Behavioural responses to ectoparasites in pied flycatchers Ficedula hypoleuca: an experimental study

Nests of cavity‐nesting birds usually harbor some species of haematophagous ectoparasites that feed on the incubating adults and nestlings. Given the negative impact of ectoparasites on nestlings there will be selection on hosts to reduce parasite infestations through behavioural means. We have experimentally reduced the abundance of all ectoparasites in nests of pied flycatchers Ficedula hypoleuca to explore both whether there are changes in the frequency and duration of putative anti‐parasite behaviours by tending adults, as well as whether such anti‐parasite behaviours are able to compensate for the deleterious effects that parasites may have on nestlings. Heat treatment of nests substantially decreased the density of ectoparasites, and thereby positively affected nestling growth. The frequency and intensity of female grooming and nest sanitation behaviours during the incubation and nestling periods decreased as a consequence of the experimental reduction of ectoparasite infestation. Although nestlings begged more intensely in infested nests, the experiment had no significant effect on parental provisioning effort. Reduction of parasites resulted in larger nestlings shortly before fledging and increased fledging success. This study shows a clear effect of a complete natural nest ectoparasite fauna on parental behaviour at the nest and nestling growth in a cavity‐nesting bird. Although ectoparasites induce anti‐parasite behaviours in females, these behaviours are not able to fully remove parasite's deleterious effects on nestling growth and survival.     

Do parasites and antioxidant availability affect begging behaviour,growth rate and resistance to oxidative stress?

Early‐life trade‐offs faced by developing offspring can have long‐term consequences for their future fitness. Young offspring use begging displays to solicit resources from their parents and have been selected to grow fast to maximize survival. However, growth and begging behaviour are generally traded off against self‐maintenance. Oxidative stress, a physiological mediator of life‐history trade‐offs, may play a major role in this trade‐off by constraining, or being costly to, growth and begging behaviour. Yet, despite implications for the evolution of life‐history strategies and parent–offspring conflicts, the interplay between growth, begging behaviour and resistance to oxidative stress remains to be investigated. We experimentally challenged wild great tit (Parus major) offspring by infesting nests with a common ectoparasite, the hen flea (Ceratophyllus gallinae), and simultaneously tested for compensating effects of increased vitamin E availability, a common dietary antioxidant. We further quantified the experimental treatment effects on offspring growth, begging intensity and oxidative stress. Flea‐infested nestlings of both sexes showed reduced body mass during the first half of the nestling phase, but this effect vanished short before fledging. Begging intensity and oxidative stress of both sexes were unaffected by both experimental treatments. Feeding rates were not affected by the experimental treatments, but parents of flea‐infested nests fed nestlings with a higher proportion of caterpillars, the main source of antioxidants. Additionally, female nestlings begged significantly less than males in control nests, whereas both sexes begged at similar rates in vitamin E‐supplemented nests. Our study shows that a parasite exposure does not necessarily affect oxidative stress levels or begging intensity, but suggests that parents can compensate for negative effects of parasitism by modifying food composition. Furthermore, our results indicate that the begging capacity of the less competitive sex is constrained by antioxidant availability.     

Parasites as mediators of heterozygosity-fitness correlations in the Great Tit (Parus major)

Positive correlations between heterozygosity and fitness traits are frequently observed, and it has been hypothesized, but rarely tested experimentally, that parasites play a key role in mediating the heterozygosity-fitness association. We evaluated this hypothesis in a wild great tit (Parus major) population by testing the prediction that the heterozygosity-fitness association would appear in broods experimentally infested with a common ectoparasite, but not in parasite-free broods. We simultaneously assessed the effects of parental and offspring heterozygosity on nestling growth and found that body mass of nestlings close to independence, which is a strong predictor of post-fledging survival, increased significantly with nestling levels of heterozygosity in experimentally infested nests, but not in parasite-free nests. Heterozygosity level of the fathers also showed a significant positive correlation with offspring body mass under an experimental parasite load, whereas there was no correlation with the mothers' level of heterozygosity. Thus, our results indicate a key role for parasites as mediators of the heterozygosity-fitness correlations.     

