Breeding birds actively modify the initial microclimate of occupied tree cavities

Abstract

The microclimate of cavities used by endothermic animals may depend on dynamic relationships between a cavity's physical properties and the heating activity of cavity users, but the rudiments of these relationships are unclear. I compared the temperature and relative humidity of active tree cavities that were occupied by nesting marsh tits Poecile palustris with the conditions in vacant tree cavities previously used for breeding by this species. I tested how presence of active nests modified initial cavity microclimate, and if this modification changed with nest progression or cavity insulation. In 2013-2014, mean daily internalambient temperature differences averaged 1.5-4.1°C higher and relative humidity 8-10% lower, in active cavities relative to vacant sites, with greatest differences in the late nestling period. Compared to vacant cavities and relative to respective ambient values, the greatest daily minimum temperature increase was in active cavities located in the thinnest trees, which insulated least efficiently. As daily minimum temperatures were elevated to a similar level relative to outside within all active cavities, birds appeared to compensate for heat loss from cavities by warming the air within in a homeostatic manner. Similar to vacant cavities, the differences between daily maximum internal and ambient temperatures decreased with tree girth in active cavities, indicating that daily temperature maxima were systematically moderated in the thickest trees. The study demonstrates the modifying effect of birds' breeding activity on tree-cavity microclimate and highlights the role of a cavity's thermal properties in reducing the energy expenditure and risk of overheating for cavity users.

Increased microclimatic variation in artificial nests does not create ecological traps for a secondary cavity breeder

“…However, we found similar differences between nest-types in both empty and occupied nests (Table A2). These results are in line with previous studies that found that even a much smaller bird, great tits Parus major , increased interior temperature and temperature buffering capacity in nest-boxes (Maziarz & Wesołowski, 2013; Vel’ký et al, 2010) as well as marsh tits Poecile palustris in natural cavities (Maziarz, 2019). Similarly to Maziarz 2019, and Maziarz and Wesołowski (2013), we found that humidity rates were lower in occupied compared to empty cavities (Table A2).…”

“…3): the presence of birds in the nest could buffer the humidity level to a medium level, in between the dry empty nest-boxes and the humid empty cavities. The effect of the presence of birds in the nest on microclimate underlines the greater energetic cost for birds using nest-boxes which showed greater microclimatic variations compared to natural cavities in our study site (Maziarz, 2019).…”

“…Overall, our results found that roller fitness was not negatively impacted in the nest-boxes they selected, at least in terms of breeding success. However, the least favourable microclimate in nestboxes probably increases energy expenditure of rollers, as also suggested in the great tit (Maziarz, 2019), which could have other fitness consequences, and affect traits such as chick growth (Andreasson, Nord & Nilsson, 2018), body condition (Catry, Franco et al, 2011), or post-fledging survival (Greño, Belda & Barba, 2008), which would be valuable to explore in future studies. Nonetheless, according to our conceptual framework (Fig.…”

“…The models included the interaction of nest type (in order to control for nest characteristics disparities between nest types) and real occupation of the nest (i.e. nest occupation during sensor deployment) as additive factor (in order to control for the potentially induced modification of the microclimate (Maziarz, 2019; Vel’ký, Kaňuch, & Krištín, 2010)) (see ‘Results’). All the other nest characteristics were available only for natural cavities and hence their correlation with microclimate was tested solely on the subset of natural cavities, with real occupation of the nest as additive factor in the models.…”

Thermal ecosystem engineering by songbirds promotes a symbiotic relationship with ants

“…We hypothesise that Myrmica ants are attracted to bird nests to exploit the heat generated by the hosts and/or the enhanced insulation properties of the nest structure, to raise their broods in more advantageous (warmer and stable) thermal conditions than in the ants' own nests. To test this hypothesis, we examine how the endothermic activity of birds modifies the thermal conditions of their nests in relation to the stage of the nesting cycle and/ or ambient temperatures, possibly affecting the amount of heat generated by the birds 16,17,31,32 . Next, we explore whether thermal ecosystem engineering by the wood warblers may have potential benefits for ants raising their broods within the birds' nests.…”