Manipulation of parental effort affects plumage bacterial load in a wild passerine.

It has been suggested that plumage microorganisms play an important role in shaping the life histories of wild birds. Some bacteria may act as pathogens or cause damage to feathers, and thereby reduce individual fitness. Intense parental care in birds can result in a reduction of self-maintenance and preening behavior in parents and therefore might affect the dynamics of microbiota living on their feathers. However, experimental evidence of this relationship is virtually absent. We manipulated the parental effort of wild breeding pied flycatcher (Ficedula hypoleuca) females by modifying their brood size or temporarily removing male partners. We expected that experimentally decreasing or increasing parental effort would affect feather sanitation in females and therefore also bacterial density on their plumage. In accordance with this hypothesis, manipulation affected the density of free-living bacteria: females with reduced broods had the lowest number of free-living bacteria on their feathers, while females left without male partners had the highest. However, manipulation did not have a significant effect on the densities of attached bacteria. Our results provide experimental evidence that a trade-off between self-maintenance and parental effort affects plumage bacterial densities in birds.

Plumage bacterial assemblages in a breeding wild passerine: relationships with ecological factors and body condition.

Microorganisms have been shown to play an important role in shaping the life histories of animals, and it has recently been suggested that feather-degrading bacteria influence the trade-off between parental effort and self-preening behavior in birds. We studied a wild breeding population of great tits (Parus major) to explore habitat-, seasonal-, and sex-related variation in feather-degrading and free-living bacteria inhabiting the birds' yellow ventral feathers and to investigate associations with body condition. The density and species richness of bacterial assemblages was studied using flow cytometry and ribosomal intergenic spacer analysis. The density of studied bacteria declined between the nest-building period and the first brood. The number of bacterial phylotypes per bird was higher in coniferous habitat, while bacterial densities were higher in deciduous habitat. Free-living bacterial density was positively correlated with female mass; conversely, there was a negative correlation between attached bacterial density and female mass during the period of peak reproductive effort. Bacterial species richness was sex dependent, with more diverse bacterial assemblages present on males than females. Thus, this study revealed that bacterial assemblages on the feathers of breeding birds are affected both by life history and ecological factors and are related to body condition.

Behavioral and physiological responses of nestling pied flycatchers to acoustic stress.

The extended secretion of stress hormones in fully developed animals is known to have profound consequences. However, little is known about the effects of stress on the behavior and physiology of free-living young animals, and how such responses relate to each other. We repeatedly (during 5 consecutive days, 1 h/day) exposed the nestlings of a passerine bird, the pied flycatcher (Ficedula hypoleuca), to recordings of nestling distress calls and examined their behavioral and physiological responses to the stressor on the first and the last day of the experiment (on days 9 and 13 post-hatch, respectively). In comparison with control siblings, stressed nestlings reduced the amount of time that they devoted to vocalization and locomotion and increased levels of circulating corticosterone. In 9-day-old nestlings, the level of stress-induced hormone was negatively related to locomotor activity, but not to the rate of vocalizations. The repeated presentation of the stressor increased the heterophile-to-lymphocyte ratio in nestlings but did not affect nestling growth rate. In 13-day-old nestlings, the level of stress-induced corticosterone was not related to behavioral activity. These results suggest that the high level of corticosterone released by immature nestlings in response to a stressor may promote anti-predator behavior (e.g., passive avoidance behavior). Moreover, repeatedly induced stress may have a cumulative and potentially negative effect on individual physiology.

Development of stress response in nestling pied flycatchers.

Birds respond to unpredictable events by secreting corticosterone, which induces various responses to cope with stressful situations. However, the evidence is still elusive whether altricial nestlings perceive and respond to external stressors. We investigated the development of adrenocortical stress response to handling-related stressor in nestlings of a small passerine bird, the pied flycatcher (Ficedula hypoleuca). Nestlings were held in isolation from their parents during the experiment to ensure that they indeed respond to handling, not to parental alarm calls. We found that both 9- and 13-day-old nestlings were able to elicit hormonal stress response. Although baseline as well as stress-induced corticosterone levels rose slightly with age, the magnitude of difference between the control and stress-induced levels remained similar in both age groups. However, comparison with adults showed that the stress response of nestlings prior to fledging was still incomplete and significantly lower than in adults. Overall, our results indicate that altricial nestlings do respond to acute stressors, but on the contrary to previous predictions the development of corticosterone stress response during growth period is not gradual and varies remarkably between different passerine species.