Aggression of breeding eastern bluebirds (Sialia sialis) toward their mates and models of intra-and interspecific intruders

Hypotheses regarding the evolution of aggressive patterns during the breeding season in eastern bluebirds (Sialia sialis) were tested by placing models of eastern bluebirds and house sparrows (Passer domesticus) into territories of experimentally naive breeding pairs of eastern bluebirds during three stages of the nesting cycle. The responses of the resident males to models of male bluebirds decreased from the egg-laying to the nestling stage, and this response pattern can be interpreted as support for the protection-of-parentage hypothesis or the nest-site-defence hypothesis. The aggression of resident males to resident females during trials with models of male bluebirds weakly conforms to the pattern predicted by the protection-of-parentage hypothesis. The aggressive response to models of house sparrows also decreased from the egg-laying to the nestling phase in a manner consistent with the nest-site-defence explanation. No aggression toward mates by males or females was observed during trials with house sparrow models, an observation consistent with the predictions of the protection-of-parentage hypothesis.     

Plasma levels of prolactin and gonadal steroids in relation to multiple-brooding and renesting in free-living populations of the song sparrow, Melospiza melodia

Plasma levels of testosterone (T), estradiol (E2), and prolactin were measured in samples collected from free-living song sparrows, Melospiza melodia. In males, plasma levels of T were elevated early in the season when territories were established and when females laid the first clutch of eggs. Thereafter, T levels declined and remained low throughout the remainder of the breeding cycle. However, if the first brood was lost to a predator, or by experimental removal of the nest, plasma levels of T increased as renesting, to replace the clutch, occurred. Circulating levels of prolactin in males began to rise during the egg-laying stage of the first brood, reached a maximum toward the end of the incubation stage, remained elevated until breeding was terminated, and then declined throughout the moulting stage to basal values in October. Prolactin levels remained high throughout the breeding season irrespective of whether a brood was raised successfully or whether the nest was lost and renesting occurred. In females, plasma levels of E2 were elevated prior to the egg-laying stage for each brood as is typical of multiple-brooded species. However, prolactin titers rose dramatically during egg-laying for the first clutch (slightly higher than in males) and were maximal by onset of incubation. Only females of this species incubate, although males do feed young. As in males, plasma prolactin in females remained high between broods and during experimentally induced renesting, and then declined to basal by the end of the moult stage in October. These data suggest that there are no differences in the temporal patterns of prolactin concentrations in blood between multiple-brooding and renesting. In a separate experiment, captive male song sparrows were transferred from a short day to artificial long days (18L 6D) and a control group was maintained on 9L 15D. In the long-day group, prolactin levels rose abruptly over the first 20 days, as the testes developed, and remained high well into postnuptial moult after the gonads had regressed. Prolactin remained basal in the control group. These data suggest that the temporal pattern of circulating prolactin levels throughout the breeding season is regulated at least partly by changing photoperiod. However, nonphotoperiodic factors are also important since photoperiodically induced increases in prolactin are significantly less than those seen in free-living individuals. These differences may be related to parental behavior.     

Reproduction and modulation of the stress response: an experimental test in the house sparrow

The stress response is highly variable among individuals, but the causes of this variation remain largely unknown. In response to stressors, vertebrates secrete elevated levels of glucocorticoids which enhance survival, but concurrently interfere with reproduction. We tested the hypothesis that individuals flexibly modulate their stress response with respect to the reproductive value of their brood in free-living house sparrows (Passer domesticus). We experimentally increased or decreased clutch size during the nestling period and found that parents tending enlarged clutches responded less strongly to a stressor than those tending reduced clutches. In addition, we examined whether individuals responded less strongly to a stressor as the breeding season progressed and future reproductive opportunities declined. We found that the stress response decreased with breeding date during the birds' first breeding attempt, but it remained constant during their second breeding attempt. Within-individual variability in the stress response was related to the brood size manipulations the birds received in their two consecutive breeding attempts. These results provide the first experimental support for the hypothesis that individuals actively modulate their stress response with respect to the value of current reproduction.     

