Wilsonia Citrina Research Articles

Implications of Vagrant Southeastern Vireos and Warblers in California

ABsTRAc.-An unprecedented influx of vagrant vireos and wood-warblers into California occurred in the spring and summer of 1992. The seven species involved (White-eyed Vireo [Vireo griseus], Yellow-throated Vireo [V. flavifrons], Northern Parula [Parula americana], Yellowthroated Warbler [Dendroica dominica], Worm-eating Warbler [Helmitheros vermivorus], Kentucky Warbler [Oporornis formosus], and Hooded Warbler [Wilsonia citrina]) all breed primarily in the southeastern United States, suggesting a common factor for the influx. Furthermore, all seven species have been recorded in California with increasing frequency over the past two decades, suggesting a common trend in the populations of these species. We examined five hypotheses that could explain the increasing number of California records of these species: (1) observer bias, (2) pesticide-caused misorientation, (3) shifts in winter range, (4) anomalous weather conditions (including global warming and El Ninlo-Southern Oscillation), and (5) range expansions into the western United States and/or dramatic population increases in the southeastern United States. We found little or no support for the first three hypotheses. Anomalous weather conditions probably accounted for the magnitude of the 1992 influx, but the final hypothesis is needed to explain the overall trend. Although available data are not sufficient to distinguish between summer distributional shifts and population increases within the normal breeding ranges of these species, the possibility of westward range expansion is intriguing. Received 15 March 1996, accepted 25 June 1996.

DNA fingerprinting reveals female preference for male parental care in Savannah Sparrows.

According to sexual selection theory, females choose mates to ensure access to high quality resources, male parental care, or good genes. This last hypothesis has been hotly debated on both theoretical and empirical grounds. In contrast, female preference for male parental care has received less attention, primarily because the potential benefits of paternal effort seem obvious. The fitness relations are less clear in double-brood species, however, because females can base mating decisions on their prior experience with male parental care. Here, the extent of male parental care delivered to first-brood offspring may indicate male genetic quality and/or be the target of female manipulation via her subsequent mating fidelity. In Savannah Sparrows (Passerculus sandwichensis), multi-locus DNA fingerprinting of 203 adults and young revealed substantial female infidelity in first and second broods: overall, 24 of 80 first-brood young (30%) and 13 of 80 second-brood young (16.3%) were the product of extra-pair fertilizations. Among 12 females altering fidelity between broods, absolute male feeding rate to first-brood offspring was a strong, positive predictor of change in female fidelity. Because the extent of male parental care reflects a male's viability, the data support a 'good-genes' interpretation.

Fledgling Adoption in Hooded Warblers (Wilsonia citrina): Does Extrapair Paternity Play a Role?

