Female‐ and male‐specific signals of quality in the barn owl

Most bird studies of female signalling have been confined to species in which females display a male‐ornament in a vestigial form. However, a great deal of benefit may be gained from considering phenotypic traits that are specific to females. This is because (1) sex‐specific traits may signal sex‐specific qualities and (2) females may develop a male‐ornament not because they are selected to do so, but because fathers transmit to daughters the underlying genes for its expression (genetic correlation between the sexes). We investigated these two propositions in the barn owl Tyto alba, a species in which male plumage is lighter in colour and less marked with black spots than that of females. Firstly, we present published evidence that female plumage spottiness reflects parasite resistance ability. We also show that male plumage coloration is correlated with reproductive success, male feeding rate and heart mass. Secondly, cross‐fostering experiments demonstrate that plumage coloration and spottiness are genetically correlated between the sexes. This implies that if a given trait value is selected in one sex, the other sex will indirectly evolve towards a similar value. This prediction is supported by the observation that female plumage coloration and spottiness resembled that of males, in comparisons at the level of Tyto alba alba populations, Tyto alba subspecies and Tyto species. Our results therefore support the hypothesis that sex‐specific traits signal sex‐specific qualities and that a gene for a sex‐specific trait can be expressed in the other sex as the consequence of a genetic correlation between the sexes.     

Nonrandom pairing by male barn owls (Tyto alba) with respect to a female plumage trait

In socially monogamous species it is rare for females to bemore intensely colored than males. The barn owl (Tyto alba)is one of the exceptions, as females usually exhibit more andlarger black spots on the plumage. The evolution of sexual dimorphismin plumage traits is commonly assumed to be the result of sexualselection. I therefore examined the prediction that male barnowls do not pair randomly with respect to female plumage spottinessduring a 5-year study in Switzerland. The prediction was supported,as males that changed mates acquired a new female that was similarlyspotted to the previous one, and pairing with respect to plumage spottinesswas positively assortative. Significant repeatability in male pairingwas presumably neither the consequence of sharing the same habitats withfemales displaying a given plumage spottiness nor of morphological characteristicsof the males that could influence mate sampling. A resemblance inplumage spottiness between the mates of sons and of their fathersuggests that repeatability could have resulted from sexualimprinting and/or heritable variance in male preference forspotted females. To test whether males assess female plumagespottiness, I either cut off black spots or small pieces of feathersbut not the spots of already mated females. Males mated to females withreduced plumage spottiness fed their brood at a lower cadencyand achieved a lower reproductive success than other males.This experiment further suggests that female plumage spottinessis a stimulus for males.     

Brood size and begging intensity in nestling birds

Theoretical models suggest that sibling competition should selectfor conspicuous begging signals. If so, begging intensity mightbe expected to increase with the number of competitiors. Thepurpose of our study was to examine the relationship betweenbegging intensity and brood size using nestling tree swallows(Tachycineta bicolor) as our model. Over 2 years, we videotapedbegging behavior in unmanipulated broods of different sizes.We found that begging intensity increased with brood size. Theaverage weight of nestlings in each brood did not vary withbrood size, but feeding rate per nestling decreased with broodsize, suggesting that nestlings in larger broods begged moreintensively, possibly because they were hungrier. We also conductedan experiment to examine the effect of nest mates on beggingin different-sized broods. We found that nestlings with similarweights, previous competitive environments, and food deprivationbegged more intensively in large broods than in small broods.Overall, our study indicates that begging intensity increaseswith brood size in tree swallows. This relationship may resultfrom interactions among brood mates rather than from lower feeding rates to individual nestlings in larger broods.     

Ecological conditions during winter predict arrival date at the breeding quarters in a trans-Saharan migratory bird

We studied variation in arrival date to the breeding colonies in Italy of a trans‐Saharan migratory bird, the barn swallow Hirundo rustica, in relation to variation in ecological conditions, as reflected by the normalized difference vegetation index (NDVI), in the winter quarters. Arrival date of old but not young individuals captured during consecutive breeding seasons was earlier after winters with favourable conditions. Change in arrival date in relation to change in NDVI was similar in the two sexes. Change in arrival date significantly and positively predicted change in breeding date. As a result of increased frequency of second broods determined by earlier arrival, the number of fledged offspring per season was larger after African winters with good in comparison to poor ecological conditions for barn swallows. This is the first study demonstrating phenotypic plasticity in migration phenology of a long‐distance migratory bird in relation to ecological conditions during wintering.     

