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.     

Experimental pyrethroid treatment underestimates the effects of ectoparasites in cavity‐nesting birds due to toxicity

Nest‐dwelling ectoparasites may result in costs for nestlings of cavity nesters in terms of compromised growth and condition before fledging. The reduction or elimination of nest ectoparasites to study their effects on avian hosts can be conducted through physical methods such as heat‐treatment or through chemical methods using insecticides. Pyrethroids are the most frequently used of the latter, although some studies have shown that they may compromise the development and future survival of birds. In this study conducted in central Spain we analysed the differences between a group of fumigated Pied Flycatcher Ficedula hypoleuca nests and a heat‐treated group, both rendered ectoparasite‐free by these treatments. We also compared these ectoparasite‐free nests with a control group with natural ectoparasite loads. Our aim was to test the possible effects of a pyrethroid‐based insecticide on reproductive success, parental care behaviours and body condition of adult females and nestlings. We also determined the effects of treatment on a biochemical biomarker, the total glutathione (tGSH) level, involved in detoxification of xenobiotics and considered the most important intracellular antioxidant. Although behavioural variables were not affected by treatment, results showed lighter 3‐day‐old chicks and shorter tarsi and wings in nestlings shortly before fledging in fumigated nests, together with depletion of tGSH levels in both females and nestlings. Fumigation with pyrethroids in ectoparasite load reduction experiments may introduce undesired systematic variability associated with toxicity, leading to underestimation of the effects of ectoparasites on avian hosts.     

Ectoparasitism and the role of green nesting material in the European starling

Summary The use of green nesting material is widespred among birds. Recent evidence suggests that birds use secondary chemicals contained in green plants to control ectoparasites. We manipulated green nesting material and ectoparasites of European starlings (Sturnus vulgaris) to test two hypotheses: (1) ectoparasites adversely affect prefledging survival and morphometrics or postfledging survival, and (2) green nesting material ameliorates the effects of ectoparasites. We recorded fat score, numbers of scabs, tarsal length, body mass, and hematocrit level on each nestling 17 days after hatching. We also fitted each nestling with unique patagial tags and resighted the starlings for 6–8 weeks after fledging to estimate survival and sighting rates. Nests devoid of green nesting material and dusted with the insecticide, carbaryl, had fewer high ectoparasite infestations, and nestlings had significantly lower scab scores, and significantly higher body masses than nestlings in undusted boxes. However, there was no difference in postfledging survival between birds from carbaryl-treated and undusted nests. There also was no difference in prefledging survival and morphometrics or postfledging survival between nestlings from boxes with and without green nesting material. These results do not support the hypothesis that starlings use green nesting material to control nest ectoparasites. We suggest an alternative hypothesis; green nesting material is used for mate selection or pairbonding in the starling.     

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.     

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.     

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.     

Experimental addition of greenery reduces flea loads in nests of a non-greenery using species, the tree swallow Tachycineta bicolor

Several bird species, including cavity-nesters such as European starlings Sturnus vulgaris , add to their nests green sprigs of plants such as yarrow Achillea millefolium that are rich in volatile compounds. In this field study on another cavity-nester, tree swallows Tachycineta bicolor , we tested whether yarrow reduced ectoparasite loads (the nest protection hypothesis), stimulated nestling immune systems (the drug hypothesis), or had other consequences for nestling growth or parental reproductive success (predicted by both preceding hypotheses). Tree swallows do not naturally add greenery to their nests, and thus offer several advantages in testing for effects of greenery independent of other potentially confounding explanations for the behaviour. We placed fresh yarrow in 23 swallow nests on the day the first egg was laid, replenishing every two days until clutch completion (=three times), and at 44 control nests, nesting material was simply touched. At 12 days of age, we measured nestling body size and mass, and took blood smears to do differential white blood cell counts. We subsequently determined the number and proportion of young fledging from nests and the number of fleas remaining after fledging. Higher humidity was associated with higher flea numbers whereas number of feathers in the nest was not. Our most significant finding was that an average of 773 fleas Ceratophyllus idius was found in control nests, versus 419 in yarrow nests. Possibly, parents compensate for blood that nestlings lose to ectoparasites by increasing food delivery, because we detected no differences between treatments in nestling mass, nestling leukocyte profiles, or proportion of young fledging, or relative to flea numbers. Our results provide no support for the drug hypothesis and strong support for the nest protection hypothesis.     