Adoption is a widespread phenomenon in birds and generally occurs at a low frequency within a species (reviewed in Rohwer 1986, Meek and Robertson 1991). Adoption of fledglings is common in seabirds due to the mobility of chicks soon after hatch (e.g. Hebert 1988, Morris et al. 1991, Pierotti 1991, Brown et al. 1995). In songbirds, studies of adoption have focussed on the behavior of replacement males during the nestling stage (Meek and Robertson 1991). Little is known about fledgling adoption in songbirds because the fledgling period is poorly described for many species (Smith 1978, Moreno 1984). We report two cases of fledgling adoption in the Hooded Warbler (Wilsonia citrina) observed in the course of an intensive study of fledgling care (Evans Ogden 1994). In passerines, the main hypotheses for the adaptive significance of adoption by males are: (1) increased opportunities for fathering offspring with the offspring's mother in the future (Power 1975, Rohwer 1986); and, (2) as a result of extrapair matings, the adopting bird may be the genetic parent of the young it adopts (Meek and Robertson 1991). The first hypothesis is supported by comparative evidence that adoption tends to occur in species where females are likely to renest with the adoptive male (Rohwer 1986). Recent reports that extrapair fertilizations occur at high frequency in many Temperate Zone passerines (e.g. Westneat 1987, 1993, Morton et al. 1990, Stutchbury et al. 1994) indicate that the extrapair-paternity hypothesis for adoption may be more important than previously recognized. We used DNA fingerprinting of Hooded Warblers to examine whether extrapair paternity played a role in adoption. Our study was conducted from May through August 1991-1993 in Crawford Co., Pennsylvania (41?N, 79?W). The study site is a 150-ha continuous hardwood forest that supported about 40 breeding pairs of Hooded Warblers each year. Adults were captured with mist nets, banded with U.S. Fish and Wildlife aluminum bands, and individually color banded. Each brood member that fledged also was banded with an aluminum band, and all nestlings within a brood were given color-band combinations different from those of other broods. For most adults and nestlings, we collected 30 to 100 uL of blood for use in parentage analysis. Multilocus DNA fingerprinting (with Jeffrey's probe 33.16) was used to determine actual paternity of the adopted fledgling (for detailed methods, see Stutchbury et al. 1994). Unrelated adults had a band-sharing coefficient of 0.301 ? 0.017 (n = 22), so young were considered unrelated to their social father if they had a band-sharing coefficient of less than 0.42 and had more than two novel bands when compared with their social parents. Territories of color-banded males were mapped by following singing individuals and noting border disputes. The social father was the male that defended the territory at the time of egg laying and fed the young at nests on the territory. The social mother was the female that incubated the eggs and fed the young. Each year, we systematically attempted to locate family groups after fledging to determine the period of fledgling care and food-delivery rates to fledglings. In Hooded Warblers we rarely saw both parents feeding a given fledgling. Instead, the brood was usually divided and each parent assumed full care of one or two fledglings (Evans Ogden 1994). When the social mother attempted a second brood, the male assumed full care of all fledglings from the first brood. In the forested habitat, it was difficult to see young fledglings and determine color-band combinations to confirm the nest from which they fledged. To identify whether or not a parent had adopted fledglings, we used only those cases where adults were seen feeding color-banded fledglings on more than two occasions. Using these criteria, we observed fledgling care by 17 parents from 13 different families. Two of these 17 parents (12%) were observed to adopt young (i.e. they fed young on their own territory that had fledged from a neighbor's nest). Below we describe the two case histories in detail. The cases were similar in that the social father of the fledglings was no longer present on his territory at the time of adoption, and the social mother had renested to attempt a second brood. The second case was perhaps unnatural, because the social father died accidentally during handling early in the nestling stage. In the first case, a fledgling was fed by a neighboring male on the adjacent territory, beginning when the fledgling was 28 days old postfledging. At this time, the adoptive male was also feeding two of his own young (nine days postfledgling) on his territory.

Nest Concealment and Predation in Hooded Warblers: Experimental Removal of Nest Cover

