging is costly. Based on my results, the assumption that begging is energetically costly needs to be reexamined and, until then, conclusions of models dependent on this assumption should be considered tentative. Received 6 September 1994, accepted 25 April 1995.

Foraging ecology and diet selectivity of Tree Swallows feeding nestlings

We studied the foraging ecology of a population of Tree Swallows (Tachycineta bicolor) breeding in New York State over a period of 5 years. While feeding nestlings, adult Tree Swallows tended to spend most of their time within sight of their nest box and less than 12 m above the ground. Major insect taxa captured include Diptera, Hemiptera, and Odonata, ranging in length from mainly 0-10 mm, with some individuals up to 60 mm. The sex of the parent delivering the food had no significant effect on diet composition. Selection for or against food categories was determined by comparing the proportion of insects of different types in the diet of Tree Swallows to the proportions available in the air column. Tree Swallows showed consistent selection for insects larger than 3 mm and against smaller insects, especially Diptera in the suborder Nematocera. Only minor differences in diet were observed among years, and the effects of the abundance of food available were generally small. The patterns of selectivity found in this population were consistent with those found in previous studies on this species carried out in other locations, and these patterns are likely the result of differences in the profitability or visibility of prey types.

POSTFLEDGING SURVIVAL AND MOVEMENT IN DICKCISSELS (SPIZA AMERICANA): IMPLICATIONS FOR HABITAT MANAGEMENT AND CONSERVATION

Abstract When land managers incorporate the habitat needs of grassland birds into their planning, they typically rely on management recommendations based on habitat use by adults during nesting. Habitat requirements for other critical life stages are seldom known and may differ from those of nesting adults. Using radio-telemetry, we examined survival and habitat use by juvenile Dickcissels (Spiza americana) during the postfledging period. In 2003 and 2004, we monitored 60 fledgling Dickcissels for ≤30 days after they left the nest. Mortality rates were highest during the first week after leaving the nest, and only 33% of the fledglings survived the first four weeks after leaving the nest. Estimated mean survival times were 16.9 ± 1.6 days after birds left the nest. In both years, fledgling survival was positively associated with dense vertical and horizontal structure of forbs at nests. Survival tended to be positively associated with vertical grass density on adult territories and negatively associated with patchily distributed forbs on adult territories. Fledgling habitat use was restricted to areas where Dickcissels nested and adjacent fields. Habitats used included corn and soybean fields, grasslands, and wetlands. Our results suggest that the fledgling period is a critical stage for Dickcissels and that fledglings require habitat similar to habitat used for nesting. Supervivencia Durante el Período Posterior al Emplumamiento en Spiza americana: Implicancias para el Manejo de Hábitat y Conservación

The number of visits to the nest by parents is an accurate measure of food delivered to nestlings in Tree Swallows

Abstract The number of visits parents make to their nest during chick rearing is a commonly used measure of the amount of food delivered to nestlings and an index of the quality of parental care. Use of the number of visits for these purposes assumes that parents feed nestlings when they visit and that there are no systematic differences in the amount of food delivered on each visit. These assumptions were tested in Tree Swallows (Tachycineta bicolor) breeding in nest boxes near Ithaca, New York. Video observations of parents inside their nest boxes showed that parents feed nestlings on 95–98% of visits to the nest. An average visit delivered 18.1 insects with a total dry mass of 24.1 mg, usually to a single nestling. Although females visited more frequently, the load they carried on each visit was not different than that carried by males. Load size did not differ with nestling age, brood size or the date of the feeding. Overall, there were few systematic differences in load size among nests. Therefore, the number of visits to the nest is a good measure of food delivery and parental care in Tree Swallows.

Nest-building behavior in pcb-contaminated tree swallows

Considerable attention has been directed toward documenting the effects of environmental chemicals on the endocrine systems of vertebrates, especially on development and reproduction. Given the well-documented role of hormones in controlling behavior, one would expect to see abnormal behavior in contaminated wildlife. We describe abnormal nest- building behavior in Tree Swallows (Tachycineta bicolor) breeding along the Hudson River of New York in areas highly contaminated with polychlorinated biphenyls (a group of chemi- cals known to disrupt endocrine systems). Previous studies of Tree Swallows have shown that nest quality is an important component of reproductive success. Swallows breeding in contaminated areas built smaller nests of lower quality compared with those in uncontam- inated areas. Our observations are consistent with the possibility that chemical contaminants interfered with behavior. Received 31 October 1997, accepted 11 May 1998.

