An overlap in attributes of nest cavities used by Great Tit Parus major across Eurasia suggests similar nest site preferences within the geographical range, although the drivers of these preferences are unclear. To determine whether preferred cavities provide conditions enhancing successful reproduction, we investigated the breeding performance of Great Tits in relation to tree cavity characteristics using data collected during 2008-2011 in primeval conditions (Białowie_ za National Park, Poland). Here, tree cavities are diverse and superabundant but nesting birds are at risk from a variety of predators. According to expectations, nest losses were high (60 % of Great Tit nests failed), mostly due to predation (69 % of nest failures). The risk of nest failure varied with nest cavity attributes. Compared to successful nests, failures were situated higher above the ground and placed closer to the cavity entrance. Very deep cavities with narrow entrances and strong livings walls provided effective protection against larger predators (e.g., martens, woodpeckers), unable to enter the cavity or pull out the contents. Yet, such holes were no barrier for the smallest predators (e.g., Forest Dormouse Dryomys nitedula), which were able to enter any Great Tit nest cavity and destroyed most of the nests. Avoiding small predators would give a selective advantage to the birds, but this seems hardly possible to achieve. We conclude that tree cavities preferred by the tits show a combination of properties which are a compromise for avoiding predation (the strongest selective pressure) and providing the minimum requirements (sufficient nest illumination, microclimate, protection against nest soaking) for development and growth of young.

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Knowledge of the breeding ecology of the Great Tit Parus major is vast, but almost exclusively concerns birds using nest-boxes. Information on birds nesting in natural conditions is scant. Here, we present the results of the first thorough study on natural nest-sites of the Great Tit. The data, including descriptions of nestcavity location and dimensions, were collected during 39 breeding seasons in the primeval forest of Białowie_ za National Park (BNP), Poland. With an excess of available tree-cavities providing a diverse choice of nesting options, Great Tits nested mainly in non-excavated, very deep and spacious cavities with elongated, narrow openings, placed at intermediate heights in living tree trunks. Different sets of tree species were used in different habitats. The pattern of nest-site utilisation by Great Tits in BNP overlapped with that recorded in other areas, but showed niche separation from other non-excavating hole-breeders in BNP. This indicates that Great Tits have core nest-site preferences, which have probably evolved in response to selective forces such as, e.g., risk of predation, flooding, sufficient nest illumination and/or efficient air ventilation. Keywords Parus major Á Tree cavities Á Nest placement Á Cavity dimensions Á Primeval conditions Zusammenfassung Natürliche Nistplätze der Kohlmeise in einem Urwald (Białowie_ za Nationalpark, Polen) Das Wissen über die Brutökologie der Kohlmeise (Parus major) ist sehr groß, bezieht sich aber fast ausschließlich auf Vögel, die in Nistkästen brüten. Informationen über Vögel, die unter natürlichen Bedingungen brüten, sind rar. Hier stellen wir die Ergebnisse der ersten umfassenden Untersuchung vor, die sich mit natürlichen Nistplätzen der Kohlmeise beschäftigte. Die Daten, darunter Beschreibungen der Lage und Dimension der Nisthöhlen, wurden während 39 Brutzeiten im Urwald des Białowie_ za Nationalparks (BNP) in Polen gesammelt. Bei einem Ü berschuss an verfügbaren Baumhöhlen, der eine vielfältige Auswahl von Nistmöglichkeiten zur Verfügung stellte, brüteten die Kohlmeisen hauptsächlich in natürlichen, sehr tiefen und geräumigen Höhlen mit länglichen, schmalen Ö ffnungen, in mittlerer Höhe von Stämmen lebender Bäume. In unterschiedlichen Habitaten wurden andere Baumarten gewählt. Das Muster der Nistplatzwahl der Kohlmeisen im BNP überlappte mit dem in anderen Gebieten aufgezeichneten, zeigte aber eine Nischentrennung von anderen Höhlenbrütern, die in natürlichen Höhlen brüteten. Das zeigt, dass Kohlmeisen grundlegende Nistplatzpräferenzen haben, die sich wahrscheinlich als Antwort auf einen Selektionsdruck entwickelt haben, wie zum Beispiel Prädationsrisiko, Ü berschwemmungsgefahr, ausreichende Ausleuchtung der Bruthöhle und/oder Ventilation. Communicated by F. Bairlein.

