Quantifying patterns of nest survival is a first step toward understanding why birds decide when and where to breed. Most studies of nest survival have relied on generalized linear models (GLM) to explore these patterns. However, GLMs require assumptions about the models' structure that might preclude finding nonlinear patterns in survival data. Generalized additive models (GAM) provide a flexible alternative to GLMs for estimating linear and nonlinear patterns in data. Here we present a comparison of GLMs and GAMs for explaining variation in nest-survival data. We used two different model-selection criteria, the Bayes (BIC) and Akaike (AIC) information criteria, to select among simple and complex models. Our study was focused on the analysis of Redwinged Blackbird (Agelaius phoeniceus) nests in the Rainwater Basin wetlands of south-central Nebraska. Under BIC, our quadratic model of nest age had the most support, and the model predicted a concave pattern of daily nest survival. We found more model-selection uncertainty under AIC and found support for additive models with ordinal effects of both day and age. These models predicted much more temporal variation than did the linear models. Following our analysis, we discuss some of the advantages and disadvantages of GAMs. Despite the possible limitations of GAMs, our results suggest that they provide an efficient and flexible way to demonstrate nonlinear patterns in nest-survival data.

We describe a new enantiornithine bird, Huoshanornis huji gen. et sp. nov., from the Early Cretaceous Jiufotang Formation in Chaoyang, western Liaoning Province, China. This new bird is distinguished from other known enantiornithines in possessing a considerably reduced alular digit, a broad intermetacarpal space, a relatively long phalanx of the minor digit of the manus, and a remarkable triangular expansion at the distal end of the lateral trabecula of the sternum. The morphology of the manus may also suggest that at low flight speeds the new bird's maneuverability was exceptional.

Floating bird eggs to estimate their age is a widely used technique, but few studies have examined its accuracy throughout incubation. We assessed egg flotation for estimating hatch date, day of incubation, and the embryo's developmental age in eggs of the American Avocet (Recurvirostra americana), Black-necked Stilt (Himantopus mexicanus), and Forster's Tern (Sterna forsteri). Predicted hatch dates based on egg flotation during our first visit to a nest were highly correlated with actual hatch dates (r = 0.99) and accurate within 2.3 ± 1.7 (SD) days. Age estimates based on flotation were correlated with both day of incubation (r = 0.96) and the embryo's developmental age (r = 0.86) and accurate within 1.3 ± 1.6 days and 1.9 ± 1.6 days, respectively. However, the technique's accuracy varied substantially throughout incubation. Flotation overestimated the embryo's developmental age between 3 and 9 days, underestimated age between 12 and 21 days, and was most accurate between 0 and 3 days and 9 and 12 days. Age estimates based on egg flotation were generally accurate within 3 days until day 15 but later in incubation were biased progressively lower. Egg flotation was inaccurate and overestimated embryo age in abandoned nests (mean error: 7.5 ± 6.0 days). The embryo's developmental age and day of incubation were highly correlated (r = 0.94), differed by 2.1 ± 1.6 days, and resulted in similar assessments of the egg-flotation technique. Floating every egg in the clutch and refloating eggs at subsequent visits to a nest can refine age estimates.

In many avian species with synchronously hatching young, incubation and embryonic development begin prior to the completion of the clutch. However, mechanisms regulating synchronous hatching and more rapid embryonic development in eggs laid later in the clutch are poorly understood. We measured physical characteristics of eggs from complete clutches of Canada Geese (Branta canadensis) to investigate whether variation in eggshell conductance, shell thickness, shell porosity, or rates of yolk accumulation during egg formation might support the observed increase in metabolic rate of eggs laid later. Canada Geese lay large clutches (7–9 eggs) and initiate incubation before completing them (approximately with egg 3), but the young hatch and leave the nest synchronously. Both conductance (from ∼15 mg H₂O torr^-1 day^-1 for the first to ∼25 mg H₂O torr^-1 day^-1 for the eighth egg in the clutch) and porosity (from ∼2.5 pores cm ² for the first to ∼3.0 pores cm ^-2 for the eighth egg in the clutch) increased linearly with the sequence in which the eggs were laid. Neither shell thickness nor rates of yolk accumulation were related to the egg's position in the sequence. We hypothesize that, in waterfowl and other species that lay large clutches and initiate incubation prior to completion of the clutch, changes in the shell gland during egg laying lead to the production of progressively more porous eggs that promote vascularization of the chorioallantoic membrane of the embryo to sustain the higher metabolic and developmental rates necessary for synchronous hatching.