Changes in plasma levels of testosterone during male-male interactions in the song sparrow,Melospiza melodia: time course and specificity of response

Summary Previous investigations have shown that malemale interactions over territory boundaries result in an increase in plasma levels of testosterone (T). In order to determine the time course of this increase following an agonistic challenge, male song sparrows, Zonotrichia (= Melospiza) melodia, were exposed to simulated territorial intrusions. Responding males were captured at intervals after onset of the intrusion for up to 60 min. Plasma levels of T were elevated by 10 min after onset of the challenge, and remained high in birds sampled 10–60 min after onset of the intrusion. In addition, plasma levels of luteinizing hormone (LH) increased and were significantly elevated after 10 min of stimulation. The results suggest that at least the prolonged elevation of T secretion precipitated by a male-male interaction was mediated via an increase in LH release. However, the initial surge of T levels in plasma may be LH independent. p Investigations of captive birds confirmed that circulating LH and T levels increased when male song sparrows were challenged by a conspecific male. Challenges involving a male house sparrow, Passer domesticus, had no effect suggesting that responses to male intrusions were species characteristic and not a result of general arousal. Further, auditory stimuli (vocalizations) alone, and visual stimuli (a devocalized male) alone were less effective in stimulating an increase in secretion of T than a combination of auditory and visual stimuli.Abbreviations DHT dihydrotestosterone - LH luteinizing hormon; - T testosterone     

Corticosterone treatment has no effect on reproductive hormones or aggressive behavior in free-living male tree sparrows, Spizella arborea

We examined the effect of corticosterone on plasma levels of reproductive hormones (testosterone, dihydrotestosterone, and luteinizing hormone) and territorial defense behavior in male tree sparrows, Spizella arborea. Birds receiving Silastic implants filled with corticosterone (B) had significantly higher plasma levels of B than control birds, which received empty implants, and exhibited pectoral muscle wastage and a decrease in body mass. We evaluated the hormonal and agonistic responses of the two implanted groups of birds using a simulated territorial intrusion (STI) 2 to 4 days after they were implanted. Corticosterone-treated and control birds did not differ in their circulating levels of reproductive hormones or in their behavioral responses to STI (latency to respond to intrusion, number of songs, and closest approach to a decoy and tape recording). Unlike previous studies of north temperate passerines, high physiological levels of exogenous B had no effect either on circulating levels of reproductive hormones or on territorial behaviors associated with breeding. Nonetheless, untreated tree sparrows do mount a robust adrenocortical response, having a two- to fourfold increase in plasma B levels during a 1-h period of capture. Thus, adrenocortical responsiveness is maintained in these birds, but elevated levels of glucocorticoids do not suppress reproductive hormones or associated behaviors. We believe that this hormonal and behavioral refractoriness to glucocorticoids-or uncoupling of the stress response from the reproductive axis-may be advantageous for species having extreme temporal constraints on their breeding schedules.     

Supplementary feeding increases nestling feather corticosterone early in the breeding season in house sparrows

Several studies on birds have proposed that a lack of invertebrate prey in urbanized areas could be the main cause for generally lower levels of breeding success compared to rural habitats. Previous work on house sparrows Passer domesticus found that supplemental feeding in urbanized areas increased breeding success but did not contribute to population growth. Here, we hypothesize that supplementary feeding allows house sparrows to achieve higher breeding success but at the cost of lower nestling quality. As abundant food supplies may permit both high‐ and low‐quality nestlings to survive, we also predict that within‐brood variation in proxies of nestling quality would be larger for supplemental food broods than for unfed broods. As proxies of nestling quality, we considered feather corticosterone (CORT_f), body condition (scaled mass index, SMI), and tarsus‐based fluctuating asymmetry (FA). Our hypothesis was only partially supported as we did not find an overall effect of food supplementation on FA or SMI. Rather, food supplementation affected nestling phenotype only early in the breeding season in terms of elevated CORT_f levels and a tendency for more variable within‐brood CORT_f and FA. Early food supplemented nests therefore seemed to include at least some nestlings that faced increased stressors during development, possibly due to harsher environmental (e.g., related to food and temperature) conditions early in the breeding season that would increase sibling competition, especially in larger broods. The fact that CORT_f was positively, rather than inversely, related to nestling SMI further suggests that factors influencing CORT_f and SMI are likely operating over different periods or, alternatively, that nestlings in good nutritional condition also invest in high‐quality feathers.     

