Wojciech Bielański

Territory choice during the breeding tenure of male sedge warblers

A territorial male can shift the location of its territory from year to year in order to increase its quality. The male can base its decision on environmental cues or else on its breeding experiences (when territory shift is caused by breeding failure in previous seasons). We tested these possible mechanisms of territory choice in the sedge warbler (Acrocephalus schoenobaenus), a territorial migrating passerine that occupies wetlands. This species bases its territory choices on an environmental cue: tall wetland vegetation cover. We found that the magnitude of territory quality improvement between seasons (measured as the area of tall wetland vegetation) increased throughout the early stages of a males breeding career as a result of territory shifts dependent on the earliness of arrival. The distance the territory was shifted between seasons depended negatively on the previous years territory quality and, less clearly, on the previous years mating success. On the other hand, previous mating or nesting success had no influence on territory quality improvement between seasons as measured in terms of vegetation. The results imply that tall wetland vegetation is a long-term, effective environmental cue and that a preference for territories in which this type of landcover prevails has evolved into a rigid behavioral mechanism, supplemented by short-term individual experiences of breeding failure.

Nesting preferences of common buzzard Buteo buteo and goshawk Accipiter gentilis in forest stands of different structure (Niepolomice Forest, Southern Poland)

Studies on nesting preferences of common buzzard and goshawk were carried out in two distinctly different parts of the Niepolomice Forest (S Poland): deciduous (oak-hornbeam wood) and coniferous. Characteristics of nest sites were determined on three spatial scales, separately for: (1) nest tree; (2) nest tree area (0.07 ha circle centred at nest tree) and (3) nest stand (15 ha circle centred at nest tree). Nesting preferences discovered for the nest tree and its surroundings, included height and diameter of trees, age of the forest stand, distance to the nearest open area or forest road and occurrence of open areas in the vicinity of the nest. In the diverse habitat of oak-hornbeam wood, more similar to natural woods, nest site selection operated on several levels, possibly starting at the most extensive end of the scale before narrowing to the selection of a particular nest tree. In the more homogeneous habitat of commercially exploited coniferous forest, the surrounding of the nest were found to be insignificant, and the nesting decisions were likely to be based principally on individual characteristics of a tree i.e. its shape and size, being suitable for nesting.

On the song resumption, polyterritorial behaviour and their population context in the Sedge Warbler Acrocephalus schoenobaenus

Mating tactics in species with facultative polygyny seem to be very flexible and to depend on local environmental conditions. We analysed the habitat and population contexts of territorial behaviour, associated with polygyny, in a population of Sedge Warblers Acrocephalus schoenobaenus inhabiting natural wetlands. Nearly one-third of all breeding males (46 individuals) resumed song after completing their mating with the first female, in order to attract another one. Resuming males were usually the earliest arrivals. There was a continuity in the territorial behaviour between resuming song on first territory and polyterritorial behaviour. Fifty-nine percent of resuming males set up second territories, clearly separated from the first. The second territory was usually located close to the primary female activity area. The quality of the first and second territories were correlated, with second territories being significantly inferior. However, their quality was not significantly different from the territories of non-breeding males. The quality of the second territories was also negatively related to the distance from the first territories. Polyterritorialism was influenced by population numbers: the higher the number of territorial males, the lower the number of second territories settled and the lower their average quality. Although the polygyny frequency in the studied population was very low, polyterritorial males were significantly more likely to be polygynous. We concluded that polygyny frequency can be significantly influenced by population numbers, which might be one of the main factors responsible for the variability in the mating system in this species.

Blood parasites shape extreme major histocompatibility complex diversity in a migratory passerine

Pathogens are one of the main forces driving the evolution and maintenance of the highly polymorphic genes of the vertebrate major histocompatibility complex (MHC). Although MHC proteins are crucial in pathogen recognition, it is still poorly understood how pathogen‐mediated selection promotes and maintains MHC diversity, and especially so in host species with highly duplicated MHC genes. Sedge warblers (Acrocephalus schoenobaenus) have highly duplicated MHC genes, and using data from high‐throughput MHC genotyping, we were able to investigate to what extent avian malaria parasites explain temporal MHC class I supertype fluctuations in a long‐term study population. We investigated infection status and infection intensities of two different strains of Haemoproteus, that is avian malaria parasites that are known to have significant fitness consequences in sedge warblers. We found that prevalence of avian malaria in carriers of specific MHC class I supertypes was a significant predictor of their frequency changes between years. This finding suggests that avian malaria infections partly drive the temporal fluctuations of the MHC class I supertypes. Furthermore, we found that individuals with a large number of different supertypes had higher resistance to avian malaria, but there was no evidence for an optimal MHC class I diversity. Thus, the two studied malaria parasite strains appear to select for a high MHC class I supertype diversity. Such selection may explain the maintenance of the extremely high number of MHC class I gene copies in sedge warblers and possibly also in other passerines where avian malaria is a common disease.