Distance sampling along a line transect is used commonly for monitoring changes of birds' abundance at sea. A critical yet rarely tested assumption of line-transect-sampling theory is that all birds along the transect line (i.e., directly in front of the boat) are detected or that probability of detecting a bird on the line can be estimated. As part of a long-term research and monitoring program for the Kittlitz's Murrelet (Brachyramphus brevirostris), we tested the assumption of complete detection of murrelets on the water along a transect line directly in front of a moving boat. Following standard survey procedures, we approached groups of murrelets (n = 57) at sea and recorded their distance, response (diving or flying), and duration of response. Flying murrelets (n = 27) were easily detected, but diving birds (n = 30) were more difficult to detect because of the duration of their dive. The probability that a bird dove and remained underwater long enough to avoid detection was low because birds that dove more than 150 m from the boat surfaced before the boat passed whereas birds that “waited” to dive near the boat were easily detected prior to diving. The greatest probability of nondetection was for birds diving at 55 m (diving long enough for the boat to pass) but was only 0.032 ± 0.007 (P SE). These experiments quantifying detection probability along the transect line could be applied to any species surveyed from a boat.

Successful foraging by avian predators is influenced largely by prey availability, which encompasses not only the density of prey but also its vulnerability to capture. For wading birds (Ciconiiformes), habitat features such as water depth and density of vegetation are thought to affect the vulnerability of their aquatic prey. In January and April 2007 we experimentally manipulated the depth of water and density of submerged aquatic vegetation (SAV) in enclosures (10 × 10 m) with equal densities of fish to determine their effects on wading birds' selection of foraging habitat and foraging success. Analysis of the results with Manly's selection index showed that wading birds preferred habitat with shallow water and SAV. The two habitat components had little effect on the birds' foraging success, however, as capture rate did not vary with water depth or SAV density. Capture efficiency did not vary by SAV density and was actually lower in shallow water, contrary to our expectations. Our results suggest that birds selected habitat on the basis of environmental cues such as water depth and SAV but that these factors did not affect foraging success strongly. We hypothesize that wading birds were selecting habitat with shallow water and SAV because of an anticipated benefit to foraging through elevated density and vulnerability of prey, but the relatively high and uniform density of prey stocked in the enclosures, as well as the scale of the enclosures, effectively equalized the vulnerability of prey across treatments.

Estimating indices of abundance of threatened species is crucial to preserving biodiversity. Over the last few decades, noninvasive genetic sampling has proven to be a more straightforward and less expensive approach than capture—mark—recapture analyses. In particular, molted feathers have become extremely popular for the monitoring of bird populations. Diagnostic molecular markers such as microsatellites, however, are still not available for many avian species of conservation concern. Highly polymorphic genes of the major histocompatibility complex (MHC), on the other hand, have become reasonably accessible during the last few years. We tested the suitability of MHC profiles as DNA fingerprints to assist the identification of individuals of a scavenger difficult to monitor through traditional approaches, the Andean Condor (Vultur gryphus). To achieve this aim, we isolated polymorphic and putatively functional genes of MHC class I (exon 3, 6 alleles) and MHC class IIB (exon 2, 11 alleles). Single-strand conformational polymorphism and direct sequencing of MHC genes, combined with molecular sexing and inference of age class from feather color, allowed us to identify 80 different individuals from 110 molted feathers collected at roost sites. Inferred sex and age ratios were concordant with previous studies relying on direct observations. Among adults, the number of males was double that of females; among juveniles, this ratio was inverted. Besides providing valuable data regarding genetic variation at functionally important genes related to resistance to pathogens, we demonstrate additional potential of polymorphic MHC loci beyond their well-known role in evolutionary ecology.

Probably always rare and local because of its ecology and specialized habitat, the small, isolated population of the Orange-breasted Falcon (Falco deiroleucus) in Belize and Guatemala, likely numbering fewer than 40 territorial pairs, appears to be in steep decline in Belize. Territory occupancy (n = 12 eyries) in the population we studied in Belize declined from 83% (1992 to 1997) to 54% (2003 to 2009), and occupancy in 2009 was only half the mean in the prior decade. Mean annual production of fledglings per territorial pair declined 35% from 0.77 to 0.50. Mean annual population productivity, which measures the combined effect of occupancy and fecundity, declined 57% from 0.90 to 0.38. In contrast, neither occupancy nor fledging success in Guatemala (n = 7 eyries) declined over the same time period. Historical records and recent surveys suggest that the Orange-breasted Falcon has been extirpated from much of Central America and southern Mexico and that its range is contracting in South America, despite large areas of apparently suitable habitat. We identify factors we think are contributing to this decline, especially human-related conflicts as well as competition for nest sites and depredation by increasing numbers of Black Vultures (Coragyps atratus). We suggest potential management solutions, including genetic restoration and the creation of safe harbors for nesting.