An experimental test of predator–parasite interaction in a passerine bird

Experimental studies incorporating multiple trophic levels are scarce but of increasing interest for understanding ecological communities. Here we investigated interactive effects of perceived predation risk and parasite pressure on life‐history traits in a hole‐nesting bird, and the effects of predation risk on parasite success. In a 3 × 2 experimental design we increased perceived predation risk for breeding great tits Parus major via simulations of either nest‐predators (woodpeckers) or post‐fledging predators (sparrowhawks) close to nests, and used a non‐predatory species (song thrush) as a control. Concurrently, half of the nests in each treatment were either infested with ectoparasites, or kept parasite‐free. Regarding the predation risk – parasite interaction, exposure to nest‐predators tended to lower wing and sternum growth rates of nestlings in the absence, but not the presence, of parasites. In the presence of parasites, exposure to a post‐fledging, but not to a nest‐predator, led to significantly reduced wing growth. Mass and tarsus length were not affected by predator exposure, but ectoparasites had slight positive effects on mass gain. In the last third of the nestling period, overall nestling size was significantly smaller when exposed to a post‐fledging predator than to a nest‐predator, but neither differed from the control. Parental feeding rates were not affected by the treatments, but parents became less selective towards food items under either predation risk. Hen‐flea population sizes (adult or larvae) in nests were not affected by predation risk treatment of hosts. In summary, we found some evidence for an interactive effect of predation risk and parasite pressure on nestling growth. The complexity of the interaction, combined with certain inconsistencies of the effects and potential statistical artifacts, prevent however a straightforward interpretation of the results. The insights from the study are useful for designing additional experiments to further investigate the complexity of predator–parasite interactions in wild populations.     

Age‐related prenatal maternal effects and postnatal breeding experience have different influences on nestling development in an altricial passerine

Reproductive success and nestling performance are related to the age of parents across several vertebrate taxa. However, because breeding experience and prenatal maternal investment in reproduction often covary, the source of these age‐related differences can be difficult to determine. In this study, we evaluated the influence of prenatal maternal effects and postnatal breeding experience on the performance of nestling tree swallows Tachycineta bicolor by conducting a carefully controlled partial cross‐fostering experiment. We swapped half‐broods of nestlings between the nest of a young first‐time breeding female and the nest of a female known to have previously raised and fledged young. Our manipulation did not influence the within‐brood nestling hierarchies, and controlled for the effects of egg laying order. We found that nestlings of older females were heavier just prior to fledging regardless of the breeding experience of the attending female. In addition, fledglings raised by experienced females grew their flight feathers faster, and had greater probability of fledging. Our study demonstrates that prenatal investment in reproduction by older females can have long‐term consequences on nestling mass, and suggests limited potential for compensatory mass gains prior to fledging. Because our analyses controlled for feeding rates, our results also suggest that foraging quantity and quality are not the only benefits nestlings gain by being raised by an experienced female.     

Behavior of adult and young grassland songbirds at fledging

The behavior of adults and young at the time of fledging is one of the least understood aspects of the breeding ecology of birds. Current hypotheses propose that fledging occurs either as a result of parent‐offspring conflict or nestling choice. We used video recordings to monitor the behavior of nestling and adult grassland songbirds at the time of fledging. We observed 525 nestlings from 166 nests of 15 bird species nesting in grasslands of Alberta, Canada, and Wisconsin, USA. Overall, 78% of nestlings used terrestrial locomotion for fledging and 22% used wing‐assisted locomotion. Species varied in propensity for using wing‐assisted locomotion when fledging, with nestling Grasshopper Sparrows (Ammodramus savannarum) and Henslow's Sparrows (Centronyx henslowii) often doing so (47% of fledgings) and nestling Song Sparrows (Melospiza melodia), Common Yellowthroats (Geothlypis trichas), and Chestnut‐collared Longspurs (Calcarius ornatus) rarely doing so (3.5% of fledgings). For 390 fledging events at 127 nests, camera placement allowed adults near nests to be observed. Of these, most young fledged (81.5%) when no adult was present at nests. Of 72 fledging events that occurred when an adult was either at or approaching a nest, 49 (68.1%) involved feeding. Of those 49 fledgings, 30 (62.1%) occurred when one or more nestlings jumped or ran from nests to be fed as an adult approached nests. The low probability of nestlings fledging while an adult was at nests, and the tendency of young to jump or run from nests when adults did approach nests with food minimize opportunities for parents to withhold food to motivate nestlings to fledge. These results suggest that the nestling choice hypothesis best explains fledging by nestlings of ground‐nesting grassland songbirds, and fledging results in families shifting from being place‐based to being mobile and spatially dispersed.     