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.     

Nest‐dwelling ectoparasites influence the start and duration of the first pre‐basic moult in the European starling Sturnus vulgaris

Nest‐dwelling ectoparasites represent an early stressor for birds as they impair the development of nestlings, which can adaptively respond by adjusting their growth rate to current conditions. While nest ectoparasites have long‐term effects on nesting adults, no study has examined if they also have an impact on the moult patterns of juveniles. Herein, we investigated whether the presence of ectoparasites in the nest influences the start and duration of the first pre‐basic moult in the European starling. To do so, we experimentally removed nest‐dwelling ectoparasites from a group of nests and used another group of unmanipulated (i.e. naturally infested) nests as the control. The moult began at an earlier age and lasted longer in birds from the ectoparasite‐free nests compared to their control counterparts. The timing of the moult was also affected by the hatching date (i.e. birds that fledged later had shorter moults) and the brood size (i.e. birds in larger broods started their moult at an older age). We also found evidence that the removal of nest ectoparasites influenced the condition of individuals during the course of the moult. In the control birds, we observed a decrease in hematocrit levels, but these were unaltered in the birds fledged from the ectoparasite‐free nests. Our study shows that nest‐dwelling ectoparasites adversely affected the timing of the moult and the individual condition of juvenile starlings, with possible major consequences for their subsequent life‐history events.     

Do parents play a role in the timing and process of fledging by nestling Mountain Bluebirds?

What causes young birds to leave nests remains unclear for almost all altricial species. For many years, the assumption was that parents often controlled the time of fledging by coaxing young from nests, e.g., by holding food within view, but out of reach, of nestlings. This assumption, though, was based solely on scattered anecdotal reports of such behavior. We used continuous video‐recording of nests to assess the role of parents, if any, in the timing and process of fledging of cavity‐nesting Mountain Bluebirds (Sialis currucoides). We placed perches ~50 cm in front of nest‐box entrances to give parents ample opportunity to display food to nestlings. We found no evidence that parents routinely initiated the fledging process. On the day of fledging, parents did not perch on supplemental perches with food more often, or for longer periods of time, than on the day before fledging. Also, after going to nest‐box entrances, parents never held food away from a nestling reaching for the food. Parents were usually absent (16 of 19 cases) when the first nestling fledged. In the remaining three cases, a parent perched with food in view of a nestling for 8, 15 and 65 s, respectively, just before that nestling fledged. Although these might have appeared to be attempts at coaxing, in each case, the parent was encountering, for the first time, a nestling partially emerging from the nest entrance. Parents may simply have hesitated to approach nests because the nestling's position prevented parents from delivering food in the normal manner. Finally, the rate at which parents fed nestlings on the day of fledging did not differ from the rate the day before, suggesting that parents do not try to use hunger to induce fledging. Our results are consistent with previous research suggesting that, in Mountain Bluebirds, it is a nestling that initiates fledging, typically when it reaches some threshold state of development.     

Maternal effects reduce oxidative stress in female nestlings under high parasite load