Trace-element analysis has been suggested as a tool for the study of migratory connectivity because (1) trace-element abundance varies spatially in the environment, (2) trace elements are assimilated into animals tissues through the diet, and (3) current technology permits the analysis of multiple trace elements in a small tissue sample, allowing the simultaneous exploration of several elements. We explored the potential of trace elements (B, Na, Mg, Al, Si, P, S, K, Ca, Ti, Cr, Mn, Ni, Cu, Zn, As, Sr, Cs, Hg, Tl, Pb, Bi, Th, and U) to clarify the migratory connectivity of shorebirds that breed in North America and winter in southern South America. We collected 66 recently replaced secondary feathers from Red Knots (Calidris canutus) at three sites in Patagonia and 76 from White-rumped Sandpipers (C. fuscicollis) at nine sites across Argentina. There were significant differences in trace-element abundance in shorebird feathers grown at different nonbreeding sites, and annual variability within a site was small compared to variability among sites. Across Argentina, there was no large-scale gradient in trace elements. The lack of such a gradient restricts the application of this technique to questions concerning the origin of shorebirds to a small number of discrete sites. Furthermore, our results including three additional species, the Pectoral Sandpiper (C. melanotos), Wilson's Phalarope (Phalaropus tricolor), and Collared Plover (Charadrius collaris), suggest that trace-element profiles change as feathers age. Temporal instability of trace-element values could undermine their application to the study of migratory connectivity in shorebirds.

We studied dispersal, mate retention, apparent survival, and renesting in a subarctic breeding population of the Semipalmated Plover (Charadrius semipalmatus) near the southern limits of its breeding range on Akimiski Island, Nunavut, Canada, from 2002 to 2007. The risk of predation at this site is higher than in more northern parts of this species' range. Dispersal of breeding birds was biased toward females, as found also at Churchill, Manitoba, a more northerly location. Mate retention was low both within (33.3%) and between (6.5%) seasons and much lower than previous estimates from Churchill. Return and encounter rates of adult males were higher than those of females, but apparent survival of adult males and females did not differ and was lower than that reported for Churchill. Within a season, renesting after a failed nest attempt was common (53%) with some pairs nesting three times in a season. Differences between the two study areas in rates of renesting may help to account for persistence of the southern population. Differences in weather at the two latitudes affect the duration of the breeding season and appear to have significant consequences for the strength of social monogamy but not for general patterns of dispersal.

The population decline of the Western Snowy Plover (Charadrius alexandrinus nivosus) along the Pacific coast of the U.S., has been attributed, in part, to the spread of European beachgrass (Ammophila arenaria), which degrades nesting habitats. We compared Ammophila cover at the plover's courtship scrapes and nest sites with that at random locations in coastal northern California. Ammophila cover around nests and scrapes was significantly less than random points at several spatial scales (≤100 m) of analysis; cover around nests was also less than around scrapes. Incubating plovers ceased incubation and left nests when an observer approached to within a mean distance of 80 ± 33 m (n = 8). We conclude that the plover's selection of open habitats for courtship and nesting may facilitate early detection of predators. Our results indicate a minimum size for restoration projects and a distance at which fencing around nests should be placed to ameliorate the effects of human disturbance on incubating plovers.

San Francisco Bay is a wintering area for shorebirds, including American Avocets (Recurvirostra americana). Recently, a new resident population of avocets has emerged, presumably because of the development of tidal marshes into salt-evaporation ponds. In habitat restoration now underway, as many as 90% of salt ponds will be restored to tidal marsh. However, it is unknown if wintering and resident avocets coexist and if their requirements for space and habitat differ, necessitating different management for their populations to be maintained during restoration. We captured and radio-marked wintering avocets at a salt pond and a tidal flat to determine their population status (migrant or resident) and examine their space use and habitat selection. Of the radio-marked avocets, 79% were migrants and 21% were residents. At the salt pond, residents' fidelity to their location of capture was higher, and residents moved less than did migrants from the same site. Conversely, on the tidal flat, fidelity of residents to their site of capture was lower, and residents' home ranges were larger than those of migrants from the same site. Habitat selection of migrants and residents differed little; however, capture site influenced habitat selection far more than the birds' status as migrants or residents. Our study suggests that individual avocets have high site fidelity while wintering in San Francisco Bay, although the avocet as a species is plastic in its space use and habitat selection. This plasticity may allow wintering migrant and resident avocets to adapt to habitat change in San Francisco Bay.

