Tyrannus Tyrannus Research Articles

Maternal characteristics and the production and recruitment of sons in the eastern kingbird (Tyrannus tyrannus)

Sex allocation theory predicts that if variance in reproductive success differs between the sexes, females who are able to produce high-quality young should bias offspring sex ratio towards the sex with the higher potential reproductive success. We tested the hypothesis that high-quality (i.e., heavy) female eastern kingbirds (Tyrannus tyrannus) that bred early in the breeding season would produce male-biased clutches. A significant opportunity for sexual selection also exists in this socially monogamous but cryptically polygamous species, and we predicted that successful extra-pair (EP) sires would be associated with an excess of male offspring. Although population brood sex ratio did not differ from parity, it increased significantly with female body mass and declined with female breeding date, but was independent of the morphology and display (song) behavior (correlates of reproductive success) of social males and EP sires. Male offspring were significantly heavier than female offspring at fledging. Moreover, the probability that male offspring were resighted in subsequent years declined with breeding date, and was greater in replacement clutches, but lower when clutch size was large. Probability of resighting female offspring varied annually, but was independent of all other variables. Given that variance in reproductive success of male kingbirds is much greater than that of females, and that male offspring are more expensive to produce and have a higher probability of recruitment if fledged early in the season, our results support predictions of sex allocation theory: high-quality (heavy) females breeding when conditions were optimal for male recruitment produced an excess of sons.

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. ...

Density‐dependent age at first reproduction in the eastern kingbird

Theory predicts that maximal fitness is obtained by individuals who begin to breed immediately upon reaching sexual maturity. However, delayed breeding occurs regularly in some taxa, and in birds and mammals is most often associated with long lifespan and/or limited access to suitable habitats. Delayed breeding is not expected among relatively short‐lived species such as migratory passerine birds, but this assumption remains untested in many species. Here we quantify age at first reproduction in an eastern kingbird Tyrannus tyrannus population breeding in an ecological island, and through both observational and experimental approaches, investigate the potential causes for the high frequency of delayed breeding that occurs in this population. Nearly half of the fledged nestlings that returned to the breeding grounds did not breed in their first potential breeding season. Some non‐breeders occupied territories, for at least some period, but most remained as non‐territorial ‘floaters’. Parentage analysis failed to show any reproductive success for female floaters, and only limited success for male floaters, indicating that floating was not a successful reproductive tactic. On the other hand, a strong negative relationship existed between population size and the proportion of young birds that bred in their first year, and non‐territorial birds of both sexes quickly filled territory vacancies created by experimental adult removals. Limited breeding habitat and territorial behavior of older birds thus appear to be the main causes of delayed breeding in kingbirds. The frequency of delayed breeding in most species is unknown but of potential significance because failure to incorporate accurate estimates of age at first reproduction in population models may lead to flawed population projections.

Age- and sex-dependent spring arrival dates of Eastern Kingbirds

ABSTRACT Among migratory passerines, the first birds to arrive on the breeding grounds are usually older males. Early arrival by older birds may be driven by experience, age-dependent changes in body condition, age-dependent access to resources during the nonbreeding period, or latitudinal segregation by age. Males may arrive earlier than females (protandry) because males maintain better condition due to greater access to resources during the winter, or because selection favors early arriving males that acquire the best territories or experience enhanced mating opportunities. During a 4-yr study (2004–2007) in Oregon, we found that older Eastern Kingbirds (Tyrannus tyrannus), regardless of sex, arrived nearly a week before younger birds and that males arrived about 5 d before females. Age- and sex-dependent arrival dates do not appear to be related to differences in body condition, social dominance in winter, or latitudinal segregation, and protandry is unrelated to the ability of early-arriving males to acquire high-quality territories. Instead, we propose that young birds have less to gain from early arrival because of their probable inability to displace experienced birds from prime territories and that protandry evolved due to enhanced mating opportunities for early arriving males that arise from the high rates of extra-pair matings in our population of Eastern Kingbirds. Entre los migrantes paserinos, los primeros individuos en arribar a las áreas de reproducción son usualmente los machos viejos. El arribo temprano de los individuos viejos puede estar influenciado por la experiencia, cambios en la condición corporal dependiente de la edad, acceso a recursos durante la época no reproductiva dependiente de la edad, o segregación latitudinal por edades. El arribo temprano de los machos en comparación con las hembras (protandria) existe posiblemente porque los machos mantienen una mejor condición debido a un mayor acceso a recursos durante el invierno, o puede haber evolucionado porque los machos que arriban temprano adquieren los mejores territorios o presentan incremento en oportunidades de apareamiento. Durante un estudio de cuatro años (2004–2007) en Oregon, encontramos que individuos viejos del Eastern Kingbirds (Tyrannus tyrannus), sin importar el sexo, arribaron casi una semana antes que los individuos jóvenes y que los machos arribaron al rededor de cinco días antes que las hembras. Las fechas de arribo dependientes de la edad y el sexo no parecen estar relacionados con diferencias en las condiciones corporales, dominancia social durante el invierno o segregación latitudinal y la protandria no esta relacionada con la habilidad de los machos de arribar tempranamente para adquirir territorios de alta calidad. En vez, nosotros proponemos que las aves jóvenes tienen menos que ganar con el arribo temprano debido a la probable inhabilidad de desplazar a los individuos experimentados de los habitas primarios, y que la protandria evoluciono para incrementar oportunidades de apareamiento de los machos que llegan temprano los cuales sobresalen por sus altas tasas de copula extra pareja en nuestra población de Eastern Kingbirds.

