Michael T. Murphy

Temporal Components of Reproductive Variability in Eastern Kingbirds (Tyrannus Tyrannus)

The influences of weather and food supply on Eastern Kingbird (Tyrannus tyrannus) reproduction were examined using data on annual (1980-1983) and seasonal variation in timing of breeding, clutch size, and egg mass in a population of kingbirds breeding in eastern Kansas, USA. Timing of breeding differed significantly among years and appeared to be related to insect abundance. Clutch size declined significantly with breeding date in all years, but differed significantly only between 1980 and 1981. A severe drought in 1980 likely depressed food availability, whereas an emergence of periodical cicadas in 1981 provided a superabundant food source. Across all years, however, clutch size varied inversely with the mean breeding date of the population, whereas average egg mass varied directly with food availability. Maximum annual differences in mean clutch and egg size were small compared to variation in food availability. I examined 10 hypotheses that have been proposed to explain seasonal changes of clutch size in birds. All but three were rejected as possible explanations for kingbirds. The tendency of older females to lay both earlier and larger clutches is probably responsible for most of the seasonal decline of clutch size, but reduced probability of survival for offspring fledged late in the breeding season and avoidance of stress in adults just prior to migration may also be contributing factors. Reduced clutch size late in the breeding period should permit females to hatch young sooner, to raise them faster, and to fledge them at higher masses. These features should increase survival of both juveniles and adults since postfledging care is long in kingbirds.

Survivorship, breeding dispersal and mate fidelity in Eastern Kingbirds

I measured annual survivorship, breeding dispersal and mate/site fidelity in a color-banded population of Eastern Kingbirds (Tyrannus tyrannus) breeding in central New York from 1989 through 1994. I also sought to identify the factors that led to breeding dispersal and to test whether breeding philopatry was based on site or mate fidelity. Survivorship of males did not differ between the first (0.64) and all subsequent years of banding (0.73), and overall, males returned at a higher rate than females. Female survival in the first year following banding (0.68) exceeded that for all subsequent years (0.51). The difference between the sexes and between banding classes of females may be related to high costs of reproduction in older females. Breeding-site fidelity was very high (96% and 72.5% of males and females, respectively, reused former territories), but females dispersed significantly farther than males between breeding seasons. Site fidelity of males was not reduced significantly by poor nest success in the past year, and was independent of whether or not the female returned to breed. Females often dispersed between seasons after nest failures, but were even more likely to move if the male failed to return. Furthermore, 86.5% of pairs reunited when both partners were known to have returned from migration. Female philopatry thus seemed to be influenced heavily by mate fidelity. Females that renested with a former mate advanced their breeding date by about four days for every year of experience together, but they did not lay larger clutches or fledge more broods than females that nested with a new male. Reunited pairs thus made a rapid transition from migration to egg-laying.

Dawn song of the eastern kingbird: an honest signal of male quality?

To be used as honest signals of quality, phenotypic traits must vary markedly within populations and show repeatable differences between individuals. Eastern kingbirds, Tyrannus tyrannus, sing a distinctive dawn song in the predawn darkness, and one hypothesis for its function is that it serves as an honest signal of male quality that females use during choice of within-pair or extrapair mates. Over a 2-year period, we quantified dawn songs of males to measure between- and within-season repeatability of the timing (start and end time) and length of song bouts, and song rate. We also measured morphological characters to measure repeatability and describe individual differences in body size and plumage quality. All song traits varied considerably among males, start time and song rates were repeatable between years, and all song rates were repeatable within both years. All morphological characters were significantly repeatable between years. Moreover, early singing males were larger and had relatively long flight feathers, and males that sang at high rates had relatively long flight feathers. Early singing males were also paired to the earliest breeding females, and in one year, male song rate was positively correlated with their social mates clutch size. Relatively long flight feathers in other species are typical of older males and individuals of higher body condition, suggesting that, despite being suboscine passerines with relatively simple and innate songs, eastern kingbirds use song performance as an honest signal of male quality that females may use during mate choice.