Parasites favour intermediate nestling mass and brood size in cliff swallows

A challenge of life‐history theory is to explain why animal body size does not continue to increase, given various advantages of larger size. In birds, body size of nestlings and the number of nestlings produced (brood size) have occasionally been shown to be constrained by higher predation on larger nestlings and those from larger broods. Parasites also are known to have strong effects on life‐history traits in birds, but whether parasitism can be a driver for stabilizing selection on nestling body size or brood size is unknown. We studied patterns of first‐year survival in cliff swallows (Petrochelidon pyrrhonota) in western Nebraska in relation to brood size and nestling body mass in nests under natural conditions and in those in which hematophagous ectoparasites had been removed by fumigation. Birds from parasitized nests showed highest first‐year survival at the most common, intermediate brood‐size and nestling‐mass categories, but cliff swallows from nonparasitized nests had highest survival at the heaviest nestling masses and no relationship with brood size. A survival analysis suggested stabilizing selection on brood size and nestling mass in the presence (but not in the absence) of parasites. Parasites apparently favour intermediate offspring size and number in cliff swallows and produce the observed distributions of these traits, although the mechanisms are unclear. Our results emphasize the importance of parasites in life‐history evolution.     

Heterozygosity is linked to the costs of immunity in nestling great tits (Parus major)

There is growing evidence that heterozygosity–fitness correlations (HFCs) are more pronounced under harsh conditions. Empirical evidence suggests a mediating effect of parasite infestation on the occurrence of HFCs. Parasites have the potential to mediate HFCs not only by generally causing high stress levels but also by inducing resource allocation tradeoffs between the necessary investments in immunity and other costly functions. To investigate the relative importance of these two mechanisms, we manipulated growth conditions of great tit nestlings by brood size manipulation, which modifies nestling competition, and simultaneously infested broods with ectoparasites. We investigated under which treatment conditions HFCs arise and, second, whether heterozygosity is linked to tradeoff decisions between immunity and growth. We classified microsatellites as neutral or presumed functional and analyzed these effects separately. Neutral heterozygosity was positively related to the immune response to a novel antigen in parasite‐free nests, but not in infested nests. For nestlings with lower heterozygosity levels, the investments in immunity under parasite pressure came at the expenses of reduced feather growth, survival, and female body condition. Functional heterozygosity was negatively related to nestling immune response regardless of the growth conditions. These contrasting effects of functional and neutral markers might indicate different underlying mechanisms causing the HFCs. Our results confirm the importance of considering marker functionality in HFC studies and indicate that parasites mediate HFCs by influencing the costs of immune defense rather than by a general increase in environmental harshness levels.     

Philornis infection in blue‐black grassquits: impact on nestlings and risk factors involved

Parasitic botfly larvae (Philornis ssp., Diptera: Muscidae) are found in nests of several bird taxa, although prevalence and impact on nestling survival vary considerably among species. Here we describe patterns of botfly infestation in blue‐black grassquit Volatinia jacarina nestlings. We identified the most typically affected nestling body parts and assessed parasite prevalence, impact on nestling survival, and changes in nestling body shape. Additionally, we tested whether climatic conditions, nest morphology and habitat characteristics are associated with larvae abundance. Blue‐black grassquits had low breeding success (16% of eggs/nestlings survived to fledged; 19% of the nests fledged at least one), but most failures resulted from predation by vertebrate predators. We estimated that 1% of nestlings died due to botfly infestation, and the number of subcutaneous larvae (range 1–18) in a nestling's body did not predict fledging success. Infected chicks exhibited higher tarsus asymmetry. Thus, we argue that although botflies had a small impact on offspring survival, they may reduce fitness in adulthood. There was no evidence that environmental conditions and nest morphology are linked to the number of larvae on nestlings. Nesting areas with higher food supply had lower infestation rates. Possibly, food‐rich habitats allow parents to invest more time in offspring care (brooding nestlings), thus protecting them from fly attacks. Alternatively, vegetation composition could influence local invertebrate diversity, which could provide a natural trophic buffer against adult Philornis. The present study brings to light new perspectives concerning bird–botfly interaction.     

