I studied the systematic relationships of the three large owlet-nightjars (Aegothelidae) to determine the taxonomic status of a fawn-colored lowland form currently classified as Aegotheles insignis tatei. I examined most of the existing specimens of A. i. insignis (n = 158) and A. crinifrons (n = 23) and all known specimens of A. i. tatei (n = 4). I also examined specimens of A. albertisi (n = 70), A. archboldi (n = 25), A. bennettii (n = 55), A. cristatus (n = 50), A. savesi (n = 1), and A. wallacii (n = 21). Aegotheles i. tatei was distinguishable from A. i. insignis and A. crinifrons by its small size and in seven plumage characters. Aegotheles i. tatei was further distinguishable from one or the other of these taxa by four additional characters. Unique among owlet-nightjars, A. i. tatei has the shortest tarsi, does not have recurved filamentous tips on its facial feathers, and has stiffer feathers on the auricular area and throat. My search of museums revealed two new specimens of A. i. tatei, expanding the known geographic range of this taxon 1,000 km eastward along the southern coast of Papua New Guinea from the upper Fly River to Nunumai, near the Ulamanu River. Unlike the montane A. i. insignis, A. i. tatei inhabits lowland forests where rivers emerge from foothills of the main cordillera. I propose that tatei be elevated to species status and that the name Starry Owlet-Nightjar be adopted based on the bird's markings. Aegotheles crinifrons, A. insignis, and A. tatei pass through a previously unrecognized but distinctive rufous juvenal plumage. These are the only owlet-nightjars known to exhibit this plumage, which calls for reexamination of generic limits within the Aegothelidae.

Whether or not migrants gain mass at a stopover site is an index of site quality. Previous studies have examined mass gain of recaptured birds, and of short-term stopovers by regressing mass at first capture on hour of day. I developed an extension of the latter method using multiple regression to examine the effects on mass gain of hour of day, date, and year. I then used the method to compare the quality of three stopover sites at Long Point, Ontario, for Magnolia Warblers (Dendroica magnolia). At the peak of fall migration, warblers at all three sites gained sufficient mass for a net gain over 24 h, but they gained mass at only two of three sites during spring. Mass gain varied significantly over the course of the day, by date in the season, and among years. The earliest spring migrants lost mass at all sites, but rate of mass gain increased as the season progressed. Similar information for many more species and stopover sites might aid in habitat conservation for migrants.

We compared sequences of mitochondrial cytochrome-b and ND5 genes in a phylogenetic analysis of seven species of jacanas, representing all six genera and including the Greater Painted-snipe (Rostratula benghalensis) as an outgroup. When analyzed separately by parsimony and maximum-likelihood bootstrapping, the two genes produced consistent trees, although the ND5 tree was better resolved than the cytochrome-b tree. When combined, the data from the two genes produced a fully resolved tree that was identical to the ND5 tree. This tree had the following form: ((((Irediparra, Microparra), Metopidius), Actophilornis), ((Jacana jacana, J. spinosa), Hydrophasianus)), Rostratula. The phylogeny consists of two major clades that were known to traditional and phylogenetic taxonomists. It also contains sister taxa that are geographically disjunct: the New World Jacana and Asian Hydrophasianus, and the African Microparra and Australian Irediparra. We postulate that this biogeographic pattern results from the extinction of intervening African and Asian taxa, respectively.

Sea ducks generally are diurnal feeders, but large numbers winter above the Arctic Circle where day lengths decrease dramatically in winter. To determine how sea ducks cope with short day lengths, we studied different aspects of the behavior of three sympatric wintering species (Common Eider [Somateria mollissima], King Eider [S. spectabilis], and Oldsquaw [Clangula hyemalis]) at 70°N where day length is reduced to less than 4.5 h of twilight in midwinter. Numbers of both eider species remained fairly constant throughout winter, whereas Oldsquaws moved out of the area in midwinter. As day length decreased, eiders extended their feeding period into lower light intensities. Common Eiders and Oldsquaws spent a higher proportion of the day diving (underwater) when days were short, whereas King Eiders did not. As the days lengthened, King Eiders and Oldsquaws increased their total time diving at similar rates, which were faster than those of Common Eiders. Feeding at lower light intensities and increased proportions of time spent diving did not offset reduced feeding time in midwinter, and estimated time spent underwater during daylight on the shortest days was only 35% of that on the longest days in King Eiders, 51% in Common Eiders, and 39% in Oldsquaws. The ability to survive when days are short might be explained by use of stored nutrient reserves, night feeding, or high prey availability.