Kleptoparasitism is defined as the stealing by one animal of food that has been caught by another. It is a well-known foraging tactic used opportunistically by many seabirds. Our study describes qualitatively and quantitatively kleptoparasitism of gulls on terns and compares the effect of prey quality (prey type and size) and environmental conditions (tide, wind direction and intensity) triggering such behavior. The rate of kleptoparasitism by Kelp Gulls (Larus dominicanus) was higher on Royal (Thalasseus maximus) than on Cayenne Terns (T. sandvicensis eurygnatha). However, the percentage of successful attacks on both species was similar (∼42%). We used an information-theoretic approach to determine the relative importance of prey quality and environmental conditions in triggering kleptoparasitism. We found that more valuable prey triggered kleptoparasitism whereas the environmental conditions included in the models didn't affect the rate of such behavior significantly. Our study shows the importance of prey quality in triggering kleptoparasitism and how this behavior can remove an important portion of food brought to the tern colony.

Anthropogenic subsidies are used by a variety of predators in areas developed for human use or residence. If subsidies promote population growth, these predators can have a negative effect on local prey species. The Glaucous Gull (Larus hyperboreus) is an abundant predator in northern Alaska that is believed to benefit from garbage as a supplemental food source, but this supposition has never been tested. In summer 2008 and 2009, we recorded the Glaucous Gull's diet and reproduction at 10 breeding colonies in northern Alaska. Colonies were in industrial, residential, and undeveloped areas and ranged from 5 to 75 km from the nearest landfill. By colony, garbage occurred in zero to 85% of pellets and food remains produced during the chick-rearing period, and the average number of chicks fledged per pair ranged from zero to 2.9. Random-forest analysis indicated that percent occurrence of garbage in the diet was the second most important factor (after number of eggs per pair) explaining variance in fledging rate. There was a significant positive correlation between percent occurrence of garbage in the diet and fledging rate in each year. If this correlation reflects a causal relationship, it suggests that human development that increases gulls' access to garbage could result in increased local gull populations. Such an increase could affect the gulls' natural prey species, including at least 14 species of shorebirds and waterfowl of conservation concern.

Patch-occupancy models offer a realistic approach to monitoring populations of nocturnal owls. However, because most owls are relatively rare, increasing the probability of detecting an owl at an occupied site will make estimates of occupancy more precise. We investigated the influence of temporal, biological, and environmental factors on rates of detection of forest owls in southeastern Alaska, 2005–2006. Following MacKenzie et al. (2006), we modeled probabilities of detection of the Northern Saw-whet Owl (Aegolius acadicus), Western Screech-Owl (Megascops kennicottii), and Barred Owl (Strix varia). We conducted 479 point counts over 100 days and detected owls 147 times. Sound broadcast increased detections 21–86% over silent surveys. During peak detection (9 April–8 May), probabilities (SE) of detection were 0.39 (0.13) for the Western Screech, 0.44 (0.16) for the Saw-whet, and 0.54 (0.25) for the Barred. For the Barred and Saw-whet, estimated occupancy probabilities (ψ;) were constant (i.e., did not vary with covariates), but for the Western Screech, ψ; was a function of whether large owls had been detected at a site, with estimated ψ; about 66% lower at sites with large owls. For the Western Screech detection probability increased after sunset. For the Barred, the pattern of detection probability was nonlinear in relation to time after sunset, being high near sunset and late at night. For the Saw-whet detection probabilities were most influenced by weather covariates, primarily precipitation and wind. We provide recommendations on allocating survey effort and increasing probabilities of detection of these three owls in southeastern Alaska.