Public information facilitates habitat selection of a territorial species: the eastern kingbird

The study of breeding habitat selection has focused mainly on physical characteristics of the environment such as nest sites and food abundance, both of which are required for successful reproduction. However, individuals may have limited time to make settlement decisions, and thus, their knowledge of resource availability may be incomplete. Furthermore, food abundance during settlement may not reflect levels later in the breeding season. An alternative perspective is that the most direct measure of habitat suitability is derived from social information such as the presence of conspecifics (conspecific attraction) or their reproductive performance (habitat copying). We tested resource and socially based hypotheses of habitat selection in a population of eastern kingbirds, Tyrannus tyrannus, at Malheur National Wildlife Refuge in southeastern Oregon, U.S.A. Kingbirds nested in areas with a greater proportion of suitable nest trees than what was generally available, but many areas with suitable numbers of trees went unused. Kingbirds also settled in areas of relatively high insect abundance, but only in years when overall abundance was low. We also found support for the socially based hypotheses. Relative to where they nested previously, kingbirds tended to settle in areas of higher conspecific density. But, we found stronger evidence in favour of habitat copying, suggesting that dispersing individuals settled in areas that produced more fledglings in the previous year. Kingbirds thus appeared to select breeding habitat based, in part, on resource availability, but dispersal decisions may be influenced by social information, primarily past reproductive success of conspecifics.

Moving riparian management guidelines towards a natural disturbance model: An example using boreal riparian and shoreline forest bird communities

Forest harvesting strategies that approximate natural disturbances have been proposed as a means of maintaining natural species’ diversity and richness in the boreal forests of North America. Natural disturbances impact shoreline forests and upland areas at similar rates. However, shoreline forests are generally protected from harvest through the retention of treed buffer strips. We examined bird community responses to forest management guidelines intended to approximate shoreline forest fires by comparing bird community structure in early (1–4 years) post-burned and harvested boreal riparian habitats and the adjacent shoreline forest. We sampled riparian areas with adjacent: (1) burned merchantable shoreline forest ( n = 21), (2) burned non-merchantable shoreline forest ( n = 29), (3) 10 m treed buffer with 25% retention in the next 30 m ( n = 18), and (4) 30 m treed buffer ( n = 21). Only minor differences were detected in riparian species’ abundance and bird community composition between treatments with greater differences in these parameters occurring between post-fire and post-harvest upland bird communities. Indicators of all merchantable treatments were dominated by upland species with open-habitat species and habitat generalists being typical upland indicator species of burned merchantable habitats and forest specialists typical upland indicators of harvested treatments. Riparian species indicative of burned riparian habitats were Common Yellowthroat ( Geothlypis trichas), Le Conte’s Sparrow ( Ammodramus leconteii) and Eastern Kingbird ( Tyrannus tyrannus) and indicators of 30 m buffers were Alder Flycatcher ( Empidonax alnorum) and Wilson’s Warbler ( Wilsonia pusilla). Multivariate Redundancy Analysis (RDA) of the overall (riparian and upland birds) community showed greater divergence than RDA with only riparian species suggesting less effect of fire and forestry on riparian birds than on upland birds. Higher natural range of variability (NRV) of overall post-fire bird communities compared to post-harvest communities emphasizes that harvesting guidelines currently do not achieve this level of variability. However, lack of a large negative effect on common riparian species in the first 4 years post-disturbance allows for the exploration of alternative shoreline forest management that better incorporates bird community composition of post-fire riparian areas and shoreline forests.