Yellow-bellied Sapsuckers (Sphyrapicus varius) obtain phloem sap from clusters of holes that they peck in living trees. I examined trees that sapsuckers used for sap extraction in northern Michigan and tested several hypotheses to explain why they choose specific trees for attack and why they cluster their holes in one place on the bole of each of these focal trees. Sapsuckers preferentially attacked individuals of paper birch (Betula papyrifera), red maple (Acer rubrum), juneberry (Amelanchier sp.) and bigtooth aspen (Populus grandidentata). They made clusters of sap holes an average of 7.13 m from the ground and within 1 m of a live branch; most clusters were located above old holes or other wounds from previous years. Each new sap incision through the bark to the phloem stream was made above a previous one and was enlarged for an average of 3.1 days before being abandoned for a new, higher hole. The resulting long vertical chains of holes were made alongside others to form tight clusters of holes at a single spot on the tree. Sapsuckers did not select trees for sap extraction based on location relative to nesting sites or on microclimate conditions of water availability and tree density. Individual trees used for sap extraction did not have thinner bark, more moisture in bark samples, or larger crowns, but they did score lower in an index of overall tree health. Experimental evidence suggested that sapsuckers cluster their holes to induce the accumulation of sap in bark that they will attack for future sap extraction. Thus, sapsuckers appear to overcome some of the difficulties in obtaining phloem sap by choosing specific species and individuals, clustering sap holes above previous wounds, and possibly by farming their resource throughout the season by girdling the tree's phloem stream with each successive sap incision. I attempted to duplicate the wounding techniques and patterns of wounding of sapsuckers but was unable to induce sap flow from the same or similar trees in the area.

In contrast to the Swamp Sparrow (Melospiza georgiana) and Song Sparrow (M. melodia), vocal studies of the congeneric Lincoln's Sparrow (M. lincolnii) are essentially lacking. To provide comparative data on song variation in this species, we obtained and analyzed recordings from 58 males (4,537 songs) breeding in montane meadows of the Sierra Nevada and San Bernardino Mountains, California. Males sang from one to six song types, and repertoire size averaged 3.7 types. No two males shared an identical song type. Males varied their types by changing the number of repetitions of a syllable or by adding, deleting, or substituting one or more syllables (i.e. by changing syllable composition). The number of variants, identified on the basis of differences in syllable composition, averaged 2.6 per song type (range 1 to 12). Individual variability was highest in the terminal elements of the song. Production of variants appears to be a process of “open-ended improvisation” of song types. Similarity of songs on the basis of shared syllables, as calculated by simple matching coefficients, showed a strong pattern of concordance with geography. Pairwise similarity declined with increasing distance between meadows, and meadows from different geographic regions clustered separately in a UPGMA tree. Patterns of geographic variation in song of Lincoln's Sparrows are similar to those observed in Song Sparrows and Swamp Sparrows. Song complexity and repertoire size show different evolutionary trends within Melospiza.

In a migratory population of Black Redstarts (Phoenicurus ochruros) in central Europe, males were territorial and sang in autumn between the end of molt in early September and the abandonment of territories in October. Participants in autumn singing were adult males past their first potential breeding season; subadults rarely defended territories in autumn. Prior to the autumn singing period, unmated males and males after their first breeding season often dispersed to new locations within the study site. Pair associations with experienced female breeders still present on the breeding ground were preformed. Low winter mortality, site dominance, and fidelity to autumn territories allowed the reformation of 59% of autumnal pair bonds in the following spring. The mating pattern was assortative by age because the initiation of territory acquisition and pair formation was temporally segregated by more than six months between subadult and adult breeders. Males benefitted from mating with experienced females because they started breeding earlier and initiated more breeding attempts per season. Autumnal singing and territoriality, a phenomenon that has not attracted much attention, may play a key role in the understanding of age-related reproductive asymmetries in Black Redstarts. Age-related reproductive performance may be the underlying cause for the evolution of delayed plumage maturation in this species.

Avian populations often consist of breeding residents and nonbreeding floaters. It is usually assumed that floaters are lower-quality individuals that do not reproduce, but floater tactics and potential reproductive success have rarely been examined carefully. To assess the potential reproductive role of male floaters in Tree Swallows (Tachycineta bicolor), we compared their reproductive organs and morphology with those of resident males. Tree Swallows show high levels of extrapair paternity, but two studies attempting to find the fathers of the extrapair offspring have been remarkably unsuccessful. Floater males that father extrapair young would face intense sperm competition. Theory predicts that under intense sperm competition, selection favors males that produce more sperm. Comparative studies in birds and other taxa provide evidence that the level of sperm competition influences relative testes size and sperm production. However, intraspecific adaptations to different levels of sperm competition have received far less attention. Floater Tree Swallows did not differ from resident males in any of the characters we measured, including testes size, but floaters had significantly larger cloacal protuberances. Thus, our results do not confirm the general assumption that floaters are lower-quality individuals that do not reproduce. Furthermore, floaters showed high variation in the volume of the cloacal protuberance (reflecting sperm numbers), which suggests that they engage in copulations. We conclude that floater male Tree Swallows invest heavily in sperm production (as do resident males) to exploit breeding opportunities through takeovers or extrapair copulations.

Pigeon Guillemots (Cepphus columba) are diving seabirds that forage near shore and feed their chicks demersal and schooling fishes. During nine years between 1979 and 1997, we studied chick diet, chick growth rate, and reproductive success of Pigeon Guillemots at Prince William Sound, Alaska, to determine factors limiting breeding populations. We found evidence for prey specialization among breeding pairs and detected differences in reproductive success between specialists and generalists. Pairs that specialized on particular prey types when foraging for their chicks fledged more chicks than those that generalized, apparently because they delivered larger individual prey items. Reproductive performance also varied among guillemot pairs as a function of the proportion of high-lipid schooling fishes fed to the chicks. Pairs that delivered primarily high-lipid fishes (Pacific sand lance [Ammodytes hexapterus] and Pacific herring [Clupea pallasii]) had higher overall reproductive success than pairs that delivered primarily low-lipid demersal fishes (e.g. sculpins, blennies, stichaeids, and pholidids) and gadids. The proportion of high-lipid fishes in the diet was positively related to chick growth, suggesting that piscivorous seabird chicks benefit from eating species with high-energy densities during development. The diet of Pigeon Guillemot chicks showed high annual variation from 1979 to 1997, presumably because of fluctuations in abundance of Pacific sand lance, a high-lipid schooling fish. Regression analyses suggest that the percent occurrence of high-lipid fishes in the diet affected chick growth rate at the population level. We conclude that Pigeon Guillemots benefit by specializing when selecting prey for their chicks, and that high-lipid schooling fishes enhance chick growth and reproductive success.

We develop a statistical method that simultaneously estimates daily survival rate and observer effect. We used Monte Carlo simulation to (1) evaluate the performance of the model, (2) compare model performance with models that ignore observer effects, and (3) evaluate methods of choosing between competing models of survival. When observer effects were absent, all models produced unbiased estimates of daily survival rate. In the presence of observer effects, however, models that ignore these effects underestimated daily survival rate. In such cases, estimates of nesting success were strongly affected even when observer effects were relatively small. In contrast, estimates of daily survival rate and nesting success produced by the model that considers observer effects consistently had little bias. However, estimates of daily survival rate from this model were less precise than those from the simpler model. Objective criteria for choosing between competing models did not perform well with sample sizes of 150 to 600 because subtle but important observer effects are difficult to detect. Likelihood-ratio tests had low power for rejecting the null hypothesis of no observer effect over a wide range of levels of observer effect and with sample sizes of 150 to 600. Estimates of daily survival rate from models selected based on Akaike's Information Criterion (AIC) had higher bias than estimates from the model that estimates observer effect when observer effect was present. Estimates from AIC-selected models had lower mean squared error than estimates from the model that estimates observer effect when observer effects were small, but the pattern reversed as effects increased. We recommend that researchers estimate observer effects using the more complex model when observer effects are possible and decide whether to use estimates of daily survival from the simpler or more complex model based on analysis results and simulation or analytic results for relevant sample sizes, daily survival rates, and observer effects. To illustrate use of the analytical techniques, we analyzed field data from Dusky Flycatcher (Empidonax oberholseri) nests monitored during the nestling stage. The observer effect was estimated to be 1.003 (95% CI 0.866 to 1.162); thus, point estimates of daily survival were very similar from the simpler (0.971; 95% CI 0.957 to 0.985) and more complex model (0.970; 95% CI 0.925 to 1.000). In this case, analysis results and simulation results indicate that the simpler model is adequate and provides an estimate of daily survival rate with small potential bias and increased precision compared with an estimate from the more complex model.