Numerous bird species have advanced their breeding seasons in response to climate warming. These changes were mostly brought about by phenotypic plasticity, i.e. flexible reactions of individual birds, rather than by microevolutionary change. Knowing the limits of plasticity is thus of paramount importance in any attempt to predict possible reactions of birds to climate warming. However, the breeding performance of the same individuals in contrasting environmental conditions, necessary to answer this question, is rarely observed. Here, we provide data on the flexibility in timing of egg‐laying of individual marsh tit Poecile palustris females breeding in an extremely late (2013) and early (2014) spring in Białowieża National Park (Poland). In both years the birds stayed in the same places in the primeval old‐growth forest, free of direct human influences (no nest‐boxes, no additional food). The weather variation was within the range of conditions observed during 40 yr in the study area, and no climate warming occurred in the marsh tit's pre‐breeding period. Females (n = 16) shifted the onset of laying by 13–23 (median = 20) days between the seasons. This range of individual flexibility encompasses almost the whole latitudinal range of the breeding dates found across Europe. Such a buffer of plasticity would probably be sufficient for marsh tits to adjust the onset of egg‐laying to the forecasted range of climate change. A combination of temperature and photoperiod appears to be involved in fine tuning of the birds’ breeding times with spring conditions, but how the birds asses and integrate this information remains poorly understood.

Bird and mammal nests provide microhabitats that support a range of other species, including invertebrates. However, the variation between communities of nest-dwelling invertebrates in different nests is poorly understood. The major aim of this study was to analyze the assemblage structure of mites from the suborder Uropodina (Acari: Mesostigmata) and from superfamily Crotonioidea (Acari: Oribatida) inhabiting nests of the wood warbler, Phylloscopus sibilatrix (Aves: Passeriformes), located on a forest floor in Białowieża Forest, in eastern Poland. We also assessed the correlation between the nest material used by the birds with the assemblage structure of Uropodina mites, and compared the results with published studies of the nests of other birds and a mammal (common mole, Talpa europaea), and also with communities of mites inhabiting the soil. The field research was conducted in the strict nature reserve of the Białowieża National Park, a near-primeval European temperate forest. In 2019, immediately after the breeding period, 69 wood warbler nests and 439 soil samples were collected. Analyses revealed assemblages of Uropodina mites inhabiting the nests that consisted of 14 species, mostly common soil species. Only five species of oribatid mites from superfamily Crotonioidea were present in the nest material. Analyzed nests had a high percentage of tree leaves and grass blades, whereas moss was the least frequent component of the nest material. The Uropodina mites were more abundant in the nests that had greater amounts of grass blades, but similar relationships were insignificant for the nests with varying amounts of tree leaves or moss. The assemblages of Uropodina mites inhabiting wood warbler nests were very similar to those found in soil and nests of the common mole, but they lacked typical nest-dwelling species of Uropodina (i.e., specialized nidicoles).

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Background Interspecific interactions within ecological networks can influence animal fitness and behaviour, including nest-site selection of birds and ants. Previous studies revealed that nesting birds and ants may benefit from cohabitation, with interspecific attraction through their nest-site choice, but mutual interactions have not yet been tested. We explored a previously undescribed ecological link between ground-nesting birds and ants raising their own broods (larvae and pupae) within the birds’ nests in a temperate primeval forest of lowland Europe. We tested whether the occurrence of ant broods within bird nests resulted from a mutual or one-sided interspecific attraction that operated through nest-site choice and was modified by weather conditions. Results We found a non-random occupation of bird nests by ants raising their own broods within them, which indicated interspecific attraction driven solely by the ants. The birds’ preference to nest near tussocks of vegetation showed little overlap with the most frequent placement of ant colonies among fallen deciduous tree-leaves, dead wood and moss. Additionally, birds did not appear to select forest localities with high densities of ant colonies. The occurrence of ant broods within bird nests was also unrelated to bird nest placement near to specific habitat features. The attractiveness of bird nests to ants appeared to increase with the thermal activity of the birds warming their nests, and also during cool and wet weather when the occurrence of ant broods within bird nests was most frequent. Ants often remained in the nests after the birds had vacated them, with only a slight reduction in the probability of ant brood occurrence over time. Conclusions The natural patterns of bird nest colonisation by ants support the hypothesis of ants’ attraction to warm nests of birds to raise their broods under advantageous thermal conditions. Similar relationships may occur between other warm-blooded, nest-building vertebrates and nest-dwelling invertebrates, which depend on ambient temperatures. The findings advance our understanding of these poorly recognised interspecific interactions, and can inform future studies of ecological networks.

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