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 mate's 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.

Extrapair paternity and the opportunity for sexual selection in a socially monogamous passerine

Effects of alternate mating strategies on the opportunity for sexual selection are widely debated, and recent studies have concluded that the effects of extrapair (EP) paternity on the opportunity for sexual selection may have been overstated due to 1) methodological limitations of empirical studies and 2) the potential for males to gain from additional within-pair (WP) reproductive opportunities. We therefore examined the impact of EP paternity on the opportunity for sexual selection in the socially monogamous and single-brooded eastern kingbird (Tyrannus tyrannus). EP paternity was common in all 3 years of our study (61% of 89 broods, 47% of nestlings) and realized reproductive success (EP + WP young) ranged from 0 to 9 young/male/year. A total of 31% of males lost all WP paternity (24% sired neither WP nor EP young, whereas 7% sired EP but not WP young), and variance in male realized reproductive success was more than 9 times greater than that of apparent reproductive success. Nearly half of EP mates were not nearest neighbors, and many were separated by 3 or more territories (>1000 m). EP success was independent of nest defense behavior, but early singing males and males with high song rates were most successful at both a population level and when cuckolders and cuckoldees were compared. EP paternity contributed significantly to the opportunity for sexual selection in kingbirds, and we suggest that this is probably due to the low potential for WP variation in reproductive success, apparent long-distance movements of one or both sexes, and consequent absence of reciprocal cuckoldry.

Lifetime Reproductive Success of Female Eastern Kingbirds (Tyrannus Tyrannus): Influence of Lifespan, Nest Predation, and Body Size

AbstractI report on the lifetime reproductive success (LRS) of female Eastern Kingbirds (Tyrannus tyrannus) in central New York. I investigated the major correlates of LRS and specifically tested the hypothesis that small body size yields reproductive benefits. Lifetime reproductive success varied widely: 15–20% of females failed to fledge young over their life, whereas 50% of young were fledged by 20% of females. Female lifespan varied between one and eight years, and females that died after one breeding season tended to be smaller-bodied than long-lived females (≥2 seasons). I therefore conducted analyses of LRS for the entire sample and for longer-lived females separately. As in other species, lifespan was the strongest predictor of LRS, followed by the proportion of eggs laid that resulted in fledged young (P). Lifetime reproductive success varied positively with clutch size and, as predicted, inversely with body size (i.e., tarsus length) of females. However, variance partitioning indicated that most variation in LRS was attributable to the effects of lifespan and P, but that a substantial negative covariance existed between lifespan and P. The latter result was consistent with experimental evidence of a cost of reproduction in Eastern Kingbirds. Analysis of the correlates of lifespan, P, and clutch size showed that over a female's lifetime, (1) the longest-lived birds fledged an intermediate proportion of the eggs that they laid, (2) the most productive birds were of intermediate wing length, and (3) females with small tarsi produced the largest clutches and lost the fewest nests to predators. Hence, although lifespan was the dominant influence on LRS, negative effects of large female size appeared to be expressed through the influence of body size on other demographic parameters that contribute to LRS.Éxito Reproductivo Completo de Vida de Hembras de Tyrannus tyrannus: Influencia de la Duración de la Vida, la Depredación de Nidos y el Tamaño Corporal

Nest Reuse by Eastern Kingbirds: Adaptive Behavior or Ecological Constraint?

AbstractThe reuse of old nests by open-cup nesting passerines is a seemingly rare but potentially adaptive behavior if, as a consequence, females begin to breed earlier, lay larger clutches, or fledge more young. We report an unusually high rate of nest reuse (~10% of 341 nests) for Eastern Kingbirds (Tyrannus tyrannus) breeding at Malheur National Wildlife Refuge, Oregon. We found no difference in availability of nesting habitat or food abundance in territories in which nests were and were not reused. We also found no support for the hypotheses that kingbirds benefited from nest reuse by breeding earlier, laying larger clutches, or fledging more young, and, contrary to expectations, females that reused nests laid significantly smaller eggs than females who built their own nests. Nest reuse was independent of age: a roughly equal number of females for which we had multiple years of data both reused nests and built new nests, but at different points in their lives. Competition for nest sites seems high at Malheur National Wildlife Refuge because many open-cup nesting species utilize similar nest sites in the limited zone of riparian vegetation. A shortage of high-quality nest sites, coupled with interspecific competition, may underlie the high frequency of nest reuse in this kingbird population.