The composition of depot fat in Western Sandpipers (Calidris mauri) was dominated by C₁₆ and C₁₈ fatty acids (palmitate, 16:0; palmitoleate, 16:1; stearate, 18:0; oleate, 18:1), which together comprised 80 to 90% of all fatty acids analyzed. Significant seasonal variation occurred in the relative proportion of specific fatty acids and the ratio of saturated to unsaturated fatty acids in relation to the migratory cycle of the sandpipers. In adults of both sexes, the proportion of 16:0 and 18:0 fatty acids decreased from winter (December) to premigration (March) to spring migration (May), whereas the proportion of 16:1 and 18:1 fatty acids increased over these same periods. Consequently, total unsaturated fatty acids increased by 34% between the winter and premigratory phase, and by a further 22% between premigration and spring migration. Therefore, biochemical modification of adipose tissue in Western Sandpipers begins during the premigratory period in preparation for long-distance flight but continues as migration progresses, perhaps reflecting a training component of physiological modulation for migration. Juveniles showed similar changes to adults in the composition of their depot fat during the “premigratory” period, even though most juveniles do not undergo hyperphagia or rapid fattening at this wintering site. Potential prey species collected from a wintering site in Panama had a higher proportion of saturated fatty acids, less 16:1 and 18:1, and more 18:0 fatty acid compared with those from a migratory stopover site in British Columbia, paralleling the seasonal changes in depot fatty-acid composition observed in Western Sandpipers. However, the fact that Western Sandpipers show an increase in level of unsaturation and in the amounts of 16:1 and 18:1 fatty acids in their fat stores on the wintering ground prior to migration suggests that seasonal variation in fatty-acid composition is not entirely diet dependent.

Little is known about the structure and function of hummingbird vocalizations. We studied the vocalizations of Blue-throated Hummingbirds (Lampornis clemenciae) at two sites in southeastern Arizona. Songs were produced by males and females. Male songs consisted of arrays of notes organized in clusters of “song units.” Within sites, all males shared the same song units. Individual differences occurred in some temporal aspects of song, and slight but consistent differences in note structure occurred between the two sites. The organization of units within songs was marked by rigid syntax, and long songs were produced by agglutination of units. Male songs may function in territorial advertisement and mate attraction. Female songs were very different acoustically from those of males and typically were given when females were within a few centimeters of a male. In these situations, the female's song often overlapped temporally with the male's song. Of the hummingbird species studied so far, the Blue-throated Hummingbird has the most complex songs and is the only known species with complex female songs. Blue-throated Hummingbirds show convergence with oscines in vocal complexity, song organization, song function, and possible learning of some song elements.

Ratios of the stable carbon (δ¹³C) and hydrogen (δD) isotopes in newly grown remiges of Ruddy Ducks (Oxyura jamaicensis) are influenced by the isotopic character of food types and ambient water. Systematic isotopic foodweb and water differences between coastal wintering grounds and prairie breeding grounds of Ruddy Ducks provide the basis for using isotopic analyses of feathers to trace the location and timing of molt. Anecdotal evidence, based largely on captive birds, suggests that Ruddy Ducks replace their remiges twice each year (once each on the breeding and wintering grounds), but a recent literature analysis indicates that this phenomenon is rare. Thus, we investigated the extent to which a biannual molt of the remiges occurs in the wild and at the population level. We analyzed the stable isotopes of carbon (n = 57 birds) and hydrogen (n = 50 birds) in flight feathers to estimate the prevalence of the so-called “double molt” of remiges in free-living Ruddy Ducks. Our data showed that natural populations of Ruddy Ducks express an overwhelming unimodal distribution of isotope ratios in their remiges, suggesting that they undergo a single molt at or near the breeding grounds. Only 3 to 6 of 50 birds from Manitoba showed isotopic evidence consistent with growing remiges on the wintering grounds. Feathers from Ruddy Ducks harvested during the fall in the Mississippi Flyway had isotopic profiles consistent with growth on northern freshwater breeding sites. Thus, our results confirm that the replacement of remiges twice each year by Ruddy Ducks is rare, and they suggest that this dual stable-isotope technique can be used to infer general molting origins of North American waterfowl.

