Breeding birds actively modify the initial microclimate of occupied tree cavities

Area of Science:

• Ecology
• Animal Behavior
• Environmental Science

Background:

• Cavity microclimates are crucial for endothermic animals but their dynamics with user activity are poorly understood.
• Tree cavity properties and animal heating activity influence internal conditions.

Purpose of the Study:

• To compare microclimate conditions in active versus vacant tree cavities used by marsh tits (Poecile palustris).
• To investigate how nesting activity modifies cavity microclimate and if this changes with nest development or cavity insulation.

Main Methods:

• Compared temperature and relative humidity in active nesting cavities versus vacant cavities of the same species.
• Analyzed microclimate modifications in relation to nest progression and tree insulation properties.

Main Results:

• Active cavities showed higher internal-ambient temperature differences (1.5-4.1°C) and lower relative humidity (8-10%) than vacant ones.
• The greatest temperature increase occurred in active cavities within the thinnest trees, suggesting birds actively warmed the air.
• Temperature moderation was observed in thicker trees for both active and vacant cavities.

Conclusions:

• Bird nesting activity significantly modifies tree cavity microclimates.
• Cavity thermal properties play a key role in reducing energy expenditure and preventing overheating for cavity users.
• Homeostatic warming by birds helps maintain stable internal cavity temperatures.