Barred Owls (Strix varia) are highly vocal and perform a diverse array of vocalizations. They are often monitored by acoustic surveys, yet Barred Owl vocalizations and vocal behavior are poorly described. We present a detailed analysis of Barred Owl vocal behavior with four goals: (1) to provide a quantitative description of Barred Owl vocalizations, including those given within duets, (2) to examine diel variation in vocal output for multiple vocalizations, (3) to explore the use of vocalizations inside and outside of duets, and (4) to identify sex-specific vocal characteristics and discuss their utility in identifying an owl's sex. Adult Barred Owls produced 13 distinct vocalizations that could be assigned to call type on the basis of fine-structural measurements. Barred Owls vocalized throughout the day but were more vocal at night with peaks in vocal activity from 18:00 to 20:00 and 02:00 to 05:00. Most calls were produced throughout the night, but some (e.g., alarm calls) were more common at particular times. Two types of vocalizations were produced only within duets, and one type of vocalization occurred primarily outside duets. Calls of females were higher in pitch with longer terminal notes and more vibrato than those of males. Using these differences, cluster analysis assigned owls to two groups corresponding to predicted sex with 91% accuracy. We discuss possible functions of certain vocalizations and how understanding the vocal repertoire and sex differences of this species' acoustic signals will benefit behavioral studies and monitoring, including Spotted Owl (Strix occidentalis) conservation.

The White-throated Barbtail, Premnoplex tatei, is a poorly known furnariid endemic to the northeastern mountain ranges of Venezuela. Although currently considered a distinct species, it has often been treated as a subspecies of its widespread congener, the Spotted Barbtail, P. brunnescens. Here we use mitochondrial and nuclear DNA sequences to assess the taxonomic status of P. tatei and its phylogenetic relationships. We found P. tatei to be the sister species of P. brunnescens, but the genetic divergence between the two species is large in comparison to that within other genera of the Furnariidae. This result is consistent with the plumage, vocal, and ecological differences used to support the recognition of P. tatei as a distinct species. Our results also corroborate previous studies in finding Premnoplex and Margarornis as sister genera. A close relationship between P. tatei and Roraimia adusta, a scenario hypothesized from the systematic affinities of taxa distributed in the northeastern mountain region of Venezuela and the Pantepui region, was not supported by this study.

In a first attempt to formally test competitive interactions among army-ant-following birds, we explored the patterns of occurrence of birds following swarms of army ants in the Atlantic rain forest of southeastern Brazil. We contrasted the frequency of occurrence of birds at ant swarms at a lowland and a highland site. Additionally, we assessed the patterns of co-occurrence of bird species at ant swarms, using null models to test for the possibility of competitive exclusion. Despite the larger number of attending bird species at the highland site, we detected no differences between the sites in either the frequency of attendance or the number of bird species per swarm. Analyses of co-occurrence did not reveal evidence of negative association of bird species attending ant swarms at the lowland site but did at the more species-rich highland site. Pairwise analyses showed that this negative association invariably involved two of the species attending ant swarms most frequently, Pyriglena leucoptera and Trichothraupis melanops, suggesting a competitive interaction. Possibly, although birds that are more dependent upon army ants avoid competitive exclusion by spatial segregation at the swarm's front, birds that follow army ants less frequently are occasionally excluded from the swarms.

The activation and maintenance of the immune system demand a cost of nutrition and energy, and this demand may result in a trade-off with other energetically demanding activities. To test for such a trade-off, in the cerrado of central Brazil, we compared the immune profile and body condition of the resident Plain-crested Elaenia (Elaenia cristata) in the rainy nonbreeding season with that in the dry breeding season. Then we compared the immune profiles and body conditions of the Plain-crested Elaenia and Lesser Elaenia (Elaenia chiriquensis), a migrant, during the breeding season. Finally, we compared the immune profiles of the Plain-crested Elaenia at four stages of molt. The Plain-crested Elaenia's immunosuppression profile was typical (decreased lymphocyte number, increased heterophil number, and H:L ratio) during the dry season but not at other seasons. Condition measured as a body-mass index did not vary significantly by season. The Plain-crested Elaenia's immunosuppression profile did not change within the breeding season, but the Lesser Elaenia's did. We found weak correlations between immune profile and body condition in both species. The immune profile of the Plain-crested Elaenia did not vary significantly by stage of molt, but it did from the wet to the dry, potentially energetically stringent season, possibly because of shortage of food. Variation in the immune profile of the Lesser Elaenia is apparently associated with migration. This study is the first to evaluate the immune profile of a wild passerine throughout the year and to compare congeneric species that differ in their migratory status.

We analyzed the structure and variation of the songs of the Brownish-flanked Bush Warbler (Cettia fortipes), a species common in southeastern Asia, including southwestern China, site of our study. During the breeding season of 2009, we investigated the possibility of distinguishing individuals by song. Most Brownish-flanked Bush Warblers we studied had a unique song repertoire composed predominantly of two song types. Whether singing spontaneously or in response to playback, the birds deliver the two types alternately. We defined type alpha as a song consisting of two notes, type beta as a song consisting of three notes. Both song types varied from individual to individual. Discriminant analysis revealed that the songs of individual Brownish-flanked Bush Warbler songs were distinct; rates of correct classification were 98% for the alpha song and 99% for the beta song.