Artificial nests are commonly used to investigate relative rates of nest predation in birds, but several methodological considerations need to be addressed before results from natural and artificial nests can be compared. Using field and laboratory experiments, we examined responses of predators to visual and olfactory cues that were associated with wicker nests and their contents. Avian predators did not discriminate between wicker nests dipped in mud and those covered by a camouflage fabric, whereas mammalian predators showed a weak tendency to depredate camouflaged nests. Nests containing plasticine eggs were depredated more often than nests containing only quail eggs and finch eggs, although no response to number of plasticine eggs in nests was found. The higher predation of nests with plasticine eggs may have resulted because small mammals, relying on olfactory cues, comprised a large portion of the predator assemblage. Field results were supported in tests where captive deer mice (Peromyscus maniculatus) were attracted to assortments of egg types that included plasticine. Time required by captive deer mice to penetrate quail eggs and finch eggs versus plasticine eggs varied as a function of egg size and shell thickness and strength. Overall, domestic finch eggs provided a better alternative to quail eggs because they were small enough to allow detection of predation events by small mammals and did not have an unnatural odor like plasticine. Potential problems with nest concealment, egg visibility, egg odors, and other factors must be resolved to enhance the design and reliability of artificial nest experiments.

We estimated nesting success at real and artificial nests of grassland birds to test the influence of nest type, nest position, and egg size on predation rates. We distributed wicker nests and realistic woven-grass nests baited with a clay egg and either a Northern Bobwhite (Colinus virginianus) egg or a House Sparrow (Passer domesticus) egg in four grasslands that were part of the Conservation Reserve Program in east-central Illinois. Nesting success averaged 86.5% for 12 days of exposure for artificial nests. For real nests, nesting success was markedly lower, averaging 39% over the entire nesting cycle and 59% during approximately 12 days of incubation. Wicker nests were depredated more often than woven-grass artificial nests (18% vs. 8%), and nests baited with House Sparrow eggs were depredated more often than nests baited with Northern Bobwhite eggs (22% vs. 9%). Elevated and ground nests were depredated at the same rate. Patterns of nest predation on wicker nests were markedly different from depredation patterns on real nests over time and among fields. In contrast, patterns of nest predation on realistic woven-grass nests corresponded much more closely with predation rates of real nests over time and among fields. We suggest that future artificial nest studies use nests and eggs that mimic as closely as possible the real nests and eggs of target species. Use of unrealistic artificial nests and eggs, at least in grasslands, may result in patterns of predation that do not accurately reflect those of real nests. Artificial nests of any type appear to underestimate predation rates on nests of grassland birds, possibly because of a lack of snake predation on artificial nests.

The taxonomy of the Australasian teals has been particularly unstable. Australasian Grey Teal (Anas gracilis) and Chestnut Teal (A. castanea) are widely viewed as specifically distinct, but the taxonomy of the New Zealand teals remains unsettled. Because conservation status is affected by taxonomic rank, it is important to resolve the status of the rare subantarctic teals. To estimate phylogenetic relationships of teals, we sequenced three mitochondrial DNA genes (12S, and ATPase 6 and 8). The resultant phylogeny unequivocally groups the Chestnut Teal with the Grey Teal, rather than with the New Zealand teals as has traditionally been held (Fleming 1953). A greater level of sequence divergence occurred within the New Zealand teals than between the Grey and Chestnut teals. This diversity, together with morphological and behavioral differences, implies that the New Zealand teals should be accorded specific status as A. aucklandica, A. nesiotis, and A. chlorotis. Although it is most likely that the teal that colonized the Auckland Islands and Campbell Islands originated in New Zealand, our data do not allow us to determine whether the ancestors of the Campbell Island Teal came from mainland New Zealand or the Auckland Islands. This uncertainty arises because, as our data show, the colonization events were separated by a short period of time.

