Geographic and temporal variation in the consumption of bats by European barn owls

Capsule We report a review of the occurrence of bats in the Barn Owl diet Tyto alba in Europe. Based on 802 studies reporting 4.02 million prey items identified in pellets, 4949 were bats (0.12%). We found that bat predation decreased during the last 150 years, is more frequent on islands than mainland, and is higher in eastern than western Europe and in southern than northern Europe. Although Barn Owls usually capture bats opportunistically, they can sometimes specialize on them.

Nestling barn owls assess short-term variation in the amount of vocally competing siblings.

Assessing the amount of rivals is crucial to optimally adjust investment into a contest. If laboratory animals show numerical abilities, little is known about the ecological implications particularly in young animals. The two to nine barn owl (Tyto alba) siblings vocally compete for priority of access to food resources before parents actually deliver them. In dyads, the individual that vocalizes at the highest rate in the absence of parents deters its siblings from competing for next delivered prey. We tested the novel hypothesis that to optimally adjust vocal investment, barn owl nestlings assess how many of their siblings are currently competing. To singleton owlets, we broadcasted a fixed global number of calls emitted by one, two or four pre-recorded unfamiliar nestlings. We could thus distinguish the independent effect on singletons' vocal behavior of the global number of calls produced by a brood from the number of competitors that produced these calls. Overall, nestlings retreated more from vocal contest when facing more competitors. However, in front of one highly motivated competitor, nestlings refrained from vocalizing to a larger extent than when competing against more but less motivated individuals. Therefore, young animals assess variation in the number of currently competing siblings based on individual-specific vocal cues.

Om revideringen av Bryssel- och Luganokonventionerna

Festskrift till Per Lindholm

A link between eumelanism and calcium physiology in the barn owl.

In many animals, melanin-based coloration is strongly heritable and is largely insensitive to the environment and body condition. According to the handicap principle, such a trait may not reveal individual quality because the production of different melanin-based colorations often entails similar costs. However, a recent study showed that the production of eumelanin pigments requires relatively large amounts of calcium, potentially implying that melanin-based coloration is associated with physiological processes requiring calcium. If this is the case, eumelanism may be traded-off against other metabolic processes that require the same elements. We used a correlative approach to examine, for the first time, this proposition in the barn owl, a species in which individuals vary in the amount, size, and blackness of eumelanic spots. For this purpose, we measured calcium concentration in the left humerus of 85 dead owls. Results showed that the humeri of heavily spotted individuals had a higher concentration of calcium. This suggests either that plumage spottiness signals the ability to absorb calcium from the diet for both eumelanin production and storage in bones, or that lightly spotted individuals use more calcium for metabolic processes at the expense of calcium storage in bones. Our study supports the idea that eumelanin-based coloration is associated with a number of physiological processes requiring calcium.

Extra-pair paternity, testes size and testosterone level in relation to colour polymorphism in the barn owl Tyto alba

In many bird populations, individuals display one of several genetically inherited colour morphs. Colour polymorphism can be maintained by several mechanisms one of which being frequency-dependent selection with colour morphs signalling alternative mating strategies. One morph may be dominant and territorial, and another one adopt a sneaky behaviour to gain access to fertile females. We tested this hypothesis in the barn owl Tyto alba in which coloration varies from reddish-brown to white. This trait is heritable and neither sensitive to the environment in which individuals live nor to body condition. In Switzerland, reddish-brown males were observed to feed their brood at a higher rate and to produce more offspring than white males. This observation lead us to hypothesize that white males may equalise fitness by investing more effort in extra-pair copulations. This hypothesis predicts that lighter Coloured males produce more extra-pair young, have larger testes and higher levels of circulating testosterone. However, our results are not consistent with these three predictions. First, paternity analyses of 54 broods with a total of 211 offspring revealed that only one young was not sired by the male that was feeding it. Second, testes size was not correlated with male plumage coloration suggesting that white males are not sexually more active. Finally, in nestlings at the time of feather growth testosterone level was not related to plumage coloration suggesting that this androgen is not required for the expression of this plumage trait. Our study therefore indicates that in the barn owl colour polymorphism plays no role in the probability of producing extra-pair young.

Rättsvetenskapens identitet - normativ och deskriptiv

Oikeustieteen päivät 20.-21.12.1996