Reproductive endocrinology of free-living nestling and juvenile starlings, Sturnus vulgaris; an altricial species

Plasma concentrations of luteinizing hormone (LH), prolactin and testosterone, and pituitary content of LH and prolactin, were measured in free-living starlings, Sturnus vulgaris , from hatching until 12 weeks of age.
Plasma LH concentrations were elevated in both sexes until four days after hatching, then they decreased. Throughout the period, plasma LH levels were low compared to those in breeding adults but were comparable to levels in post-breeding photorefractory adults. Pituitary LH content increased until 12 days after hatching, but this increase was due to physical growth during this period. Plasma prolactin concentration and pituitary prolactin content increased dramatically during the nestling period. The increase in pituitary prolactin content was in excess of that accounted for by increasing size. Plasma prolactin remained high during the immediate post-fledging period, but had started to decrease by 12 weeks after hatching. Plasma testosterone concentrations were lower than those in breeding adults, but generally higher than in post-breeding photorefractory adults. The gonads of both sexes remained regressed.
These results suggest that the reproductive system of nestling and juvenile starlings is in a similar state to that of post-breeding photorefractory adult starlings. The comparatively high levels of testosterone may reflect involvement in sexual differentiation.     

Effects of sexual experience, season, and mating stimuli on endocrine concentrations in the adult ram.

Two behavioral trials were conducted to determine the endocrine response of cortisol (C), luteinizing hormone (LH), testosterone (T), prolactin (PRL), and growth hormone (GH) in adult rams during exposure to estrous ewes during the breeding and nonbreeding seasons. One-half of the rams in each season were sexually experienced (SE) and the remainder were sexually inexperienced (SI). All SE rams (100%) achieved at least one ejaculation, but only 33% (summer) and 67% (fall) SI rams achieved ejaculation. In the fall, mean C, T, and GH concentrations were elevated (P less than .001) compared to values measured in the summer, whereas LH and PRL levels were higher (P less than .01) in the summer. Overall levels of C, LH, T, and PRL were higher (P less than .05) in SE rams than in SI rams. Mean GH concentration was higher (P less than .10) in SI than in SE rams during restricted and complete access to estrous ewes. In general, LH, PRL, and GH responses were similar during restricted and complete access to females for both SE and SI rams. Cortisol levels were higher (P less than .06) during periods of mating and T levels were higher (P less than .001) during periods where activity was limited to courtship behavior (nasogenital investigation). Correlations of hormones to reproductive behaviors indicated that mounting and intromission were associated with elevations in C and PRL, whereas elevated LH and T tended to be associated with courtship behaviors. Correlations between GH and behaviors were inconsistent. However, there was an increased coincidence between time of female exposure and hormonal response that occurred in the fall; brief exposure to estrous ewes resulted in increases in concentrations of all hormones examined. The most consistent response was observed in sexually experienced rams during restricted access to females during the breeding season. These results provide new information on the effects of season and level of sexual experience upon hormonal and behavioral characteristics of the ram during mating activity.     