-It is commonly assumed that passerine birds with open nests reduce the risk of predation by concealing their nests. At a forest study site in northwestern Pennsylvania, about 50% of the Hooded Warbler (Wilsonia citrina) nests were depredated-the primary cause of reproductive failure. To test the hypothesis that concealed nests are less likely to be depredated than conspicuous nests, we examined seven characteristics of the warbler's nest and nest microhabitat in relation to nest predation: nest visibility, vegetation density surrounding the nest, height of nest, height of nest substrate, nest-substrate species, height of cryptic dead-leaf base of nest, and proximity of nest to a microedge. Successful and depredated nests (n = 97) did not differ in any of these variables. In addition, we performed a vegetationremoval experiment in which manipulated nests (n = 15) with surrounding vegetation removed were on average 86% more visible overall than control nests (n = 15). The highly visible manipulated nests did not suffer higher predation than control nests. Contrary to our hypothesis, our results show that nest concealment is not an important factor in predator avoidance for Hooded Warblers. This outcome would be expected if nest predation is the result of nonspecialist predation that occurs by chance and is based on the location of available food in general. Received 19 September 1994, accepted 25 April 1995. THE MAJOR FACTOR influencing reproductive success of open-nesting birds is nest predation (Skutch 1949, Nice 1957, Ricklefs 1969, Gates and Gysel 1978, Martin 1992). Consequently, there should be selective pressure for birds to place their nests in sites that deter predation. It is clear that birds select nest sites nonrandomly (e.g. Peterson and Best 1985, Bekoff et al. 1987), and there is evidence that they can identify nest sites with characteristics that reduce the risk of predation (Stauffer and Best 1986, Martin and Roper 1988, Marzluff 1988, M0ller 1988). Nest concealment and characteristics of nest microhabitat, such as vegetation density and height I Present address: 3662 Kinter Hill Road, Edinboro, Pennsylvania 16412, USA. of nest from the ground, have been related to predation risk (Caccamise 1977, Murphy 1983, Martin and Roper 1988, Yahner and Scott 1988, Kelly 1993, Martin 1993) and, thus, could serve as criteria in nest-site selection. Although concealment of a nest by selecting an inconspicuous site is commonly assumed to reduce the risk of predation, there have been conflicting results in testing this hypothesis. Martin (1992) concluded in a review of 56 studies that dense foliage reduces the probability of predation by concealing a nest. However, of the 11 studies on passerines, 7 reported a negative correlation between predation and nest concealment, and 4 reported no correlation. Pertinent research, including those studies reviewed by Martin (1992), was conducted by Nice (1937), Nolan (1978), Martin and Roper

A Hooded Warbler Record in the Eastern Qu’Appelle Valley

A Hooded Warbler Record in the Eastern Qu’Appelle Valley

Does Male-like Coloration of Female Hooded Warblers Increase Nest Predation?

We tested the hypothesis that male-like coloration of female birds increases the risk of nest predation in a sexually dimorphic songbird, the Hooded Warbler (Wilsonia citrina). Female Hooded Warblers vary greatly in the extent of the black hood, ranging from no black in yearlings to an almost complete black hood in some older females. However, the frequency of nest predation did not vary significantly with extent of male-like plumage during the egg stage, nestling stage, or over all nest stages. In addition, yearling females whose crown and throat was dyed black did not suffer a higher nest predation rate than unmanipulated and control yearlings. We found no evidence that breeding experience of yearling versus older females affects nest predation rate. In addition, yearling females did not tend to build more visually conspicuous nests. These results strongly suggest that predators did not use conspicuous plumage coloration as a cue for locating nests.

Individual recognition of song repertoires in two wood warblers

We examined recognition of individual neighbors in hooded warblers (Wilsonia citrina) and Kentucky warblers (Oporornis formosus) by comparing responses to playbacks of neighbors' songs from appropriate and inappropriate boundaries of the subjects' territories. Kentucky warblers, which sing one song type each, responded more strongly to the songs of neighbors broadcast from incorrect than from correct boundaries. Hooded warblers use their repertoires of three to nine song types in two distinct modes of singing: repeat mode involves repeated presentation of one song type; mixed mode involves presentation of two or more song types in irregular sequence. Playbacks of neighbors' mixed-mode songs from appropriate and inappropriate boundaries indicated capabilities of individual recognition similar to those reported previously for repeat-mode songs (Godard 1991) and to those of Kentucky warblers. Repertoires of moderate size, therefore, have no pronounced influence on eventual recognition of individual neighbors in hooded warblers.