Mothers can adjust the phenotype of their offspring to the local environment through a modification of their egg investment and/or nestling provisioning. However, offspring health may be severely impaired if the conditions experienced by nestlings do not match with those anticipated by the mother. If maternal effects differentially affect the sexes or if one sex is more strongly affected by an environmental stressor, fitness benefits may also differ between male and female offspring. Here, we study maternal effects in male and female great tit Parus major nestlings by means of an ectoparasite treatment before egg‐laying combined with a partial cross‐foster experiment between broods of infested and uninfested nests. Nestlings that were raised in their own nest experienced the same conditions before and after cross‐fostering (either in parasite infested or uninfested nests), while cross‐fostered ones experienced different conditions (either changing from infested to uninfested or the other way around). We measured effects on nestling plasma levels of oxidative stress [reactive oxygen metabolites (ROMs) and total antioxidant capacity (OXY)], body condition (body size and mass) and post‐fledging survival. Daughters, but not sons, from matching conditions showed the lowest ROM and high OXY levels when exposed to parasites, while there was no effect of parasite exposure in any of both sexes in case of a mismatch. In contrast, body condition and post‐fledging survival were not (or only slightly) affected by any of the experimental treatments. Results of this study show that maternal effects can affect oxidative stress levels of nestlings in a sex‐specific way and that the outcome depends on the exposure to environmental stressors, such as parasites.     

Physiological ecology of Mediterranean blue tits (Parus caeruleus L.): effects of ectoparasites (Protocalliphora spp.) and food abundance on metabolic capacity of nestlings

The consequences of nest ectoparasites, such as Protocalliphora larvae, on nestling birds have been the subject of numerous studies. Despite observed reductions in mass and hematocrit of chicks from parasitized nests, no studies have found any effect of Protocalliphora on nestling survival, suggesting that fitness consequences of Protocalliphora are either weak or occur after fledging. From experiments on the metabolic performance of chicks, we found that parasitized chicks suffer from reduced thermogenic and metabolic capacities as a result of decreased mass and hematocrit. Hence, Protocalliphora may potentially affect nestling survival after fledging, when energetically costly activities such as flight and moult are undertaken. Previous studies have demonstrated an increase in parental feeding rate to compensate for the detrimental consequences of parasite infestation. We tested whether parasite effects on nestling aerobic capacity were dependent on food availability during the feeding period. Measures of caterpillar densities and experimental manipulations of parasite loads allowed us to investigate relationships among host, parasite, and environment. A positive relationship between chick aerobic and thermogenic performances and caterpillar density suggests that negative effects of parasitism may be offset by increased food availability. This study provides the first measurement of the effects of an ectoparasite on metabolic competence in wild birds and documentation of the effect of food availability on ectoparasite virulence using a quantitative measure of food abundance.     

A review of available methods and description of a new method for eliminating ectoparasites from bird nests

Bird nests offer an ideal situation to manipulate ectoparasites and study how they impact hosts. Several methods are available to eliminate parasites from nests and each has its own suite of advantages and disadvantages. For example, recent toxicity research has revealed that some commonly used insecticides may not be suitable for use in experiments with nestlings. This highlights the need for investigators to control for the effects of methods used to eliminate nest parasites within experimental designs. Methods that can be used across treatment groups are also often needed to study the effects of variation in parasite intensity. To aid investigators in deciding which method(s) to use, we provide a comprehensive review of available methods for eliminating nest ectoparasites and also describe a new heat gun method. We tested the effectiveness of the heat‐gun method with nests of Barn Swallows (Hirundo rustica) to which 100 nest mites were added and then quantified the number of surviving mites and other naturally occurring arthropods. We found that fully heated nests had significantly fewer mites and other arthropods than partially heated or control nests. Use of the heat gun had no negative effects on nestling growth or mortality rates. In studies of avian nest ectoparasites, investigators need to consider methods that can be used across treatment groups to ensure that unaccounted for toxicity effects are not influencing results and leading to underestimation of the often subtle effects of ectoparasites on birds.     