A Cautionary Tale: Cryptic Sexual Size Dimorphism in a Socially Monogamous Passerine

AbstractAmong socially monogamous birds, standard metrics suggest that males are only ∼5% larger than females. An untested assumption is that, with the exception of reproductive systems, males and females are scaled mirror images of one another. I used external morphological and skeletal data, and information on muscle mass and organ size, to test this assumption in a population of breeding Eastern Kingbirds (Tyrannus tyrannus). Male and female Eastern Kingbirds exhibited no differences in body mass or standard measures of size, except in a longer (∼6%) wing chord and tail in males. However, keel length, a character rarely measured in the field, was 9.7% larger in males. In principal component analysis, overall body size (PC1) failed to differ between the sexes unless keel length was included. Analysis of 16 skeletal characters also showed that only the bones associated with flight were larger in males. However, the most significant differences between the sexes was that lean dry pectoral muscle mass (LDPMM) was nearly 30% greater in males, whereas the alimentary tract was 27.5% heavier in females. Females also carried more fat. In both sexes, LDPMM scaled in a positive allometric manner with body mass (i.e., slope > 1.0), but the significantly higher slope of males suggested especially strong selection for large muscle mass and, presumably, greater power generation during flight. Eastern Kingbirds thus exhibit pronounced cryptic sexual size dimorphism, but it is not clear whether natural or sexual selection is responsible. These data call for a broader perspective for measuring and a re-evaluation of sexual size dimorphism in other socially monogamous species.Un Llamado de Atención: Dimorfismo Críptico en el Tamaño en un Paserino Socialmente Monógamo

A CAUTIONARY TALE: CRYPTIC SEXUAL SIZE DIMORPHISM IN A SOCIALLY MONOGAMOUS PASSERINE

Abstract Among socially monogamous birds, standard metrics suggest that males are only ∼5% larger than females. An untested assumption is that, with the exception of reproductive systems, males and females are scaled mirror images of one another. I used external morphological and skeletal data, and information on muscle mass and organ size, to test this assumption in a population of breeding Eastern Kingbirds (Tyrannus tyrannus). Male and female Eastern Kingbirds exhibited no differences in body mass or standard measures of size, except in a longer (∼6%) wing chord and tail in males. However, keel length, a character rarely measured in the field, was 9.7% larger in males. In principal component analysis, overall body size (PC1) failed to differ between the sexes unless keel length was included. Analysis of 16 skeletal characters also showed that only the bones associated with flight were larger in males. However, the most significant differences between the sexes was that lean dry pectoral muscle mass (LD...

NEST REUSE BY EASTERN KINGBIRDS: ADAPTIVE BEHAVIOR OR ECOLOGICAL CONSTRAINT?

Abstract The reuse of old nests by open-cup nesting passerines is a seemingly rare but potentially adaptive behavior if, as a consequence, females begin to breed earlier, lay larger clutches, or fledge more young. We report an unusually high rate of nest reuse (~10% of 341 nests) for Eastern Kingbirds (Tyrannus tyrannus) breeding at Malheur National Wildlife Refuge, Oregon. We found no difference in availability of nesting habitat or food abundance in territories in which nests were and were not reused. We also found no support for the hypotheses that kingbirds benefited from nest reuse by breeding earlier, laying larger clutches, or fledging more young, and, contrary to expectations, females that reused nests laid significantly smaller eggs than females who built their own nests. Nest reuse was independent of age: a roughly equal number of females for which we had multiple years of data both reused nests and built new nests, but at different points in their lives. Competition for nest sites seems high at...