Symposium: Brood parasitism

Brown-headed Cowbirds Molothrus ater are very social birds. Males have a rich vocal repertoire, and much of their social behavior is mediated by vocalizations. For example, subtle differences in song reflect dominance status and are used by females during mate choice. Differences in song result from feedback from social interactions, and, in the absence of normal social feedback, subordinate males may produce inappropriate dominant-like songs (1) and above-normal testosterone levels (2). Another vocalization, the flight whistle, is equally interesting. It occurs in dialects, and can be used in long distance communication. In some populations parts of the flight whistle are produced antiphonally, probably to designate specific recipients. While the importance of intraspecific vocalizations to cowbird biology is well-established, surprisingly little attention has been given to the possibility that cowbirds also may attend to host vocalizations, which could act as one cue in locating nests. Given that the most frequently parasitized cowbird hosts are highly vocal oscines, it is reasonable that cowbirds could use host vocalizations to discover nests (cf. 3). Preliminary data suggest that, in one host species, median vocal rates are higher in parasitized than in unparasitized pairs (UYEHaRA, unpubl.). Further, some birds produce vocalizations that are associated with nest-building, and cowbirds may use these to detect nests. For example, female Hooded Warblers Wilsonia citrina produce a particular vocalization during nest construction. Hooded Warbler nests were heavily (20/25 nests) and selectively parasitized in one songbird community, probably due to the parasite's use of the host's nest-building vocalization (STuTCHBUt~Y, unpubl.). Other females have nest departure calls (4) that could also be used to discover nests. Another host vocalization that could be used to locate nests are nestling distress calls. These are produced when nestlings are in grave and immediate danger, such as when in the grasp of a predator. Female M. ater quickly approach playback of tape-recordings of distress calls, and often are the only species to respond. Approaching cowbirds do not mob, nor do they vocalize. Instead, they peer quietly and intently in the direction of the playback. This may represent an attempt by female cowbirds to determine the general location of a nest that is about to be lost, and it may alert her to a territory where nest building activity (and a new opportunity for brood parasitism) may occur within a few days. Thus, it represents a new "function" for distress calls. Additional study of the cowbird use of host vocalizations is warranted.

Response of a forest-interior songbird to the threat of cowbird parasitism

Abstract. The expansion of the breeding range of the brown-headed cowbird, Molothrus ater, into eastern North America has resulted in many historically naive populations of songbirds becoming victims of brood parasitism. To test whether an eastern forest-interior songbird, the hooded warbler, Wilsonia citrina, recognizes brown-headed cowbirds as a threat to the nest, a mount of a cowbird or a control, the veery, Catharus fuscescens, was presented to 25 incubating females. Female hooded warblers were significantly more aggressive towards the cowbird than the veery model. Total time spent off the nest, rate of alarm calling and chipping, time spent within 2 m of the model, and number of close passes at the model were significantly greater for the cowbird trials. Most yearling females had moderate or strong discrimination between cowbird and veery models, and there was no significant difference between yearling and older females in the intensity of their response towards cowbirds. The ability of naive yearling females to recognize cowbirds suggests some non-learned (i.e. genetic) basis to this behaviour. Hooded warblers have developed the ability to recognize cowbirds as a threat to their nests, suggesting that host aggression can evolve relatively quickly even for non-learned behaviour.

Competition for Winter Territories in a Neotropical Migrant: The Role of Age, Sex and Color