HABITAT‐SPECIFIC DEMOGRAPHY OF A LONG‐DISTANCE, NEOTROPICAL MIGRANT BIRD, THE EASTERN KINGBIRD

I used data from a 10-yr study of Eastern Kingbirds (Tyrannus tyrannus) breeding in three habitats in the Charlotte Valley of central New York to describe population trends, and to examine how population dynamics varied with habitat-specific estimates of reproductive output, adult dispersal, and survival. Although breeding date was slightly earlier on the floodplain than the upland, my other comparisons showed that clutch size, egg mass, or the number of young lost to starvation or predators did not vary with habitat. However, floodplain pairs had the lowest annual productivity because significantly more of their eggs were eaten by predators during incubation. Males tended to exhibit higher survival and site fidelity than females, but in both sexes, survival was highest on the floodplain and lowest in the upland. Most dispersing individuals of both sexes moved toward the creek (significant in males). As a result of this and the moderately high survival of adults that bred along the creek, the creek population remained stable between 1989 and 1998, whereas the floodplain, upland, and total population tended to decline (2.5%/yr). Counts of kingbirds from Breeding Bird Surveys (1989–1996) conducted within 100 km of the study site also showed significant declines. Population size within the Charlotte Valley was positively associated with local adult male survival and seasonal productivity, and the declines in population size seemed to be linked most strongly to a drop in male survival during the latter half of the study. Calculation of habitat-specific population growth rates suggested that all three habitats, and thus the entire Charlotte Valley system, was a population sink whose numbers were supplemented substantially by outside immigration. However, productivity along the creek (and upland) was density dependent, suggesting also that the creek may function as a “pseudosink.” My results suggest that spatial and temporal (i.e., successional) changes in habitat quality may have led to increased permanent emigration of adults, and declines in population size. The importance of outside immigration also suggests that population dynamics can only be understood by evaluating local trends within a larger geographic context.

Nestling Eastern Kingbird Growth: Effects of Initial Size and Ambient Temperature

Growth, defined in terms of mass gain and increase in lengths of tarsus and ninth primary wing feather, was studied in nestling Eastern Kingbirds in relation to a suite of environmental variables. These data indicate that many factors influence survival probabilities and final size attained by nestlings, though initial mass (egg size) seems of primary importance. The impact of weather variables was, however, nearly as important. Mass gain throughout the first half of the nestling period depended most on initial mass on the day of hatching. During the latter half of the nestling period, variation in ambient air temperature was the variable best able to explain mass variation. Nestlings lost mass on days with high air temperatures. Nestling mortality due to heat stress was also high on hot days, but only for nestlings with poorly developed powers of thermoregulation, i.e., those < 10 d of age. Tarsal and primary lengths at fledging were directly correlated with hatching tarsal length and mass at day 14. Primary growth was also directly correlated with ambient air temperature.

Sex roles, parental experience and reproductive success of eastern kingbirds, Tyrannus tyrannus

We quantified parental behaviour of eastern kingbirds during the incubation and nestling periods to determine parental roles, and to examine the impact of previous breeding experience (defined as having bred on the territory in the past) on behaviour and reproductive success. Females performed all incubation, while males spent more than 60% of their time in vigilant or nest guarding behaviour during incubation. Parental roles were not defined as sharply during the nestling period. Females spent more time vigilant, but males provisioned young at only 54% of the rate of females. Vigilance and nest watching were still primarily male duties. Male and female behaviour did not vary with the pairs combination of experience (e.g. experienced-experienced versus inexperienced-inexperienced in previous-current breeding season, respectively) during either phase of reproduction, but experienced males were more vigilant during incubation and fed young relatively more than inexperienced males. Experienced females were also more efficient foragers. Although behaviour did not differ among the four combinations of pair experience, inexperienced pairs none the less lost the most young to starvation and predation. Consequently, inexperienced pairs fledged one less nestling per nesting attempt than did pairs with at least one experienced breeder. Our results suggest that having at least one experienced breeder substantially improved a pairs reproductive success. We propose that female site fidelity is a safeguard to avoid the lower breeding success a female would incur if she were to move to a new territory and breed with an inexperienced male. Copyright 1999 The Association for the Study of Animal Behaviour.

Reproductive correlates of spring arrival date in the Eastern Kingbird Tyrannus tyrannus

Harsh weather in spring presents energetic challenges to birds during migration and upon reaching the breeding grounds, and yet, birds often arrive well before breeding begins. We studied a population of Eastern Kingbirds in eastern Oregon from 2004 through 2007. Early arriving kingbirds faced the poorest weather conditions, and therefore we predicted that benefits of early arrival must exist to balance the presumed costs. Early-arriving kingbirds were more likely to both acquire a high-quality territory and to replace nests after failure. Early-arriving birds also bred early, and early breeding led to larger clutches and greater production of young. Early-arriving males also sired more extra-pair young than later arrivers. Our data suggest that arrival date is in part influenced by individual quality, and that arrival date has reproductive consequences, with the primary benefits of early arrival being the acquisition of a high-quality territory, early breeding, and increased probability of replacing failed initial nests.