LIFETIME REPRODUCTIVE SUCCESS OF FEMALE EASTERN KINGBIRDS (TYRANNUS TYRANNUS): INFLUENCE OF LIFESPAN, NEST PREDATION, AND BODY SIZE

Abstract I report on the lifetime reproductive success (LRS) of female Eastern Kingbirds (Tyrannus tyrannus) in central New York. I investigated the major correlates of LRS and specifically tested the hypothesis that small body size yields reproductive benefits. Lifetime reproductive success varied widely: 15–20% of females failed to fledge young over their life, whereas 50% of young were fledged by 20% of females. Female lifespan varied between one and eight years, and females that died after one breeding season tended to be smaller-bodied than long-lived females (≥2 seasons). I therefore conducted analyses of LRS for the entire sample and for longer-lived females separately. As in other species, lifespan was the strongest predictor of LRS, followed by the proportion of eggs laid that resulted in fledged young (P). Lifetime reproductive success varied positively with clutch size and, as predicted, inversely with body size (i.e., tarsus length) of females. However, variance partitioning indicated that mos...

Dawn song of eastern kingbirds: intrapopulation variability and sociobiological correlates

AbstractWe quantify the singing behaviour of eastern kingbirds (Tyrannus tyrannus) in the 'dawn song' period to describe variation in performance, identify sources of variation, and identify the main recipient(s) of the information conveyed in the display. All males (N = 87) participated in dawn song, but the probability that a male did not sing increased if he was relatively isolated, and it was late in the season. The earliest and longest song bouts were made by males with many neighbours and when population level female fertility was high. High song rates were typical of males in dense nesting assemblages, early in the season and nest cycle, and when social mates were infertile. However, among males with active nests, song rates varied only with neighbour density and either date or population level female fertility. Given the strong inverse correlation between population fertility and date, we suspect that population fertility drove the relationships. The association of start time and song rate with population fertility, continuation of dawn song after territory establishment, and depressed song rate when mates were fertile, are all consistent with the hypothesis that the availability of extra-pair females is the primary factor selecting for male participation in dawn song.

Intrapopulation variation in reproduction by female eastern kingbirds Tyrannus tyrannus: the impacts of age, individual performance, and breeding site

I used data from a 13‐year study of eastern kingbirds Tyrannus tyrannus from central New York, USA, to evaluate the relative impact of female age and body size on reproduction. I also calculated repeatabilities of reproductive traits for both females and the sites where they bred in an attempt to evaluate the relative contribution of each to intrapopulation variation in reproduction. Female age had a strong influence on timing of breeding (breeding date advanced by one day for each year of life), but was not a significant source of variation for clutch size, egg mass, number of young to hatch or fledge, or total seasonal production. Repeatabilities of breeding date for females and sites were both significant (0.284 and 0.181, respectively), but the only other significant repeatabilities were for female clutch size (0.282) and female egg mass (0.746). Among‐year repeatabilities of breeding date for females who bred at two or more sites over their lifetime were as high as those for females that were site faithful. Thus, breeding date was probably affected independently by the female and site. No measure of productivity exhibited a repeatable pattern in comparisons made among females or sites. All reproductive traits were entered as dependent variables in a series of stepwise multiple regression analyses in an attempt to identify female properties (size, lifespan and condition) that might be linked proximately to differences in breeding statistics. I found that (a) large birds tended to breed the earliest, (b) clutch size was independent of female size, condition and lifespan, (c) female body size and egg size were correlated positively, but (d) production of young was independent of all measured female properties. Reproduction appears to be linked more closely to the female than to the site. Body size accounts for a portion of the repeatable portion of breeding date and egg mass, but most of the intrapopulation variation in these and other traits remained unexplained.

New Species and Records of Cheyletoid Mites (Acari: Cheyletoidea) from Birds in Canada

Four new species of cheyletoid mites (Acari: Cheyletoidea) are described from birds in Canada: Harpyrhynchoides vulgaris sp. nov. (Harpirhynchidae) from Purple Finch, Carpodacus purpureus (Passeriformes: Fringillidae) and American Redstart, Setophaga ruticilla (Passeriformes: Parulidae); Syringophilopsis porzanae sp. nov. (Syringophilidae) from Sora, Porzana carolina (Gruiformes: Rallidae); Syringophilopsis tyrrani sp. nov. from Eastern Kingbird, Tyrranus tyrranus (Passeriformes: Tyrranidae); Niglarobia skorackii sp. nov. (Syringophilidae) from Greater Yellowlegs, Tringa melanoleuca (Charadriiformes: Scolopacidae). Harpyrhynchoides zumpti Fain, 1972 is recorded from Dark-eyed Junco, Junco hyemalis (Passeriformes: Emberizidae), for the first time. A key to species for females of the genus Syringophilopsis and a table of their associated hosts are provided.