-Hooded Warblers (Wilsonia citrina) defend long-term feeding territories during the nonbreeding season. Experimental removals of territory owners early in the fall in Mexico resulted in rapid replacement by nonterritorial individuals (floaters). The sex ratio of replacement birds was similar to that of territory owners in the same habitat, but virtually all replacements were hatching-year (HY) birds. Females were able to acquire and defend territories in the presence of male floaters. Loss of wintering habitat would likely increase the proportion of nonterritorial birds and, therefore, increase overwinter mortality rates, especially of HY birds. The extent of malelike coloration of females was not correlated with habitat, indicating that dark females were not more likely to obtain territories in forested habitat where males predominate. Furthermore, the aggressive response of male territory owners toward models did not depend on intruder color. These results suggest that malelike coloration in females does not function in competitive interactions with males. Intense competition for winter territories coupled with reduced survivorship of winter floaters could constrain breeding strategies if young produced late in the summer have little chance of obtaining winter territories. Received 5 April 1993, accepted 30 September 1993. NEOTROPICAL MIGRANT songbird populations have declined in recent years (Askins et al. 1990). Habitat loss and fragmentation in the tropics is a major potential cause of this decline (Robbins et al. 1989). If habitat loss has a detrimental effect on wintering populations, then densities must be high relative to the carrying capacity of the habitat (Winker et al. 1990). Although many migrants are territorial on their wintering grounds (e.g. Greenberg 1986, Holmes et al. 1989), little is known about the intensity of competition for those territories (Morton et al. 1987, Winker et al. 1990) or the effects of such competition on population dynamics (Marra et al. 1993). When resources become limiting, are females at a disadvantage in competing for territories with males? Do yearling birds have difficulty obtaining territories? Age or sex biases in competitive ability for winter territories could have important implications for overwinter survival and productivity on the breeding grounds (Marra et al. 1993). The existence of nonterritorial populations on the wintering grounds has been well documented only for the Wood Thrush (Hylochichla mustelina; Rappole et al. 1989, Winker et al. 1990) and American Redstart (Setophaga ruticilla; Marra et al. 1993), and has been reported anecdotally for the Blackthroated Blue Warbler (Dendroica caerulescens; Holmes et al. 1989) and Hooded Warbler (Wilsonia citrina; Rappole and Warner 1980, Morton et al. 1987). Experimental removal of territory owners is an important tool for studying floating populations (e.g. Krebs 1971), but has been performed on the wintering grounds for only a few species (Rappole and Warner 1980, Morton et al. 1987, Winker et al. 1990, Marra et al. 1993). I conducted removal experiments with wintering Hooded Warblers to determine: (1) the size of the nonterritorial floater population; and (2) the relative abilities of birds in particular age/ sex classes to obtain territories. The winter ecology of the Hooded Warbler is among the best studied of any Neotropical migrant (Rappole and Warner 1980, Lynch et al. 1985, Morton et al. 1987, Morton 1990, Morton et al. 1993). There is habitat segregation by sex, such that most birds in the forest are males; however, both males and females are found in scrub habitat (Lynch et al. 1985). When territorial males are removed from forest habitat, females in adjacent scrub habitat do not move onto those territories even when such territories remain undefended, indicating that females are not being excluded from forest habitat (Morton et al. 1987). Experiments with hand-raised individuals and field data suggest that habitat segregation results from innate habitat preferences by each sex (Morton 1990, Morton et al. 1993). During winter females must compete directly

Hooded Warbler (Setophaga citrina)

We examined nestling paternity, fate of nests, and prior experimental manipula- tions in Hooded Warblers (Wilsonia citrina) to test the hypothesis that those factors influenced the warblers' fidelity to their previous breeding site (general breeding area), territory (specific nesting and feeding area within general breeding area), and mate the following year. Field re- search was conducted at Hemlock Hill Biological Research Area in northwestern Pennsylvania from 1991 to 1998. Hooded Warblers returned to Hemlock Hill at an average annual rate of 47% over seven years. Significantly more males than females returned in 1992 and 1995, but overall difference between male and female return rates was not significant. Frequency of males re- turning to their former territory (58%) was greater than frequency of females returning to their former territory (29%). When mates from the previous year returned, remated pairs (n = 13) oc- cupied their former territory more often than they occupied a new territory. Average number of young fledged per nest tended (P = 0.06) to be greater for females that returned to Hemlock Hill than for females that did not return. There was also a tendency (P = 0.09) for females that returned to the breeding site to have more nests with extrapair young than those that did not return. Territory fidelity and mate fidelity for both males and females were not related to prior nest fate or paternity of young. During those years, we conducted two relatively long-last- ing experimental manipulations: vegetation removal from nest sites and radiotagging. Those experimental manipulations did not significantly affect future selection of breeding site, ter- ritory, or mate. Thus, whereas male Hooded Warblers demonstrated greater territory fidelity than females, only females were possibly affected by factors that we examined from the previ- ous year, specifically extrapair fertilization and prior reproductive success or failure. Received