Growth of yellow-headed blackbird Xanthocephalus xanthocephalus nestlings in relation to maternal body condition, egg mass, and yolk carotenoids concentrations

Condition‐dependent resource allocation to eggs can affect offspring growth and survival, with potentially different effects on male and female offspring, particularly in sexually dimorphic species. We investigated the influence of maternal body condition (i.e., mass‐tarsus residuals) and two measures of female resource allocation (i.e., egg mass, yolk carotenoid concentrations) on nestling mass and growth rates in the polygynous and highly size dimorphic yellow‐headed blackbird Xanthocephalus xanthocephalus. Egg characteristics and carotenoid concentrations were obtained from the third‐laid egg of each clutch and were correlated with the mass and growth rates of the first two asynchronously hatched nestlings. Maternal body condition was associated with the growth of first‐hatched, but not second‐hatched nestlings. Specifically, females in better body condition produced larger and faster growing first‐hatched nestlings than females in poorer body condition. As predicted for a polygynous, size‐dimorphic species, females that fledged first‐hatched sons were in better body condition than females that fledged first‐hatched daughters. Associations between egg mass, yolk carotenoid content, and nestling growth were also specific to hatching‐order. Egg mass was positively correlated with the mass and growth rates of second‐hatched nestlings, and yolk concentrations of β‐carotene were positively correlated with second‐hatched nestling mass. Surprisingly, the relationship between yolk lutein and hatchling growth differed between the sexes. Females with high concentrations of yolk lutein produced larger and faster growing first‐hatched sons, but smaller first‐hatched daughters than females with lower lutein concentrations. Mass and growth rates did not differ between first‐ and second‐hatched nestlings of the same sex, despite asynchronous hatching in the species. Results from this study suggest that maternal body condition and the allocation of resources to eggs have carotenoid‐, sex‐, and/or hatch‐order‐specific effects on yellow‐headed blackbird nestlings.     

A parasite-induced maternal effect can reduce survival times of fleas feeding on great tit nestlings

Parents can increase their reproductive success by assisting their neonate offspring in parasite defence. In birds, parental tactics include post-hatching parental responses such as increased parental care and pre-hatching maternal effects such as the transfer of maternal antibodies via the egg. These parasite-induced parental responses are known to reduce the effects of parasites on offspring, but their costs for the parasite are largely unknown. In two separate experiments on great tits Parus major we assessed these costs for hen fleas Ceratophyllus gallinae . Half of the parents where exposed to fleas during egg-laying to induce the parental response, while control nests were left flea-free. In experiment 1 parents raised their own young and we measured the effect of combined pre- and post-hatching parental effects, while in experiment 2 a cross-foster design allowed us to assess the effects of pre-hatching maternal effects alone. In both experiments we let fleas take a blood meal on nestlings from either flea-exposed or unexposed parents. We then measured flea-feeding duration, the quantity of extracted blood, and the fleas' subsequent survival time. We found in both experiments that on the largest nestlings of a brood flea survival was significantly reduced by the parental effects, whereas on the smaller nestlings it was independent of parental effects. The pre- and post-hatching parental responses did neither affect duration nor size of a flea blood meal. These results suggest first that the pre-hatching maternal effects, i.e. the substances transferred to the nestling via the egg, have the potential to harm fleas without reducing flea feeding capacity, and second that the strength of the maternal response varies between the nestlings, either because maternal products are unequally distributed among eggs within a clutch, or because large nestlings can build up a response that enhances the effect of the maternal products.     