Migration Timing and Wintering Areas of Three Species of Flycatchers (Tyrannus) Breeding in the Great Plains of North America

ABSTRACT. Descriptions of intra- and interspecific variation in migratory patterns of closely related species are rare yet valuable because they can help assess how differences in ecology and life-history strategies drive the evolution of migration. We report data on timing and location of migration routes and wintering areas, and on migratory speed and phenology, of Eastern Kingbirds (Tyrannus tyrannus) from Nebraska and Oklahoma and of Western Kingbirds (T. verticalis) and Scissor-tailed Flycatchers (T. forficatus) from Oklahoma. Eastern Kingbirds primarily departed the breeding site in September, migrating to the Amazon Basin (Bolivia and Brazil), >6,400 km from their breeding site, then used a second wintering site in northwestern South America (Colombia, Ecuador, and Peru) before returning to the breeding site in April. Western Kingbirds left Oklahoma in late July, migrating >1,400 km to northwestern Mexico, then to central Mexico and finally to Central America before returning to Oklahoma in April. Scissor-tailed Flycatchers departed Oklahoma mainly in mid-October, migrating to Central America (El Salvador, Guatemala, Honduras, and Nicaragua), ∼2,600 km from the breeding site, remaining there until early April before returning to Oklahoma. Timing of migration appears to be tightly linked to molt. Early departure of Western Kingbirds from the breeding site appears to be timed so that they molt in the Sonoran Desert region during the monsoon, whereas Scissor-tailed Flycatchers remain at their breeding site to complete molt in late summer, when insect prey are abundant. Eastern Kingbirds delay molt until reaching South America where, possibly, abundant fruit supports molt.

Parental Investment Theory and Nest Defense by Eastern Kingbirds

Abstract Nest defense represents a parental strategy to maximize fitness by enhancing prospects of offspring survival. We used a taxidermic mount of a nest predator to measure nest defense of Eastern Kingbirds (Tyrannus tyrannus) at Malheur National Wildlife Refuge in southeastern Oregon in 2003 and 2004 to test the hypotheses that intensity of nest defense (1) was individually repeatable, (2) differed between males and females, and (3) predicted nest success. We also predicted that (4) intensity of defense would increase with age and number of young, but decline over the breeding season. Intensity of nest defense was significantly repeatable for male kingbirds. Male response was twice as strong as female response during incubation and the nestling period, but nest success was independent of defense scores of males and females. Simple paired comparisons suggested female responses did not change between incubation and the nestling period, whereas males tended to defend nestlings more vigorously than eggs. Multivariate analyses demonstrated strong individual differences were the main source of variation in nest defense. Intensity of nest defense by males and females increased with age of young, declined seasonally, but was not related to number of young. Kingbird nest defense is a repeatable behavior that differs between males and females and, as predicted by parental investment theory, nests of the greatest value (older young and earlier broods) were defended most aggressively.

IS REPRODUCTION BY TREE SWALLOWS COST FREE

Abstract We manipulated brood sizes of Tree Swallows (Tachycineta bicolor) in 1996 and 1997 to test for the existence of intra- and intergenerational costs of reproduction. Modal clutch size was six eggs, but experimental brood sizes ranged from two to nine young. Nestling starvation was higher in 1996 (and dependent on brood size) than in 1997, but in both years enlargement of brood size resulted in increased productivity. Nestling mass near fledging was negatively correlated with brood size, but tarsus length and wing chord were not. Food deliveries by parents increased steadily between broods of two to six young but then remained constant between broods of six to nine young. The loss of female mass between incubation and the end of the nestling period was positively related to the pairs total feeding effort, and female mass near fledging declined with increasing brood size. The latter decline disappeared, however, when broods of nine were omitted. Adult return rate (1996 to 1997) was highest among birds that raised enlarged broods. Our results, and a review of other studies of Tree Swallows, suggest that broods of seven or eight young can be raised without costs to the parents or young, and it appears that costs associated with feeding young have not influenced annual fecundity of Tree Swallows. Rather, egg production is most likely limited by energy availability to laying females. A major cost of reproduction for Tree Swallows probably arises from nest-site competition in that early arrival in spring to obtain nest sites exposes adults to high risks of death from starvation.