Use of Two Singing Modes By Hooded Warblers as Adaptations for Signalling

Abstract Hooded warblers Wilsonia citrina use two modes of singing, repeat mode (one pattern sung repeatedly) and mixed mode (2-4 other patterns sung in irregular sequence). Intensive focal-individual sampling of 14 males documented the use of these modes of singing throughout the nesting cycle. Males of different ages (first breeding season or later) did not differ in use of the two modes. Time spent singing in repeat mode decreased markedly after acquiring a mate, but time spent singing mixed mode did not change significantly across stages of the nesting cycle. Males sang more when their neighbors sang at a distance of 25 m or more. Repeat mode increased in this situation before a male acquired a mate, while mixed mode increased afterwards. Near a neighbor (within 25 m), males avoided use of repeat mode. Nearby females before the onset of incubation evoked increased use of repeat mode. More distant, calling females elicited little response before incubation, but thereafter calling females tended to suppress all singing. Males used mixed mode proportionately more in locations nearer neighbors. There were no indications that variation in singing influenced the dates on which males acquired mates. Unmated males late in the breeding season sang persistently in repeat mode, even more than eventually mated males had early in the season before they acquired mates. These results provide support, with some reservations, for three hypotheses for the evolution of distinct modes of singing: (1) specializations for male and female listeners; (2) specializations for indicating conditional behavioral tendencies; and (3) specializations for communication in low- and high-noise situations. These hypotheses are not mutually exclusive, and all three in combination might offer the strongest explanation for the evolution of distinct singing modes in this species and other paruline warblers.

Extrapair paternity in hooded warblers

Extrapair paternity in hooded warblers

Tit for tat among neighboring hooded warblers

The “dear-enemy” relationship of territorial songbirds could be mutually beneficial to neighbors, as males who recognize neighbors and reduce their responses to these neighbors would require less time and energy for territorial defense. In order for this relationship to be evolutionarily stable, this reduction in response to a neighbor must be conditional on reciprocal restraint by that neighbor. This study examined the possibility of such conditional responses in hooded warblers (Wilsonia citrina). Responses of territorial hooded warblers to playbacks of neighbors' songs from shared boundaries were measured before and after playbacks that simulated intrusions of those same neighbors (NNNN treatment) or strange birds (NSSN treatment) into the subjects' territories. Each male received both treatments separated by at least 8 days. Males increased their responses to playbacks of a neighbor's songs at the boundary after simulated intrusions of that same neighbor (NNNN) but did not increase their responses to such playbacks after simulated intrusions of strangers (NSSN). This increased response to a “defecting” neighbor suggests that the relationship between neighboring territorial hooded warblers is based on a conditional strategy like tit-for-tat.

How a warbler chooses its habitat: field support for laboratory experiments

Abstract. During the non-breeding season hooded warblers, Wilsonia citrina, defend individual territories and segregate by sex into different habitats. Previous laboratory trials suggested that choice was based upon the verticality of plant features: males prefer habitat with more stems growing vertically, whereas females prefer vegetation predominated by oblique angles. To test the proximate cues used by hooded warblers to assess habitat suitability, the effect of hurricane disturbance on the distribution of the sexes was investigated. Hurricane Gilbert crossed the northern portion of the Yucatan Peninsula, Mexico in mid-September 1988 with 320 km/h winds. The abundance and distribution of each sex was compared before and after the storm. After the storm females appeared in forests previously occupied only by males. Analysis of the angles, size, and density of vegetative stems showed that females were found on territories where the verticality of the vegetation was reduced compared with the verticality of vegetation in territories still defended by males. Vegetative height did not differ in male and female territories. This result supports laboratory predictions that primary cues for habitat segregation are based on the relative amounts of vertical and oblique elements in the habitat.