Nesting structures are important for successful reproduction in most birds, and, because of this, geographic variation in nest morphology and composition are usually interpreted as adaptations to breeding in different environments. We compared the structure of nests of Yellow Warblers (Dendroica petechia) breeding in Churchill, Manitoba, and Elgin, Ontario, Canada. Churchill is subarctic in habitat and typically much colder during the breeding season than Elgin. We compared temperature, rainfall, and wind speed at these two sites and then tested whether differences in nest structure corresponded to different environments. Yellow Warblers breeding in Churchill built larger, less porous nests that retained heat better but also absorbed more water and took longer to dry than Yellow Warbler nests from Elgin. We suggest that differences in the structure of Yellow Warbler nests represent adaptations to breeding in different environments because the differences in nest morphology and properties of heat retention and water loss correspond to differences between the sites in environmental challenges.

Swainson's Warbler (Limnothlypis swainsonii) breeds in bottomland hardwood forests across the southeastern United States, where it is believed to be one of the rarest breeding songbirds. Although information on its nest-site habitat is considerable, little is known about its foraging habitat except that the species is insectivorous, with a large bill used to flip fallen leaves on the forest floor. We captured Swainson's Warblers and flushed their crops to determine their diet and sampled leaf-litter arthropods and vegetation at each location of capture. We compared the proportion of arthropod orders in the crop samples to the proportion of arthropods collected in the leaf litter to determine the warbler's prey in proportion to its availability. Although Acari (mites and ticks) and Chilopoda (centipedes) were the most abundant arthropods in the leaf-litter samples (51% and 18%, respectively), these orders rarely occurred in the warblers' crops. Conversely, Araneae (spiders) and Coleoptera (beetles) were uncommon in leaf-litter samples (2% and 5%, respectively) but were the most abundant arthropod orders in the warblers' crops. Binary logistic regression with presence or absence of Araneae as the response variable and habitat measures as the predictor variables revealed that the probability of spiders occurring in the leaf litter increased as leaf-litter depth increased. To promote foraging habitat for Swainson's Warbler, deep leaf litter should be maintained by maintaining patches of closed-canopy forests and restoring natural regimes of flooding.

Even though it is widely acknowledged that mate choice in female songbirds is likely to be based on multimodal cues such as those encoded in audiovisual song displays, procedural difficulties have limited studies of mate choice in songbirds mainly to acoustic signals. In the current study, we used audiovisual recordings of male Brown-headed Cowbirds (Molothrus ater) to test the sexual responses of females to males' song displays. Using the copulation-solicitation displays of estradiol-treated females (n = 9), we found that audiovisual playbacks of males performing wing-spread song displays were significantly more sexually stimulating for females than presentations of song accompanied by video of nondisplaying males, or song alone. Results from this study clearly showed that the visual stimulus of the wing-spread display in particular, not just the visual stimulus of a conspecific male in general, elicits the maximum observed response from females. In the Brown-headed Cowbird, females' sexual preferences for song as revealed by studies of copulation-solicitation displays have been shown to parallel males' mating success, so our results likely reflect an effect of visual displays on the mating success of wild males.

Seasonal migration of a bird from lowlands to montane habitats should induce an increase in its hematocrit value. This relationship has been studied mainly in domestic species, however, and data on wild birds are mostly correlational. We report our results on variation in hematocrit values of the Citril Finch (Serinus citrinella), which migrates altitudinally. Data were taken from wild birds trapped within the same season at different altitudes and from an experiment in which Citril Finches were transported to different altitudes (600 to 2000 m above sea level). On average, hematocrit values from Citril Finches trapped at higher altitudes in the wild were 3% higher than those of birds collected at lower levels. In the experiment in which Citril Finches were transported to different altitudes, individual birds were measured repeatedly, so that the same individuals acted alternately as controls or experimental subjects. Hematocrit values of experimental and control individuals did not vary initially but varied over the course of the experiment, decreasing when birds were moved to lower altitudes and increasing when they were moved to higher altitudes by an average of 6%. These results indicate that wild birds that migrate altitudinally must contend with an increase in hematocrit values. Transporting the birds to altitudes within the species' range allowed us to exclude the possibility that the increase in hematocrit from lower to higher altitudes was the result of the flight itself and demonstrates an adjustment in hematocrit not induced by the exercise of flight.