Individual Consistency in Parental Effort Across Multiple Stages of Care in the House Sparrow, Passer domesticus

In many avian species in which biparental care is provided to offspring, substantial variation exists within members of each sex in the level of effort contributed to various forms of parental care. Questions remain as to whether individuals that contribute more toward one parental activity also contribute more toward other activities in which they participate. We examined the contributions of male and female house sparrows (Passer domesticus) to three forms of parental care: incubation, nestling provisioning, and nest defense, and compared the investments made by individuals at each stage of care relative to other same‐sexed parents. In both males and females, nestling feeding rates were positively associated with time spent incubating, but no relationships were found between measures of nestling feeding and nest defense. The predictability of an individual's feeding behavior based on earlier incubation efforts may make incubation a good stage for individuals to evaluate the parental abilities of their partners.     

The reproductive--endocrine response of adult rams to sexual encounters with estrual ewes is season dependent

An experiment was conducted with four adult, sexually inexperienced Finnish Landrace rams during the ovine nonbreeding (July) and breeding (October) seasons to determine the influence of components of the rams' mating behavior on the secretion of luteinizing hormone (LH), follicle-stimulating hormone (FSH), prolactin (PRL), and testosterone. On four occasions in both seasons, blood was collected by jugular venipuncture at 20-min intervals during an 8-hr period while rams were (1) separated from, (2) observing with minimal direct physical contact, (3) mounting without intromission, or (4) mating estrous-induced ewes. In comparison with separation periods, mating activity in July was associated with increased mean LH (P less than 0.05) and testosterone levels and number of LH peaks, while in October, obvious increases were detected in only baseline LH levels (P less than 0.05). Circulating LH and testosterone levels either did not change (July) or were depressed (October) during the mounting and observation periods. FSH levels generally remained unaffected by engagement in the various sexual activities. Although a clear relationship between type of sexual activity and mean PRL levels was not observed in July, activities which appeared to involve the most physical exertion tended to be associated with much higher circulating PRL levels in October. These data suggest (1) the act of ejaculation is important in the induction of increases in LH and testosterone secretion that occur in rams in response to mating activity during the nonbreeding season and (2) excessively stressful sexual activities during the breeding season may alter the pattern of secretion of some reproductive hormones.     

Patterns of testosterone and prolactin concentrations and reproductive behavior of helpers and breeders in the cooperatively breeding red-cockaded woodpecker (Picoides borealis)

We examined the proximate causes of delayed breeding and alloparental behavior in the cooperatively breeding red-cockaded woodpecker by measuring plasma concentrations of testosterone (T) and prolactin (PRL) in female breeders, male breeders, and male helpers during different stages of the reproductive cycle. Male breeders and helpers have low T during the prebreeding period, highest T during copulation, and low concentrations of T during the egg-laying/incubation and nestling provisioning stages. Helpers appear physiologically capable of reproducing; their T concentrations equal that of male breeders. Helpers unrelated to the breeding female have higher T than helpers sharing a territory with their mother. Sexual inactivity by male helpers might be explained by behavioral suppression resulting from interactions of male helpers with the breeding pair that somehow differ in accordance with the helper's relatedness to the breeding female (e.g., female breeders are generally unreceptive to courtship from male helpers and least receptive to related helpers). Female breeder, male breeder, and male helper PRL levels did not differ and increased from the prebreeding stage through the copulation and egg-laying/incubation stages. During the nestling provisioning stage, male breeder and helper PRL declined, while female PRL continued to increase. Based on these results, we conclude that the physiological bases of alloparental behavior have not diverged from those that mediate parental behavior in this species.     

A study of seasonally delayed puberty in the male hare, Lepus Europaeus.

The brown hare, Lepus europaeus, has a mating season which extends from January to September. Adult males exhibit pronounced seasonal changes in the reproductive tract which are associated with changes in LH secretion. Maximum plasma levels of immunoreactive LH occur between March and June and minimal levels in the autumn non-mating period from September to December; this seasonal cycle in gonadotrophin output is reflected by the appropriate changes in the secretion of testosterone from the testes and in the activity of the accessory sex glands. Juvenile animals reach puberty only during the adult mating season, and the age of puberty thus varies with the date of birth. Males born before May reach puberty and become fertile at 3 months of age, while those born from May to July grow to a mature body size during the autumn non-mating season but puberty is delayed for several months. Since some animals experiencing delayed puberty were found to have elevated plasma levels of LH and testosterone, it is concluded that puberty is not completely suppresed by the environmental effects of the autumn, but that the developmental process is prolonged, resulting in the juveniles being synchronized with the adults in their reproductive activity.     