Females compensate for moult‐associated male nest desertion in Hooded Warblers

Uniparental offspring desertion occurs in a wide variety of avian taxa and usually reflects sexual conflict over parental care. In many species, desertion yields immediate reproductive benefits for deserters if they can re‐mate and breed again during the same nesting season; in such cases desertion may be selectively advantageous even if it significantly reduces the fitness of the current brood. However, in many other species, parents desert late‐season offspring when opportunities to re‐nest are absent. In these cases, any reproductive benefits of desertion are delayed, and desertion is unlikely to be advantageous unless the deserted parent can compensate for the loss of its partner and minimize costs to the current brood. We tested this parental compensation hypothesis in Hooded Warblers Setophaga citrina, a species in which males regularly desert late‐season nestlings and fledglings during moult. Females from deserted nests effectively doubled their provisioning efforts, and nestlings from deserted nests received just as much food, gained mass at the same rate, and were no more likely to die from either complete nest predation or brood reduction as young from biparental nests. The female provisioning response, however, was significantly related to nestling age; females undercompensated for male desertion when the nestlings were young, but overcompensated as nestlings approached fledging age, probably because of time constraints that brooding imposed on females with young nestlings. Overall, our results indicate that female Hooded Warblers completely compensate for male moult‐associated nest desertion, and that deserting males pay no reproductive cost for desertion, at least up to the point of fledging. Along with other studies, our findings support the general conclusion that late‐season offspring desertion is likely to evolve only when parental compensation by the deserted partner can minimize costs to the current brood.     

Determinants of abundance and effects of blood-sucking flying insects in the nest of a hole-nesting bird

Compared to non-flying nest-dwelling ectoparasites, the biology of most species of flying ectoparasites and its potential impact on avian hosts is poorly known and rarely, if ever, reported. In this study we explore for the first time the factors that may affect biting midge (Diptera: Ceratopogonidae) and black fly (Diptera: Simuliidae) abundances in the nest cavity of a bird, the hole-nesting blue tit Cyanistes caeruleus, and report their effects on adults and nestlings during reproduction. The abundance of biting midges was positively associated with nest mass, parental provisioning effort and abundance of blowflies and black flies, while negatively associated with nestling condition. Furthermore, a medication treatment to reduce blood parasitaemias in adult birds revealed that biting midges were more abundant in nests of females whose blood parasitaemias were experimentally reduced. This finding would be in accordance with these insect vectors attacking preferentially uninfected or less infected hosts to increase their own survival. The abundance of black flies in the population was lower than that of biting midges and increased in nests with later hatching dates. No significant effect of black fly abundance on adult or nestling condition was detected. Blood-sucking flying insects may impose specific, particular selection pressures on their hosts and more research is needed to better understand these host–parasite associations.     

Green plants in starling nests: effects on nestlings

European starlings, Sturnus vulgaris, intermingle fresh herbs, especially species rich in volatile compounds, with their otherwise dry nest material. In this field study we investigated whether these herbs reduce ectoparasites and thereby protect nestlings (the nest protection hypothesis). We also considered whether volatile compounds in herbs improve the condition of nestlings (the drug hypothesis). As measures of condition we used body mass, haematocrit levels and immunological parameters. We replaced 148 natural starling nests with artificial ones: half contained herbs and half (controls) contained grass. The ectoparasite loads (mites, lice, fleas) in herb and control nests were indistinguishable. However, nestlings in herb nests weighed more and had higher haematocrit levels at fledging than nestlings in control nests. Fledging success was similar in herb and control nests, but more yearlings from herb nests were identified in the colony the year after hatching. The response of the immune system when challenged with phytohaemagglutinin did not differ in nestlings from herb and control nests. Nestlings from herb nests had more basophils and fewer lymphocytes in their blood than those from control nests, while the eosinophil and heterophil counts did not differ. We conclude that herbs do not reduce the number of ectoparasites, but they improve the condition of nestlings, perhaps by stimulating elements of the immune system that help them to cope better with the harmful activities of ectoparasites. Copyright 2000 The Association for the Study of Animal Behaviour.     