A multi-scale analysis of species-environment relationships: breeding birds in a pitch pine–scrub oak ( Pinus rigida–Quercus ilicifolia) community

Analyses of species–environment relationships are critically needed to guide conservation of many declining species. Comprehensive investigations of these relationships must incorporate environmental variables at multiple spatial scales since species responses to the environment vary with the scale of observation. We used partial canonical correspondence analysis to associate a bird assemblage in a threatened pitch pine–scrub oak ( Pinus rigida–Quercus ilicifolia) community in Plymouth County, Massachusetts, USA, with plot, patch, and landscape level variables. The first level of analysis partitioned the amount of variance in the bird community explained by plot, patch, and landscape factors from that explained by spatial autocorrelation. The second level partitioned the amount of variance explained by plot, patch, and landscape factors alone and in combination. All three levels of environmental variables together accounted for 43% of the variance in the species data and only 5% of the variance was explainable by spatial factors alone. Landscape level factors accounted for a slightly larger amount of the explained variation (12%) than plot (11%) or patch (8%) level factors. We also examined the cumulative fit of each species to the plot, patch, and landscape partial models. Nine species of regional and/or national conservation concern had distributions that fit one model considerably better than the others. The Great Crested Flycatcher ( Myiarchus crinitus) and Black-and-white Warbler ( Mniotilta varia) were predominantly associated with plot level factors; the Whip-poor-will ( Caprimulgus vociferus) was predominantly associated with patch level factors; and the Purple Finch ( Carpodacus purpureus), Gray Catbird ( Dumetella carolinensis), Scarlet Tanager ( Piranga olivacea), Ovenbird ( Seiurus aurocapillus), Brown Thrasher ( Toxostoma rufum), and Eastern Kingbird ( Tyrannus tyrannus) were predominantly associated with landscape level factors. This study suggests that focusing conservation efforts at the landscape level would provide the most effective protection for the largest number of sensitive species.

Temporal Patterns of Bird Abundance in Cornfield Edges during the Breeding Season

Abstract Bird abundance in cornfield edges in Iowa was documented from mid-April through early August 1992. During this period the barren, sparsely vegetated fields are transformed into fields with dense plant cover; the availability of food resources (corn and arthropods) also changes. Temporal patterns of bird abundance in cornfield edges differed greatly among species—some were present in cornfield edges throughout most, if not all, of the study period (e.g., American robin and eastern kingbird), whereas others restricted their use to brief periods. Some were more abundant early in the season (e.g., killdeer and horned lark); others were more abundant later (e.g., black-capped chickadee and indigo bunting). Much of the seasonal change in bird abundance in cornfield edges was attributed to the habitat affinities of the various species and to seasonal shifts in available food resources. Birds that feed on the ground or in low herbaceous vegetation became less abundant later in the season, whereas species...

Source‐Sink Dynamics of a Declining Eastern Kingbird Population and the Value of Sink Habitats

Abstract: I developed a demographic model for species inhabiting a mosaic of habitats of varying quality and among which dispersal was limited only by territorial behavior. I used the model to describe an ongoing decline of an Eastern Kingbird ( Tyrannus tyrannus) population breeding in the Charlotte Valley of central New York and to explore the possibility that the population utilized a network of source‐sink habitats. Population size declined in upland and floodplain habitats between 1989 and 1999, but remained stable along a riparian zone (the “creek”). Population size after 10 years was predicted accurately when I used the empirically observed, habitat‐specific rates of adult survival, productivity, and adult dispersal among habitats, and assumed a 5% rate of immigration into the upland. Simulation of the effect of density‐dependent variation in reproductive success or a small increase in adult survival in all habitats failed to improve the model's predictive ability. The model indicated that none of the subpopulations was self‐sustaining over 50 years and that population decline was driven by declining adult survival. Further analyses, however, showed that small increases in adult survival along the creek changed the creek habitat into a source and reversed population trends. Simulation of realistic rates of habitat loss as a result of forest maturation did not affect predicted population size at 50 years, but, assuming that the area of pasture and grazed lands continues to decline at current rates, the kingbird population is predicted to become extinct locally within 40 years. Riparian zones appear to be source habitats for kingbirds and should be targeted for protection. Sink habitats were also important because in every year most of the kingbird population bred on the floodplain or in the upland and many floodplain birds later dispersed to the creek. Preservation of open space is thus critical for maintenance of kingbird populations in eastern North America.