Aquatic community characteristics influence the foraging patterns of Tree Swallows

During periods of inclement spring weather, Tree Swallows (Tachycineta bicolor) were observed foraging close to the surface of a series of experimentally manipulated ponds. Censuses of foraging swallows during these periods indicated that the use of ponds for foraging was positively affected by earlier additions of nutrients and the removal of fish from the ponds. Collections of emerging insects from these ponds indicated that nutrient additions and the removal of fish greatly increased the numbers of insects emerging from those ponds. The ability of Tree Swallows to exploit local concentrations of food may be critical to their ability to return to the breeding grounds before aerial insects are reliably available.

FRUIT ABUNDANCE AND LOCAL DISTRIBUTION OF WINTERING HERMIT THRUSHES (CATHARUS GUTTATUS) AND YELLOW-RUMPED WARBLERS (DENDROICA CORONATA) IN SOUTH CAROLINA

Abstract We conducted winter censuses of two short-distance migrants, Hermit Thrushes (Catharus guttatus) and Yellow-rumped Warblers (Dendroica coronata), over seven years in five different habitats to determine whether their local abundances could be predicted by fruit pulp biomass. Sampled habitats were stands of upland and bottomland hardwood, loblolly pine (Pinus taeda), longleaf pine (P. palustris), and young (<10 years) longleaf pine. Hermit Thrush abundance, which was highest in bottomland hardwood habitats, was positively related to total dry mass of fruit pulp. Those results are consistent with the hypothesis that resource availability affects the local distribution of migrant passerines on their wintering grounds. Our results also indicate that bottomland hardwood habitats in the southeastern United States may be especially important to wintering Hermit Thrushes. Yellow-rumped Warbler abundance was correlated with ripe-fruit pulp dry mass of Myrica cerifera, a major source of winter food for that species. However, because M. cerifera pulp dry mass was confounded with habitat type, we could not distinguish the relative importance of fruit resources and habitat for Yellowrumped Warblers. Our results underscore the importance of fruit to wintering birds. However, the overall percentage of variation in winter bird abundance explained by differences in ripefruit biomass was modest, indicating that other factors are also important.

Cold temperature increases winter fruit removal rate of a bird-dispersed shrub.

We tested the hypothesis that winter removal rates of fruits of wax myrtle, Myrica cerifera, are higher in colder winters. Over a 9-year period, we monitored M. cerifera fruit crops in 13 0.1-ha study plots in South Carolina, U.S.A. Peak ripeness occurred in November, whereas peak removal occurred in the coldest months, December and January. Mean time to fruit removal within study plots was positively correlated with mean winter temperatures, thereby supporting our hypothesis. This result, combined with the generally low availability of winter arthropods, suggests that fruit abundance may play a role in determining winter survivorship and distribution of permanent resident and short-distance migrant birds. From the plant’s perspective, it demonstrates inter-annual variation in the temporal component of seed dispersal, with possible consequences for post-dispersal seed and seedling ecology.

Resource availability, breeding site selection, and reproductive success of red-winged blackbirds

Abstract Red-winged blackbirds are polygynous and show strong breeding site preferences, but it is unclear which environmental factors regulate their reproductive success and are ultimately responsible for shaping their patterns of habitat selection and their mating system. We evaluated the effect of variation in insect emergence rates on the reproductive success of male and female redwings nesting on replicate ponds. The number of male and female redwings that settled on a pond varied two- to three-fold among ponds, but was not related to insect emergence rates. Insect emergence rates had a positive effect on the number of nestlings successfully fledged by females, the number of nestlings fledged from male territories, and on the mass of nestlings at fledging. Typha stem density also varied widely among ponds, and was positively related to male and female settling density and mass of nestlings at fledging, but not to the number of nestlings fledged by females or males. We conclude that alternative breeding sites differ in their ability to support redwing reproduction, and that the availability of emerging odonates is an important environmental factor influencing the reproductive success of both male and female red-winged blackbirds.

Buff-Breasted Sandpiper Density and Numbers During Migratory Stopover in the Rainwater Basin, Nebraska

Abstract The Buff-breasted Sandpiper (Tryngites subruficollis) is a shorebird of conservation concern whose migration patterns and population size are poorly known. We conducted surveys in the Eastern Rainwater Basin, Nebraska, in 2004 and 2005 using distance sampling. This survey produced density estimates of 0.09 birds per ha in 2004 and 0.04 birds per ha in 2005. Because the study area was explicitly defined by soil characteristics, we were able to extrapolate from density estimates to produce predictions of overall numbers in the study area. We produced minimum estimates of the numbers of Buff-breasted Sandpipers stopping over in the region—43 300 in 2004 and 22 924 in 2005. When we restricted our predictions to only the area adjacent to roads, numbers of birds ranged from 13 488 to 41 513, depending on the area used. These predictions indicate two important findings—that 1) the current estimate of the world population of Buff-breasted Sandpipers of 15 000 to 20 000 individuals is too low, and 2) the Eastern Rainwater Basin appears to be a primary spring stopover site for the species in the North American Great Plains.