Discrepancy between factors affecting nestling growth and survival and maternal success in Common Grackles

ABSTRACT Multiple factors potentially affect nestling survival and maternal reproductive success. However, little is known about the relative importance of different factors when operating simultaneously or whether the same factors are important for nestlings and their mothers. We determined the effect of hatching asynchrony, individual egg size, mean egg size, nestling sex, and clutch initiation date on the survival of individual nestlings and on maternal reproductive success in Common Grackles (Quiscalus quiscula) from 2004 to 2006 in central Illinois. Factors most important to maternal success differed from those important for individual nestling growth and survival. Hatching asynchrony had the greatest within‐nest influence on the fate of nestlings; the earlier a nestling hatched relative to siblings, the greater its mass and likelihood of fledging. Clutch size had the greatest influence on maternal reproductive success, with females with larger clutches fledging more young. Thus, both nestling survival and maternal success were largely determined by a single, albeit different, factor. A possible explanation for the apparent unimportance of most factors we measured in determining maternal success is that we did not consider variation among females. Individual variation in maternal attributes such as condition, size, age, experience, or mate quality may result in females tailoring clutch attributes (i.e., egg size, sex, and degree of hatching asynchrony) in ways that allow them to maximize their reproductive success. The discordance between factors that benefited mothers versus their offspring illustrates the importance of considering the maternal consequences of any factor that appears to affect offspring survival. Factors that increase the mass and survival of some offspring may not result in increased maternal reproductive success.     

Adaptive host-abandonment of ectoparasites before fledging? Within-brood distribution of nest mites in house sparrow broods

We studied the within-brood distribution of a haematophagous mite Pellonyssus reedi living on nestling house sparrows (Passer domesticus) near the time of fledging. We measured the natural level of infestation of individual nestlings, and determined the feeding efficiency of mites, by scoring their feeding status. Within-brood distribution of mite loads was unrelated to nestling body mass, tarsus length, or immunocompetence. These results did not support parasite preference for large or susceptible hosts. Mite feeding-efficiency was also unrelated to these nestling characteristics, confirming that large nestlings or nestlings with less-developed immunocompetence did not provide superior feeding conditions for mites. Therefore, our results do not support the hypothesis that within-brood distribution of avian ectoparasites is explained by the parasites' preferences for characteristics, such as large body size or low immunocompetence, that make nestlings suitable hosts. On the other hand, we found that mite loads were negatively correlated with nestling age and feather length, suggesting that nestlings closer to fledging harbored fewer mites then their less-developed nestmates. Furthermore, feather length had a stronger relationship with parasite distribution than did nestling age. We presume, therefore, that feather characteristics, i.e., length, may serve as a signal for mites to perceive the ready-to-fledge state of nestlings, inducing abandonment behavior. These results support another, largely neglected hypothesis, i.e., that the avoidance or abandonment of those nestlings that are close to fledging may also explain the parasites' distribution in a brood. This hypothesis is based on the argument that many nest-dwelling ectoparasites breed in the nest material and emerge only periodically to feed on nestlings. In such parasites, the ability to recognize and avoid mature fledglings can be adaptive because this may help the parasites to avoid their removal from the nest so they can continue to reproduce by feeding on unfledged chicks of the current or later broods. Our results suggest that adaptive host-abandonment by nest-dwelling ectoparasites can influence within-brood parasite distributions around the time of fledging.     

Intraseasonal variation in immune defence,body mass and hematocrit in adult house martins Delichon urbica

The intensity of parasite infections often increases during the reproductive season of the host as a result of parasite reproduction, increased parasite transmission and increased host susceptibility. We report within‐individual variation in immune parameters, hematocrit and body mass in adult house martins Delichon urbica rearing nestlings in nests experimentally infested with house martin bugs Oeciacus hirundinis and birds rearing nestlings in initially parasite‐free nests. From first to second broods body mass and hematocrit of breeding adult house martins decreased. In contrast leucocytes and immunoglobulins became more abundant. When their nests were infested with ectoparasites adults lost more weight compared with birds raising nestlings in nests treated with pyrethrin, whereas the decrease in hematocrit was more pronounced during infection with blood parasites. Neither experimental infestation with house martin bugs nor blood parasites had a significant effect on the amount of immune defences.     