We tested for the presence of compensatory growth (i.e. faster age-specific growth) following ephemeral periods of food restriction in altricial nestlings using the House Sparrow (Passer domesticus) as a model species. To simulate periods of poor food conditions, we raised nestlings in captivity, fed them a synthetic diet, and held them at constant body mass for 48 h beginning on either day 3 or 6 of life. Controls were fed according to an age-specific feeding schedule that yielded normal growth curves. During realimentation, restricted nestlings did not achieve a faster rate of growth than that of controls. Instead, these nestlings either died (all controls lived) or gained mass at a rate similar to that of controls. Consequently, restricted nestlings reached asymptotic mass two days later than control nestlings. Growth of culmen and tarsus was not affected, but growth of the eighth primary was reduced for several days in nestlings restricted at day 6 (i.e. late restricted), although this difference disappeared by the age of fledging. Because surviving nestlings achieved only a 15.9% increase in food consumption compared with unrestricted controls and were unable to translate it into a faster rate of growth, the nestlings may have been growing at a maximum rate. We found no differences between late-restricted and unrestricted nestlings in % water, % protein, % lipid, and % ash. The two groups were of similar maturity as measured by % body water and the water index. Our results are consistent with current theory in that periods of food restriction delayed the schedule of mass accretion by the length of the restriction period. Although House Sparrows have a labile growth rate and developmental time, our results did not support the hypothesis of compensatory growth. Based on this and one other study, compensatory growth does not appear to occur in altricial birds.

Hybrid zones between Townsend's Warblers (Dendroica townsendi) and Hermit Warblers (D. occidentalis) in the Pacific Northwest are narrow relative to estimated dispersal distances and appear to be moving, with Townsend's replacing Hermits. We examined whether the habitat-transition and parental-fitness asymmetry models can explain why these zones are narrow and moving by comparing habitat variables associated with warbler territories in the Washington Cascades hybrid zone. Habitat variables did not differ among phenotypes, suggesting that the habitat-transition model cannot explain the narrow and dynamic nature of this hybrid zone. Habitat characteristics of Hermit Warbler territories did not differ inside versus outside the hybrid zone, also suggesting that this zone is not associated with a region of habitat transition. The lack of difference in habitat use could be the result of comparing variables that are not important to pairing success. However, warblers tended to select territories on west-southwest aspects. South aspects in the southern Washington Cascades are dominated by Douglas fir (Pseudotsuga menziesii) and true fir, which is a habitat selected by female warblers when choosing among territories and males. The parental-fitness asymmetry model does not necessarily make predictions about habitat use within the hybrid zone but predicts the superiority of one parental species over the other. However, if significant overlap occurs in habitat use or niche (as in these warblers), then competition between parental species is likely to occur. To determine whether these species compete, we mapped 12 warbler territories and monitored an additional 94 territories throughout the breeding season and found that all males with neighbors compete for and hold exclusive territories. Thus, the pattern of habitat use and territoriality is consistent with the parental-fitness asymmetry model.

Accurate estimation of survival probabilities is an important component of population demographics, and it permits a test of the life-history prediction that densities influence population dynamics via suppression of survival rates. As part of a long-term study of urban-nesting Merlins (Falco columbarius), we estimated survival rates and tested for the effects of density dependence based on capture histories from 1,354 individuals (43 males and 110 females caught for the first time as adult breeding birds, and 597 males and 604 females caught for the first time as locally produced nestlings). Overall capture probabilities were 0.55 ± SD of 0.039 per year for adults, 0.10 ± 0.075 per year for juvenile males, and 0.58 ± 0.23 per year for juvenile females. Mean survival rate of adults was 0.62 ± 0.11 per year and did not differ significantly between males and females. Overall juvenile survival rates were 0.23 ± 0.032 for males and 0.055 ± 0.012 for females. Band returns suggest that the discrepancy in survival rates between juvenile males and females resulted from higher natal dispersal of females rather than from lower survival. Survival of adults (but not juveniles) was negatively density dependent, suggesting that density-dependent declines in survival exerted a regulatory effect on population size.