Long-term memory of individual neighbours in a migratory songbird

CAPABILITIES for long-term memory and recall of information have evolved in non-human animals primarily for special requirements such as for learning species-typical vocalizations and caching food1–6. Long-term memory of individual social partners has, however, not been demonstrated previously for non-human animals. The ability to recognize individuals has important consequences for the evolution of intricate social interactions7–11 and provides a basis for more sophisticated forms of cognition in animal societies12,13. Recognition of social partners has been documented for territorial songbirds, which discriminate between songs of different neighbours14–16 as well as between the songs of strangers and neighbours17. Here I show that male hooded warblers ( Wilsonia citrina, Parulinae) not only recognize their neighbours individually by song during the breeding season, but also retain the memory of neighbours' songs after an 8-month period during which they cease singing and migrate to Central America before they return to former breeding territories.

Habitat Segregation by Sex in the Hooded Warbler: Experiments on Proximate Causation and Discussion of Its Evolution

Although habitat choice has been shown to have an innate basis in some vertebrates, few experimental studies have been performed, and these most often relied on interspecific comparisons. Male and female hooded warblers (Wilsonia citrina) defend territories in markedly different habitats while wintering in the tropics. Males use forests; females use shrub or field habitats. I used naive handraised warblers in experiments designed to test for sex-related innate differences in habitat selection. Choosing between two three-dimensional artificial habitats, males chose the tall habitat with separated stems over the short habitat with dense stems, whereas females showed no preference. Field observations and a laboratory experiment with the naive birds showed no habitat preferences based on light intensity. Finally, naive warblers were exposed to a two-dimensional array of black stripes on opposite walls of an experimental room, one wall with the black stripes arranged vertically and the other with the stripes arranged at oblique angles to the floor. Males oriented toward the vertical stripes, whereas females oriented toward the oblique stripes. Since forests have more vertical structures than shrubby fields, an innate preference for cues based on vertical or oblique cues is a sufficient proximate mechanism to bring about the observed habitat segregation between the sexes in wild birds. I discuss possible sources of selection favoring the evolution of intraspecific habitat segregation by sex. A hypothesis is presented to explain the presence of a subadult female plumage and a highly variable adult female plumage as they relate to habitat segregation in the hooded warbler. I suggest that this unusual plumage variability in females resulted from changes in the habitat available to females after swidden agriculture began in their main wintering area.

Stopover and fat deposition by North American wood-warblers (Parulinae) following spring migration over the Gulf of Mexico.

Length of stopover and rate of weight gain (fat deposition) were studied in several species of passerine birds that stopped in southwestern Louisiana along the northern coast of the Gulf of Mexico after a trans-Gulf flight. Fatdepleted birds were more common among the birds that arrived at our study site in southwest Louisiana, though variability characterized our samples. Migrants that landed after encountering opposing winds or rain over the northern Gulf of Mexico were, on average, fatter than migrants that landed when weather was favorable for continued migration. Some of the variation in the energetic condition of arrivals may be explained by the location where migrants initiated crossings. Our simulation of flight over the Gulf of Mexico showed that with following winds a warbler can cross the Gulf of Mexico from Yucatan with fat reserves to spare, and stronger tailwinds make flights from as far south as Honduras energetically permissible. The length of stay after a trans-Gulf flight was related to the extent of fat-depletion upon arrival: lean birds stayed longer than fat migrants. Migrants stopped over for 1-7 days and replenished energy reserves at rates that varied from 0.19 g/d for Hooded Warblers (Wilsonia citrina) to 0.87 g/d for Ovenbirds (Seiurus aurocapillus). Within each species, most individuals gained weight at a rapid rate, though a few individuals lost or maintained weight during their stay.

Do Male Hooded Warblers Exclude Females from Nonbreeding Territories in Tropical Forest?

Do Male Hooded Warblers Exclude Females from Nonbreeding Territories in Tropical Forest?

The Hooded Warbler (Wilsonia citrina) in Ontario

The Hooded Warbler (Wilsonia citrina) in Ontario