Parasite-mediated growth patterns and nutritional constraints in a cavity-nesting bird

1. Trade-offs between growth and immunity of nestling birds can be influenced by parasites, but the magnitude of these effects may depend on availability of critical dietary nutrients. Owing to their importance for both immune system function and growth, dietary carotenoids have the potential to mediate parasite-induced developmental strategies of avian hosts. 2. The effects of ectoparasitic blow flies Protocalliphora spp. and dietary carotenoids (lutein and zeaxanthin) on immune function and patterns of growth in nestling mountain bluebirds Sialia currucoides were investigated by combining parasite removal and carotenoid supplementation treatments in a 2 x 2 design. 3. Supplemental carotenoids enhanced nestlings' T-cell-mediated immune response following intradermal injection of phytohaemagglutinin. 4. The effect of carotenoid supplementation on rate of mass gain depended on whether broods were exposed to parasites: among parasitized broods, those receiving supplemental carotenoids gained mass more rapidly than nonsupplemented broods, whereas there was no effect of supplemental carotenoids on growth of mass in broods that had parasites removed. This suggests that additional dietary carotenoids allowed nestlings to compensate for the otherwise detrimental effects of parasites on mass gain. For length of the eighth primary feather at fledging, early and late broods differed in their response to parasitism: early broods showed an increase in feather length when parasites were removed, while nestlings in late broods had shorter feathers in the absence of parasites. We suggest that this may reflect within-season variation in parasite-mediated growth strategies of nestlings. 5. Maternal condition was positively associated with mass, condition and rate of feather growth of offspring under all conditions, and also influenced nestling immunocompetence, but only in the absence of parasites. 6. We conclude that dietary carotenoids alleviate some of the detrimental effects of parasites on nestling birds; however, parasites also appear to specifically influence other growth and resource allocation strategies, and possibly constrain maternal or genetic effects on offspring phenotype, irrespective of dietary carotenoid availability.     

Avoidance,tolerance, and resistance to ectoparasites in nestling and adult tree swallows

We examined avoidance, tolerance, and resistance strategies of nestling and adult tree swallows Tachycineta bicolor in response to ectoparasitic blowflies Protocalliphora sialia. Tree swallows avoided settling in north‐facing nest boxes early in the breeding season. These boxes were more likely to be parasitized later in the season, suggesting that box selection may facilitate blowfly avoidance. After experimentally manipulating blowfly intensity, we found that nestlings were generally tolerant of parasitism. Parasites significantly reduced nestling blood hemoglobin but had no effect on nestling body mass, primary feather growth, age at fledging, or fledging success. Parents of parasitized nestlings did not increase their food provisioning rate to promote nestling tolerance. Adult female tree swallows demonstrated both tolerance and resistance: blowfly parasitism had no effect on adult hemoglobin and body mass, and those with higher P. sialia‐binding antibody levels had fewer blowfly larvae in their nests. Nestling antibodies were unrelated to blowfly intensity. Despite considerable variation among years, our results suggest that the costs of blowfly parasitism to nestling and adult tree swallows are modest, and limited to blood loss in nestlings. Future work should examine the effects of reduced blood hemoglobin on fledgling survival and the importance of parasite‐specific antibodies.     

Transgenerational immunity in a bird–ectoparasite system: do maternally transferred antibodies affect parasite fecundity or the offspring's susceptibility to fleas?

During egg formation, female birds deposit antibodies against parasites and pathogens they were exposed to before egg laying into the yolk. In captive bird species, it has been shown that these maternal immunoglobulins (maternal yolk IgGs) can protect newly hatched offspring against infection. However, direct evidence for such benefits in wild birds is hitherto lacking. We investigated (1) if nestling Great Tits Parus major originating from eggs with naturally high levels of maternal yolk IgG are less susceptible to a common, nest-based ectoparasite, (2) if maternal yolk IgGs influence nestling development and in particular, their own immune defence, and (3) if there is a negative correlation between levels of maternal yolk IgG in host eggs and the reproductive success of ectoparasitic fleas feeding on the nestlings. Counter to expectations, we found no indication that maternally transferred yolk IgGs have direct beneficial effects on nestling development, nestling immune response or nestling resistance or tolerance to fleas. Furthermore, we found no negative correlation between host yolk IgG levels and parasite fecundity. Thus, whereas previous work has unequivocally shown that prenatal maternal effects play a crucial role in shaping the parasite resistance of nestling birds, our study indicates that other egg components, such as hormones, carotenoids or other immuno-active substances, which bird females can adjust more quickly than yolk IgG, might mediate these effects.     