Rapid change in nest size of a bird related to change in a secondary sexual character

Among bird species in which males contribute to nest building,sexual selection has favored larger nests. I investigated determinantsof nest size in the barn swallow Hirundo rustica and how nestsize changed during the period 1977–2003, when tail length(a male secondary sexual character) increased by more than 1.2standard deviations. Males with short tails contributed moreto nest building than long-tailed males, signaling their futureinvestment in food provisioning of offspring. Pairs of barnswallows were consistent in nest size when build ing new neststhe same or different years, and level of phenotypic plasticityin nest size was small and could not account for temporal patternsin nest size. Offspring resembled their parents with respectto nest size, indicating a significant heritability of nestsize, independent of whether offspring were reared by theirparents or by foster parents, and there was a significant negativegenetic correlation between male tail length and outer nestvolume and amount of nest material. The temporal increase inmale tail length was associated with a decrease in nest size,with the amount of nest material in 2003 on average being lessthan a third of the amount used in 1977. Temporal change innest size could be accounted for by indirect selection on taillength causing change in nest size to match that predicted fromchange in tail length and the genetic correlation between maletail length and nest size.     

Lifetime individual plasticity in body condition of a migratory bird

Body condition is an important trait that reflects the capacity of individuals to acquire food or resist disease, eventually allowing successful reproduction and survival. We first quantified the effects of condition on life history and a secondary sexual trait using long‐term lifetime data on condition in a migratory bird species, the barn swallow, Hirundo rustica. Second, we quantified sources of individual variation in condition by separating the within‐individual from the between‐individual components of variation of environmental condition and age on body condition. Therefore, we were able to partition variation as a result of selection and phenotypic plasticity. Within‐individual variation in body condition increased in early life until middle age (i.e. 3–4 years of age) in the two sexes followed by only a slight decrease in body condition during senescence in males but not in females. After accounting for age‐dependent variation, condition could be partitioned into a within‐individual plastic response to environmental conditions during migration and a nonplastic response (i.e. a between‐individual difference) to environmental conditions experienced in the African winter quarters. Specifically, there was a within‐individual increase in body condition as environmental conditions during migration improved in both males and females, independent of age. There was a between‐individual effect of condition found in the winter quarter in body condition of females, but not in males, which was attributed to the disappearance of females in poor body condition from the study population as a result of the higher natal dispersal of low‐quality females compared to high‐quality ones during years with favourable environmental conditions in the African winter quarters. Males and females also tended to be in better body condition during the warmer springs upon arrival at the breeding grounds. There was a temporal decline in female body condition during 1991–2007, whereas no significant trend was detected in males. Therefore, both intrinsic (e.g. age and sex) and extrinsic factors (e.g. climate) affected body condition. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105 , 420–434.     

Divorce in the barn owl: securing a compatible or better mate entails the cost of re‐pairing with a less ornamented female mate

Two nonmutually exclusive hypotheses can explain why divorce is an adaptive strategy to improve reproductive success. Under the ‘better option hypothesis’, only one of the two partners initiates divorce to secure a higher‐quality partner and increases reproductive success after divorce. Under the ‘incompatibility hypothesis’, partners are incompatible and hence they may both increase reproductive success after divorce. In a long‐term study of the barn owl (Tyto alba), we address the question of whether one or the two partners derive fitness benefits by divorcing. Our results support the hypothesis that divorce is adaptive: after a poor reproductive season, at least one of the two divorcees increase breeding success up to the level of faithful pairs. By breeding more often together, faithful pairs improve coordination and thereby gain in their efficiency to produce successful fledglings. Males would divorce to obtain a compatible mate rather than a mate of higher quality: a heritable melanin‐based signal of female quality did not predict divorce (indicating that female absolute quality may not be the cause of divorce), but the new mate of divorced males was less melanic than their previous mate. This suggests that, at least for males, a cost of divorce may be to secure a lower‐quality but compatible mate. The better option hypothesis could not be formally rejected, as only one of the two divorcing partners commonly succeeded in obtaining a higher reproductive success after divorce. In conclusion, incompatible partners divorce to restore reproductive success, and by breeding more often together, faithful partners improve coordination.     

Feather micro-organisms and uropygial antimicrobial defences in a colonial passerine bird

1.  The relationship between the composition of communities of micro-organisms and their hosts remains poorly understood. We conducted extensive field studies of feather-degrading bacteria, other cultivable bacteria, and fungi on the plumage of a migratory bird, the barn swallow Hirundo rustica Linnaeus, to understand the association between micro-organisms, host sociality and host antimicrobial defences, as reflected by the size of the uropygial gland.
2.  The abundance of feather-degrading bacteria, but not other cultivable bacteria or fungi, decreased with increasing size of the uropygial gland.
3.  Females had more feather-degrading bacteria than males.
4.  Barn swallows living in larger colonies had more feather-degrading bacteria than less social conspecifics.
5.  These findings suggest that the uropygial gland plays a specific role in regulating the abundance of feather-degrading bacteria that furthermore depends on the social environment of the host.