The price of insulation: costs and benefits of feather delivery to nests for male tree swallows Tachycineta bicolor

Many species of birds line their nests with feathers, and it has been hypothesized that this functions to provide a thermally stable microenvironment for the development of eggs and nestlings. Feathers in the nest may also function as a mechanism for parasite control, providing a physical barrier that protects nestlings from ectoparasites. We tested these hypotheses by performing a feather removal and addition experiment in tree swallows Tachycineta bicolor, a species well‐known for lining their nests with feathers. While we found no evidence that quantity of feathers in nests influenced the ability of females to produce and incubate eggs, offspring in well‐feathered nests had longer flight feathers and were structurally larger just prior to fledging that those in nests with fewer feathers. Furthermore, we also demonstrated a positive correlation between feathers and the abundance of larval blow flies Protocalliphora spp. in nests, a result opposite to that predicted by the anti‐parasite hypothesis. While our study provides strong support for the insulation hypothesis, we also discuss the possibility that devoting time to feather gathering may result in males losing paternity in their nests, although manipulative studies will be necessary to fully evaluate this idea.     

Experimental evidence for host preference in a tick parasitizing songbird nestlings

Mechanisms of host preference in ectoparasites are important to the understanding of host‐parasite interactions. Since ectoparasites negatively affect the condition of their hosts, while the hosts’ condition itself may affect the parasites’ choice, separating the factors that drive host preference from parasite impact asks for experiments. We combined the data of two choice experiments to investigate the preference of the nidicolous tick Ixodes arboricola when exposed to the nestlings of a passerine bird (Parus major). In the first experiment, in which complete broods at hatching were exposed to an ecologically relevant number of ticks, the relationship between tick loads and nestlings’ developmental status was characterized by a distribution with the highest tick loads on the more developed nestlings. Host preference became more apparent at a smaller brood size, suggesting a role for host density. In a second experiment we evaluated host choice in a pairwise choice experiment, exposing pairs of siblings with contrasting developmental status to eight ticks. In the first and the second pair, a median developed nestling was linked with the most developed and the least developed nestling, respectively. Seventy‐two h after tick exposure we measured the innate constitutive humoral immunity and haematocrit. No differences were found in innate immunity, but the least developed nestlings had on average a lower haematocrit than the median and most developed nestlings. Significantly fewer ticks attached on the least developed nestling compared to the median nestling, and this difference was more pronounced when the innate immunity of the median developed nestling was higher. No difference in tick load was found among the median and best developed nestlings. The linkage between host preference and host physiological condition provide further insight in the mechanisms driving ectoparasite aggregation, which is important for the population dynamics of host, ticks and tick‐transmitted pathogens.     

Nestling sex predicts susceptibility to parasitism and influences parasite population size within avian broods

Vertebrate hosts differ in their level of parasite susceptibility and infestation. In avian broods, variation in susceptibility of nestlings to ectoparasites may be associated with non‐uniform distributions of parasites among brood mates, with parasites concentrating feeding on the most vulnerable hosts. The presence of a highly susceptible nestling in a brood can benefit the remaining young by reducing the parasite pressure they experience; however, from a parasite’s perspective, broods with fewer susceptible hosts may provide effectively fewer resources than broods of the same size containing a greater abundance of susceptible hosts, and this could limit the number of parasites that a host brood can sustain. To test whether variation in number of susceptible hosts affects the number of parasites in bird nests, we first examined the role of host sex and induced immunity (via methionine supplementation) on susceptibility of mountain bluebirds Sialia currucoides to parasitism by blow flies Protocalliphora spp. We then assessed the effect of variation in number of susceptible hosts on the number of parasites inhabiting the nest. Only females showed a benefit of methionine supplementation, gaining mass more rapidly following supplementation compared to males. This suggests that females are more susceptible to parasites in this system; this was further supported by parasite feeding trials, in which parasites extracted larger blood meals from female than male hosts. Finally, the abundance of parasites in nests was predicted by brood sex ratio: broods containing more female young harboured more parasites. Hence, within‐brood variation in host susceptibility to parasites can not only influence the costs of parasitism for individual nestlings, but may also have consequences for the size of parasite populations within nests. If patterns of maternal investment affect the abundance of nest‐dwelling parasites, these interactions may be important for understanding fitness consequences of maternal resource allocation in many vertebrate hosts.     

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.