The process of fledging in the Mountain Bluebird

Fledging is a critical event in the avian breeding cycle, but remains unstudied in almost all species. As a result, little is known about factors that cause nestlings to leave nests. We documented fledging behavior in a box‐nesting population of Mountain Bluebirds (Sialia currucoides) using radio‐frequency identification. We attached a passive integrated transponder (PIT tag) to the leg of each nestling in 40 nests. An antenna checked for the presence of a transponder signal (i.e., a nestling) at nest‐box entrances every 2 s. The time of last detection of a nestling was taken as the time that nestling fledged. We found that fledging began when the oldest nestlings were 15–22 d old. Broods that were ahead in development, as measured by primary feather length, fledged at relatively younger ages. All nestlings fledged on the same day at 33 nests (83%) and over 2 d at remaining nests. When all nestlings fledged on the same day, fledging usually began in the morning and median time between the first and last fledging was 55 min (range = 2.3 min–10.6 h). When young fledged over 2 d, fledging always began >8 h after sunrise and usually just one nestling fledged the first day, suggesting that this fledging may have been accidental. Clutches in our population often hatch asynchronously, which sets up a hierarchy within broods in developmental state, size, and competitive ability. In such situations, fledging may be initiated by one of the most‐developed and hence most‐competitive nestlings in a brood, presumably when it reaches a certain threshold state of development. Alternatively, fledging may begin when a less‐developed, less‐competitive, and probably hungrier nestling leaves the nest, presumably to gain better access to food. We used the proportion of time that a nestling was able to occupy the nest‐box entrance late in the nestling stage, waiting to intercept parents with food, as an index of nestling competitive ability. Assuming that the number of nest entrance detections reliably indicates nestling competitive ability, we found that the most‐competitive nestling fledged first at over half of all nests, supporting the notion that fledging usually begins when oldest nestlings reach a threshold state of development.     

The Process and Causes of Fledging in a Cavity-Nesting Passerine Bird, the House Wren (Troglodytes aedon)

Little is known about the process or causes of fledging or nest‐leaving in passerine birds because researchers can rarely predict when fledging will occur in a given nest. We used continuous videotaping of nests to both document the process of fledging in the house wren, Troglodytes aedon, a small, cavity‐nesting songbird, and test hypotheses as to what might cause fledging to begin. Fledging began any time from 14 to 19 d after hatching commenced. Slower‐developing broods fledged later than faster‐developing broods. Fledging typically began within 5 h of sunrise and over 80% of all nestlings fledged before noon. All nestlings fledged on the same day at 65% of nests and over two consecutive days in most other nests. We found no evidence that fledging was triggered by changes in parental behaviour. Parental rate of food delivery to nestlings did not decline during a 3‐h period leading up to the first fledging, nor was the rate of feeding just prior to the first fledging lower than the rate at the same time the day before. Moreover, parents did not slow the rate of food delivery to nests after part of the brood had fledged. Hatching is asynchronous in our study population which creates a marked age/size hierarchy within broods. At most nests, the first nestling to fledge was the most well‐developed nestling in the brood or nearly so (as measured by feather length). This suggests that fledging typically begins when the most well‐developed nestlings in the brood reach some threshold size. However, at about one‐fifth of nests, the first nestling to fledge was only moderate in size. At these nests, severe competition for food may have caused smaller, less competitive nestlings to fledge first to increase their access to food. We found no strong support for the suggestion that the oldest nestlings delay fledging until their least‐developed nestmate reaches some minimum size, although further experimental work on this question is warranted.     

Does avian malaria reduce fledging success: an experimental test of the selection hypothesis

Like many parasites, avian haematozoa are often found at lower infection intensities in older birds than young birds. One explanation, known as the “selection” hypothesis, is that infected young birds die before reaching adulthood, thus removing the highest infection intensities from the host population. We tested this hypothesis in the field by experimentally infecting nestling rock pigeons (Columba livia) with the malaria parasite Haemoproteus columbae. We compared the condition and fledging success of infected nestlings to that of uninfected controls. There was no significant difference in the body mass, fledging success, age at fledging, or post-fledging survival of experimental versus control birds. These results were unexpected, given that long-term studies of older pigeons have demonstrated chronic effects of H. columbae. We conclude that H. columbae has little impact on nestling pigeons, even when they are directly infected with the parasite. Our study provides no support for the selection hypothesis that older birds have lower parasite loads because parasites are removed from the population by infected nestlings dying. To our knowledge, this is the first study to test the impact of avian malaria using experimental inoculations under natural conditions.