Reduced reproductive performance associated with warmer ambient temperatures during incubation in a winter‐breeding,food‐storing passerine

Timing of reproduction can influence individual fitness whereby early breeders tend to have higher reproductive success than late breeders. However, the fitness consequences of timing of breeding may also be influenced by environmental conditions after the commencement of breeding. We tested whether ambient temperatures during the incubation and early nestling periods modulated the effect of laying date on brood size and dominant juvenile survival in gray jays (Perisoreus canadensis), a sedentary boreal species whose late winter nesting depends, in part, on caches of perishable food. Previous evidence has suggested that warmer temperatures degrade the quality of these food hoards, and we asked whether warmer ambient temperatures during the incubation and early nestling periods would be associated with smaller brood sizes and lower summer survival of dominant juveniles. We used 38 years of data from a range‐edge population of gray jays in Algonquin Provincial Park, Ontario, where the population has declined over 50% since the study began. Consistent with the “hoard‐rot” hypothesis, we found that cold temperatures during incubation were associated with larger brood sizes in later breeding attempts, but temperatures had little effect on brood size for females breeding early in the season. This is the first evidence that laying date and temperature during incubation interactively influence brood size in any bird species. We did not find evidence that ambient temperatures during the incubation period or early part of the nestling period influenced summer survival of dominant juveniles. Our findings provide evidence that warming temperatures are associated with some aspects of reduced reproductive performance in a species that is reliant on cold temperatures to store perishable food caches, some of which are later consumed during the reproductive period.     

Stress responses of testosterone and corticosterone-binding globulin in a multi-brooded species,Eurasian Tree Sparrows (Passer montanus): Does CBG function as a mediator?

In avian plasma, testosterone (T) and corticosterone (CORT) compete to bind with corticosterone-binding globulin (CBG). Elevation of CBG may function to "buffer" the tissues against high circulating levels of T and stress-induced levels of CORT. To demonstrate the effects of acute stress on CBG and T levels and their biological functions, we investigated seasonal changes of baseline and stress-induced T and CBG levels in Eurasian Tree Sparrows (Passer montanus) during different life stages using the capture-handling-restraint stress method. Our results show that (1) male sparrows had significantly higher baseline T levels and CBG capacities during the nest building, the first egg-laying, and the first nestling stages, and significantly decreased stress-induced T levels only during the nest building and the first egg-laying stages. They also expressed significantly increased stress-induced CBG capacities during the second nestling stage. (2) Females showed significantly higher baseline CBG capacities but significantly decreased stress-induced CBG capacities during the nest building stage, and females also showed significantly increased stress-induced CBG capacities during the second egg-laying and the second nestling stages. Therefore, the seasonal fluctuations of baseline CBG in both sexes and baseline T in males reflect their adaptive strategies for optimizing their physiological and behavioral states to the life history cycle. The different patterns of stress-induced CBG in females suggest CBG functions as an essential mediator in regulating stress response to unpredictable perturbations. Our results highlight the need for future studies of stress-induced CBG and T levels on a wide range of vertebrate species that vary in different life history stages to gain a full understanding of the mechanisms that underlie biological functions of CBG and T for unpredictable stressors.     

Temporal patterns of circulating LH and steroid hormones in a brood parasite, the Brown-headed cowbird, Molothrus afer.