The influence of phenology on double-brooding and polygyny incidence in the Sedge Warbler Acrocephalus schoenobaenus

There is growing evidence that the fitness of birds in temperate zones depends strongly on the match between the timing of breeding and local phenology. We have analysed data collected from a 15-year study on a Sedge Warbler (Acrocephalus schoenobaenus) population in its natural habitat. The highly skewed distribution of arrival dates of males indicated that they competed intensely to be the earliest arrivals. The distribution of mating dates, unlike arrival dates, was not skewed, rather it formed a bell-shaped distribution. Because females arrived much later, they could not base their mating choices on the arrival dates of males. Females, however, could use male song repertoire, a trait we found to be correlated with male arrival date. Double-brooded females mated early in the season, exactly within the peak of mating date distribution. Polygynous females, on the other hand, mated later in the season, choosing high-quality older males which had arrived significantly earlier. Male territory quality did not differ between double-brooded and polygynous females, nor did the quality of the territories occupied by an individual female change during her lifetime. Although longer lifespan and double-brooding influenced a female’s fitness over the course of her lifetime, polygyny had no significant effect. We conclude that Sedge Warbler females do not compete for territorial resources, but they do prefer to mate with high-quality early-arriving males. Polygyny is an effective strategy used by females to compensate for their late arrival and breeding onset, since they pair with high-quality males.ZusammenfassungDer Einfluss der Phänologie auf das Auftreten von Zweitbruten und Polygynie beim SchilfrohrsängerAcrocephalus schoenobaenus Es gibt zunehmend Hinweise darauf, dass die Fitness von Vögeln in gemäßigten Breiten stark davon abhängt, wie der Brutzeitpunkt zur lokalen Phänologie passt. Wir analysierten Daten aus einer 15-jährigen Studie an einer Schilfrohrsängerpopulation im natürlichen Lebensraum. Die hochgradig schiefe Verteilung der Ankunftsdaten der Männchen wies auf eine intensive Konkurrenz um das früheste Eintreffen hin. Anders als bei den Ankunftsdaten war die Verteilung der Verpaarungsdaten nicht schief, sondern entsprach eher einer Glockenkurve. Da die Weibchen viel später eintrafen, konnten sie sich bei der Partnerwahl nicht nach den Ankunftsdaten der Männchen richten. Die Weibchen konnten sich jedoch am Gesangsrepertoire der Männchen orientieren, welches, wie wir feststellten, mit den Ankunftsdaten der Männchen korrelierte. Zweimal brütende Weibchen verpaarten sich zu Beginn der Brutsaison, genau am Verteilungsmaximum der Verpaarungsdaten. Dahingegen verpaarten sich polygyne Weibchen später in der Brutsaison und wählten hochwertigere ältere Männchen, die signifikant früher eingetroffen waren. Es gab keine Qualitätsunterschiede bei den Revieren der Männchen zwischen polygynen Weibchen und solche mit Zweitbruten; auch änderte sich die Qualität der Reviere, die ein Weibchen bewohnte, im Laufe seines Lebens nicht. Obwohl eine längere Lebensspanne und Zweitbruten die Fitness eines Weibchens im Laufe seines Lebens beeinflussten, hatte Polygynie keinen signifikanten Effekt. Wir folgern daraus, dass, zumindest beim Schilfrohrsänger, die Weibchen nicht um Reviere konkurrieren. Weibchen paaren sich bevorzugt mit hochwertigen, früh eintreffenden Männchen. Polygynie ist eine effektive Strategie, mittels derer Weibchen eine späte Ankunft und Brutbeginn kompensieren können, indem sie sich mit hochwertigen Männchen verpaaren.