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Heated Nest Boxes Reduced Blue Tit Incubation Effort and Short-Term Parasite Loads

Blue tit feeding its young at a nest
Research · