Heinz Richner

Effect of an ectoparasite on reproduction in great tits

1. The effect of a haematophageous ectoparasite, the hen flea, on quality an number of offspring was experimentally investigated in the great tit. The experiment consisted of a controlled infestation of a random sample of nests with the parasitic flea and of a regular treatment of control nests with Microwaves in order to eliminate the naturally occurring fleas. 2. To assess the effects of fleas on variables related to offspring number, we considered the number of hatchlings and fledglings, the mortality between hatching and fledging, and the hatching and fledging success. For assessment of offspring quality, we measured body mass, tarsus and wing length, and calculated the nutritional condition of, nestlings as the ratio of body mass to tarsus length. A physiological variable, the haematocrit level, was also measured. 3. Hatching success and hatchling numbers did not differ between the two experimental groups. Offspring mortality between hatching and fledging was significantly higher in the infested broods (xBAR = 0.22 chicks dead per day) than in the parasite-free broods (xBAR = 0.07 dead per day). Fledging success was 83% in the parasite-free broods, but only 53% in the infested ones. The number of fledglings in infested broods (xBAR = 3.7 fledglings +/-2.1 SD) was significantly lower than in the parasite-free (xBAR = 4.9 +/- 1.1 SD) broods. 4. Body mass of chicks in the infested broods was significantly smaller than in the parasite-free broods both 14 days and 17 days after hatching. The chicks in the infested broods reached a significantly smaller tarsus length than the ones in the parasite-free broods. Close to fledging, the nutritional condition of chicks was significantly lower in infested broods. Haematocrit levels were significantly lower in the infested broods. 5. Brood size correlated differently with body mass and condition of chicks in infested and parasite-free nests. In parasite-free broods both body mass and condition of chicks at age 17 days, i.e. close to fledging, were significantly higher in small broods than in large ones. However, in the infested broods chicks were of the same body mass and condition in large as in small broods. Therefore, in parasite-free broods fitness can potentially be gained through offspring quality or number or both, whereas in infested broods it can be gained through offspring quantity only. In other words, a trade-off between quality and number of offspring is feasible only in the absence of the parasitic hen flea. 6. These results emphasize the need to study the effects of ectoparasites on ecological, behavioural and evolutionary traits of their bird hosts. A knowledge of these effects is essential for the understanding of population dynamics, behaviour and life-history traits of the hosts.

The design of artificial nestboxes for the study of secondary hole-nesting birds: a review of methodological inconsistencies and potential biases

Abstract. The widespread use of artificial nestboxes has led to significant advances in our knowledge of the ecology, behaviour and physiology of cavity nesting birds, especially small passerines. Nestboxes have made it easier to perform routine monitoring and experimental manipulation of eggs or nestlings, and also repeatedly to capture, identify and manipulate the parents. However, when comparing results across study sites the use of nestboxes may also introduce a potentially significant confounding variable in the form of differences in nestbox design amongst studies, such as their physical dimensions, placement height, and the way in which they are constructed and maintained. However, the use of nestboxes may also introduce an unconsidered and potentially significant confounding variable due to differences in nestbox design amongst studies, such as their physical dimensions, placement height, and the way in which they are constructed and maintained. Here we review to what extent the characteristics of artificial nestboxes (e.g. size, shape, construction material, colour) are documented in the ‘methods’ sections of publications involving hole-nesting passerine birds using natural or excavated cavities or artificial nestboxes for reproduction and roosting. Despite explicit previous recommendations that authors describe in detail the characteristics of the nestboxes used, we found that the description of nestbox characteristics in most recent publications remains poor and insufficient. We therefore list the types of descriptive data that should be included in the methods sections of relevant manuscripts and justify this by discussing how variation in nestbox characteristics can affect or confound conclusions from nestbox studies. We also propose several recommendations to improve the reliability and usefulness of research based on long-term studies of any secondary hole-nesting species using artificial nestboxes for breeding or roosting.

Immunocompetence of nestling great tits in relation to rearing environment and parentage

Theoretical models of host–parasite coevolution assume a partially genetic basis to the variability in susceptibility to parasites among hosts, for instance as a result of genetic variation in immune function. However, few empirical data exist for free–living vertebrate hosts to support this presumption. In a cross–fostering experiment with nestling great tits, by comparing nestlings of the same origin we investigated (i) the variance in host resistance against an ectoparasite due to a common genetic origin, (ii) the effect of ectoparasite infestation on cell–mediated immunity and (iii) the variance in cell–mediated immunity due to a common genetic origin. Ectoparasitic hen fleas can impair the growth of nestling great tits and nestling growth was therefore taken as a measure of host susceptibility. A common origin did not account for a significant part of the variation in host susceptibility to fleas. There was no significant overall effect of fleas on nestling growth or cell–mediated immunity, as assessed by a cutaneous hypersensitivity response. A common rearing environment explained a significant part of the variation in cell–mediated immunity among nestlings, mainly through its effect on nestling body mass. The variation in cell–mediated immunity was also related to a common origin. However, the origin–related variation in body mass did not account for the origin–related differences in cell–mediated immunity. The results of the present study thus suggest heritable variation in cell–mediated immunity among nestling great tits.

The quantitative genetic basis of offspring solicitation and parental response in a passerine bird with biparental care

The coevolution of parental investment and offspring solicitation is driven by partly different evolutionary interests of genes expressed in parents and their offspring. In species with biparental care, the outcome of this conflict may be influenced by the sexual conflict over parental investment. Models for the resolution of such family conflicts have made so far untested assumptions about genetic variation and covariation in the parental resource provisioning response and the level of offspring solicitation. Using a combination of cross-fostering and begging playback experiments, we show that, in the great tit (Parus major), (i) the begging call intensity of nestlings depends on their common origin, suggesting genetic variation for this begging display, (ii) only mothers respond to begging calls by increased food provisioning, and (iii) the size of the parental response is positively related to the begging call intensity of nestlings in the maternal but not paternal line. This study indicates that genetic covariation, its differential expression in the maternal and paternal lines and/or early environmental and parental effects need to be taken into account when predicting the phenotypic outcome of the conflict over investment between genes expressed in each parent and the offspring.

Host Responses to Ectoparasites: Food Compensation by Parent Blue Tits

Parental food compensation has been proposed to account for the absence or small negative effects of parasites on offspring in various bird-parasite systems. An increase in the quantity of energy and nutrients provided by the adults would therefore compensate for the offsprings loss of blood to blood-sucking ectoparasites. We studied parental food compensation in a blue tit Parus caeruleus population with experimentally controlled infestations by the bird flea Ceratophyllus gallinae. Parental feeding effort, offspring quality, and parasite reproduction were measured in randomly assigned parasite-free and infested broods. Although the ectoparasites reproduced at a high rate in infested nests, the nestlings did not suffer higher mortality or reduced body size and body condition than nestlings in parasite-free nests. Fleas had a small negative effect on feather development of nestlings. Parent blue tits of infested nests increased rate of food provisioning by 29%. The results support the parental food compensation hypothesis. No short-term costs (i.e. lowered body condition) of parasites on the parents could be detected. Although the long-term costs of parasites on parents were not measured in this study, the results lend support to previous suggestions that the absence of an effect of ectoparasites on the offspring may be due to the fact that parents bear the cost of parasitism.

Sperm of colourful males are better protected against oxidative stress

Sperm cells are highly vulnerable to free radicals, and sperm quality and male fertility are critically affected by oxidative stress. Recently, sexual ornaments, particularly carotenoid-based colourful traits, have been proposed to depend on a males capacity to resist oxidative stress, and thus to signal sperm quality. We conducted an experimental test of this hypothesis on great tits Parus major, in which adults are sexually dichromatic in carotenoid-based breast plumage. We report the first evidence that ornaments and sperm quality may be linked through oxidative stress. When experimentally subjected to oxidative stress resulting from increased workload, less colourful males suffered a greater reduction in sperm motility and swimming ability, and increased levels of sperm lipid peroxidation compared to more colourful males. Moreover, the level of sperm lipid peroxidation was negatively correlated with sperm quality. Finally, carotenoid supplementation increased sperm quality of less colourful males, suggesting that pale males are deficient in carotenoid antioxidants.

Habitat-specific growth and fitness in carrion crows (Corvus corone corone)

SUMMARY (1) The relationship between habitat, chick growth, fledging size and fitness of breeders was studied in two populations of carrion crows, Corvus corone corone L., one living in an urban and another in an agricultural environment in south-western Switzerland. The logistic equation was used to model weight gain and tarsus growth, and the growth constant (k) and the asymptote (a) computed, using an iterative least square fitting technique. The chicks in the two habitats are compared with respect to the parameters a and k of the logistic equation. The Darwinian fitness is evaluated for each habitat. (2) Chicks in the urban habitat gain weight slower, take longer to reach fledging weight and are significantly lighter at fledging than the chicks in the agricultural habitat. Thus, for the weight curves, the chicks in the urban habitat show significantly lower values for both parameters a and k. (3) Tarsus growth is slower for chicks in the urban habitat, but in contrast to weight gain, growth of tarsus is not prolonged. For tarsus growth the parameter k is identical in the two habitats, but urban chicks show significantly lower values for the parameter a. Tarsus length is fixed by the age of fledging. Chicks in the urban habitat have shorter tarsi at fledging. (4) Growth of wing is slower in the urban habitat, but, since wing length is incomplete at fledging, no curves were fitted. (5) Tarsus length of territory-holding adults was measured in both habitats and a critical minimum size for territory acquisition empirically established. Of all fledglings raised in the urban habitat, 79% fall below this critical size and will therefore be unable to acquire a territory and thus be excluded from breeding, whereas only 24% of fledglings raised in the agricultural habitat will be excluded. (6) Successful parents in the agricultural habitat fledge, on average, 2-7 chicks per year, parents in the urban habitat 1 5 chicks only. 36% of all territory holders in the agricultural habitat fledge young, 39% in the urban habitat. Since a much higher percentage of fledged chicks reach the critical body size in the agricultural habitat, these parents reach a Darwinian fitness five times higher than the parents in the urban habitat.

Benefits of Induced Host Responses against an Ectoparasite

As a consequence of the deleterious effects of parasites on host fitness, hosts have evolved responses to minimize the negative impact of parasite infection. Facultative parasite–induced responses are favoured when the risk of infection is unpredictable and host responses are costly. In vertebrates, induced responses are generally viewed as being adaptive, although evidence for fitness benefits arising from these responses in natural host populations is lacking. Here we provide experimental evidence for direct reproductive benefits in flea–infested great tit nests arising from exposure during egg production to fleas. In the experiment we exposed a group of birds to fleas during egg laying (the exposed group), thereby allowing for induced responses, and kept another group free of parasites (the unexposed group) over the same time period. At the start of incubation, we killed the parasites in both groups and all nests were reinfested with fleas. If induced responses occur and are adaptive, we expect that birds of the exposed group mount earlier responses and achieve higher current reproductive success than birds in the unexposed group. In agreement with this prediction, our results show that birds with nests infested during egg laying have (i) fewer breeding failures and raise a higher proportion of hatchlings to fledging age; (ii) offspring that reach greater body mass, grow longer feathers, and fledge earlier, and (iii) a higher number of recruits and first–year grandchildren than unexposed birds. Flea reproduction and survival did not differ significantly between the two treatments. These results provide the first evidence for the occurrence and the adaptiveness of induced responses against a common ectoparasite in a wild population of vertebrates.

Proximate mechanisms of variation in the carotenoid-based plumage coloration of nestling great tits (Parus major L.).

Abstract Many vertebrates use carotenoid‐based signals in social or sexual interactions. Honest signalling via carotenoids implies some limitation of carotenoid‐based colour expression among phenotypes in the wild, and at least five limiting proximate mechanisms have been hypothesized. Limitation may arise by carotenoid‐availability, genetic constraints, body condition, parasites, or detrimental effects of carotenoids. An understanding of the relative importance of the five mechanisms is relevant in the context of natural and sexual selection acting on signal evolution. In an experimental field study with carotenoid supplementation, simultaneous cross‐fostering, manipulation of brood size and ectoparasite load, we investigated the relative importance of these mechanisms for the variation in carotenoid‐based coloration of nestling great tits (Parus major). Carotenoid‐based plumage coloration was significantly related to genetic origin of nestlings, and was enhanced both in carotenoid‐supplemented nestlings, and nestlings raised in reduced broods. We found a tendency for ectoparasite‐induced limitation of colour expression and no evidence for detrimental effects of carotenoids on growth pattern, mortality and recruitment of nestlings to the local breeding population. Thus, three of the five proposed mechanisms can generate individual variation in the expression of carotenoid‐based plumage coloration in the wild and thus could maintain honesty in a trait potentially used for signalling of individual quality.

Ectoparasite-modulated deposition of maternal androgens in great tit eggs.

Maternal yolk androgens can promote growth and competitive abilities of nestling birds but are also suggested to increase susceptibility to parasites or suppress immune function. We tested the hypothesis that females exposed to ectoparasites during egg formation will adjust the content of androgens in the yolk. We predicted that when anticipating high levels of parasitism, females deposit (i) less androgens into all eggs of their clutch and (ii) smaller amounts of androgens in eggs late in the laying sequence to facilitate brood reduction.In a field experiment we exposed female great tits (Parus major) to hen fleas (Ceratophyllus gallinae), or kept them free of ectoparasites prior to egg laying. We collected the eggs and measured yolk concentrations of androstenedione (A4), testosterone (T) and 5∝–dihydrotestosterone (DHT) by radioimmunoassay. Among clutches, eggs of ectoparasite–exposed females contained significantly less A4 and tended to contain less T, whereas DHT content was unaffected. Within clutches, content of A4 and T increased significantly with laying order whereas DHT content significantly decreased. These patterns were unaffected by ectoparasites. In summary, our results provide no evidence for hormone–based facilitation of brood reduction under ectoparasite exposure but support the hypothesis that females exposed to ectoparasites reduce levels of T and its precursor A4 in yolk and might thereby reduce the negative effects of parasites on offspring.