The breeding behaviour of female Brown-headed cowbirds, Molothrus ater , is remarkably distinct from that of females of most other passerine species. Cowbirds are brood parasites that do not build nests, incubate eggs or feed young. Instead, they have an extended egg-laying stage, and deposit eggs in the nests of hosts within large breeding territories that are defended from other female cowbirds. In this communication, we report the changes in plasma levels of circulating hormones and in morphological characteristics that occur in free-living female Brown-headed cowbirds over a breeding season.
Plasma LH in female cowbirds increases at the onset of breeding and remains elevated for approximately eight weeks, at which time levels decline to basal values. This pattern is similar to that of follicle development, and is consistent with the prolific reproductive output by individuals of this species. Circulating E, levels are quite variable and do not demonstrate any discernible pattern, while plasma B is elevated during the egg-laying period and declines subsequently.
Testosterone titre (T) is highest early in the breeding season, when territorial interactions are generally most frequent. T levels decline slowly thereafter, reaching their lowest values at the season's end. Plasma DHT levels are slightly higher than those for T; DHT exhibits little change until late in the breeding season, when it declines. Fat scores are highest prior to egg laying, and fat reserves diminish as the breeding season progresses. In contrast, body weight increases during the reproductive season, probably as a result of developing eggs in the ovary. These results are consistent with the hypothesis that temporal patterns of hormone secretion are correlated with the expression of social systems.     

Effects of testosterone on song, aggression, and nestling feeding behavior in male great tits, Parus major

As outlined in the trade-off hypothesis of testosterone (T) secretion, fluctuations in T during the breeding season might reflect how males allocate their time and energy to competitive behaviors for mates and territories, associated with high T levels, and parental activities, associated with low T levels. In the present study, great tit, Parus major, males were implanted with T-filled or empty silastic capsules at the start of the breeding season and the behavior of these two male categories was compared during the entire breeding season. As a measure of competitive behavior we looked at song behavior and territorial responsiveness to a male decoy, during the three main stages of the breeding period (the egg-laying, incubation, and nestling stages). As a measure of parental care we looked at feeding behavior during the nestling stage. Our results only partly supported the trade-off hypothesis. T implants increased plasma androgen levels and enhanced spontaneous song activity and the production of aggressive vocalizations in response to a decoy. However, our results suggest that the degree of physical aggression might be less than fully coupled with T. First, approach to the decoy was not affected by the treatment. Second, although T levels are known to vary from high during egg laying to low while feeding young, control and T-treated males spent similar amounts of time close to the decoy in the three breeding stages. Our results thus suggest that vocal and physical aggression might be regulated differently in the great tit. Furthermore, in contrast with most other studies on temperate bird species but in agreement with a previous study on the great tit, T treatment did not affect male feeding rates. As the dose of T we used was lower than that typically used in other studies, we cannot at present completely exclude the possibility that the latter result reflects this lower dose of T rather than the species used.     

Parental Feeding Rates in the House Sparrow,Passer domesticus: Are Larger‐Badged Males Better Fathers?

Elaborated secondary sexual characteristics may reflect genetic quality or good health, either of which may be associated with an individual's competence as a parent. We examined whether female house sparrows (Passer domesticus) paired to large vs. small‐badged mates gain benefits in the form of increased parental care or improved nestling welfare. House sparrow nests where the male had been trapped and banded were observed for 1 h on at least 5 d during the peak growth period of nestlings. Male feeding shares, measured as the proportion of total feeds per chick made by the male, were marginally positively correlated with male badge size. Moreover, higher male shares of nestling feeding were associated with improved prospects for offspring survival, and a greater proportion of chicks fledged from the nests of larger‐badged males. These results suggest that females paired to large‐badged males gain direct benefits in the form of enhanced nestling survival, which presumably stem from factors associated with increases in the proportion of nestling feeding contributed by their mates.     