I monitored 278 Wood Thrush (Hylocichla mustelina) nests in 14 forest fragments (range 7 to 500 ha) in northern Indiana to explore patterns of brood parasitism by Brown-headed Cowbirds (Molothrus ater), nest predation, and reproductive success. Density of thrushes was negatively related to area of forest fragments. Cowbirds were common throughout the landscape, but I found no relationship between their abundance and forest area. Overall, 90% of the thrush nests were parasitized by cowbirds (x̄ = 2.4 cowbird eggs per nest). The number of cowbird eggs per thrush nest was not related to forest area, abundance of host species, or distance to a forest edge but was positively related to thrush abundance. Nest predation rates averaged 58% and were lower than those reported in much of neighboring Illinois but also were unrelated to forest area and distance to a forest edge. The combination of cowbird parasitism and nest predation resulted in relatively low reproductive success (x̄ = 0.6 thrush fledglings per nesting attempt). I estimated that Wood Thrushes in northern Indiana made an average of three nesting attempts per breeding season (based on 17 color-marked females) and had relatively low seasonal fecundity (x̄ = 0.9 female fledglings per adult female per season). Nonetheless, considerable annual variation in seasonal fecundity suggested that some sites exceeded the source-sink threshold in some years. Regardless, the overall landscape appeared to be part of a regional sink for Wood Thrushes, although its negative influence on regional demography was not as severe as elsewhere in the midwestern United States. The poor demographic balance in much of the agriculturally dominated Midwest suggests that conservation efforts for Wood Thrushes and other Nearctic-Neotropical migrants should be directed at preserving and enhancing possible source habitats in regions where parasitism by cowbirds and nest predation are reduced.

Using the altitudinal profiles of wind, temperature, pressure, and humidity in three flight models, we tried to explain the altitudinal distributions of nocturnal migrants recorded by radar above a desert in southern Israel. In the simplest model, only the tailwind component was used as a predictor of the most preferred flight altitude (T model). The energy model (E model) predicted flight ranges according to mechanical power consumption in flapping flight depending on air density and wind conditions, assuming optimal adjustment of airspeed and compensation of crosswinds, and including the influence of mass loss during flight. The energy-water model (EW model) used the same assumptions and parameters as the E model but also included restrictions caused by dehydration. Because wind was by far the most important factor governing altitudinal distribution of nocturnal migrants, differences in predictions of the three models were small. In a first approach, the EW model performed slightly better than the E model, and both performed slightly better than the T model. Differences were most pronounced in spring, when migrants should fly high according to wind conditions, but when climbing and descending they must cross lower altitudes where conditions are better with respect to dehydration. A simplified energy model (Es model) that omits the effect of air density on flight costs explained the same amount of variance in flight altitude as the more complicated E and EW models. By omitting the effect of air density, the Es model predicted lower flight altitudes and thus compensated for factors that generally bias height distributions downward but are not considered in the models (i.e. climb and descent through lower air layers, cost of ascent, and decrease of oxygen partial pressure with altitude). Our results confirm that wind profiles, and thus energy rather than water limitations, govern the altitudinal distribution of nocturnal migrants, even under the extreme humidity and temperature conditions in the trade wind zone.

In many birds, populations breeding at higher latitudes have larger clutch sizes. It has been hypothesized that this relationship results from the greater availability of food at higher latitudes. We examined geographic variation in clutch size of Tree Swallows (Tachycineta bicolor) in relation to latitude, longitude, laying date, breeding density, elevation, and summer actual evapotranspiration (AE). AE is a measure of terrestrial primary productivity, and we assumed that it was correlated with resource abundance. Clutch size data were compiled from nest records throughout the breeding range in the United States and Canada (n = 7,459). Summer AE estimates were made from long-term climatological records, and breeding density was estimated from Breeding Bird Surveys. All variables were averaged for each 1° block of latitude and longitude (n = 85 latilong blocks). We found a strong positive relationship between clutch size and latitude. Absolute levels of resource abundance (summer AE) were not related to clutch size but were related inversely to latitude and longitude. After controlling for the effects of confounding variables, clutch size was related positively to latitude and relative resource abundance (summer AE after controlling for breeding density). Our results are consistent with Ashmole's hypothesis that relative, rather than absolute, levels of resource abundance determine at least some of the variation in clutch size.