Territorial aggression, circulating levels of testosterone, and brain aromatase activity in free-living pied flycatchers

Testosterone (T) is a critical endocrine factor for the activation of many aspects of reproductive behavior in vertebrates. Castration completely eliminates the display of aggressive and sexual behaviors that are restored to intact level by a treatment with exogenous T. There is usually a tight correlation between the temporal changes in plasma T and the frequency of reproductive behaviors during the annual cycle. In contrast, individual levels of behavioral activity are often not related to plasma T concentration at the peak of the reproductive season suggesting that T is available in quantities larger than necessary to activate behavior and that other factors limit the expression of behavior. There is some indication from work in rodents that individual levels of brain aromatase activity (AA) may be a key factor that limits the expression of aggressive behavior, and in agreement with this idea, many studies indicate that estrogens produced in the brain by the aromatization of T may contribute to the activation of reproductive behavior, including aggression. We investigated here in pied flycatcher (Ficedula hypoleuca) the relationships among territorial aggression, plasma T, and brain AA at the peak of the reproductive season. In a first experiment, blood samples were collected from unpaired males holding a primary territory and, 1 or 2 days later, their aggressive behavior was quantified during standardized simulated territorial intrusions. No relationship was found between individual differences in aggressive behavior and plasma T or dihydrotestosterone levels but a significant negative correlation was observed between number of attacks and plasma corticosterone. In a second experiment, aggressive behavior was measured during a simulated territorial intrusion in 22 unpaired males holding primary territories. They were then immediately captured and AA was measured in their anterior and posterior diencephalon and in the entire telencephalon. Five males that had attracted a female (who had started egg-laying) were also studied. The paired males were less aggressive and correlatively had a lower AA in the anterior diencephalon but not in the posterior diencephalon and telencephalon than the 22 birds holding a territory before arrival of a female. In these 22 birds, a significant correlation was observed between number of attacks/min displayed during the simulated territorial intrusion and AA in the anterior diencephalon but no correlation was found between these variables in the two other brain areas. Taken together, these data indicate that the level of aggression displayed by males defending their primary territory may be limited by the activity of the preoptic aromatase, but plasma T is not playing an important role in establishing individual differences in aggression. Alternatively, it is also possible that brain AA is rapidly affected by agonistic interactions and additional work should be carried out to determine whether the correlation observed between brain AA and aggressive behavior is the result of an effect of the enzyme on behavior or vice versa. In any case, the present data show that preoptic AA can change quite rapidly during the reproductive cycle (within a few days after arrival of the female) indicating that this enzymatic activity is able to regulate rapid behavioral transitions during the reproductive cycle in this species.     

Age and clutch size variation in Dusky Flycatcher nest survival

Examination of spatial and temporal factors that influence nest survival can provide insight into habitat selection, reproductive decisions (e.g., clutch size), population dynamics, and conservation requirements for species. We used nest survival data for the Dusky Flycatcher Empidonax oberholseri to examine several factors that may influence nesting success. Our prediction was that the number of nest initiations would be positively associated with period nest survival. We used a model selection framework and found that nesting success was a function of clutch size and a cubic effect of age. Clutches with one, two, three, and four eggs had period survival rates of 0, 0.05, 0.33, and 0.49, respectively. Daily survival rates decreased from the onset of egg-laying and increased during the later stages of incubation before remaining relatively constant through the later portions of the nestling stage. Model-selection criterion provided support for a date effect on daily survival (i.e., daily nest survival declined across the nesting season) although the 95% confidence interval for the estimate included zero. We found that the majority of nest initiations occurred early in the nest season and declined across the season as period nest survival declined. Our prediction concerning nest survival was partially supported. In addition, we found substantial positive associations between clutch size and nest survival. While low daily survival rates for clutches with one or two eggs suggested that individuals may have reduced reproductive effort in response to nest predation risk, we did not find strong evidence that individuals reduced their clutch sizes in subsequent nest attempts. Alternative predictions, including the preferential settlement of higher quality individuals (e.g., those with the ability to lay full clutches to replace depredated nests) into high-quality habitat and differences in behavior patterns (e.g., number of visits to provision nestlings), may provide more consistent explanations for these patterns.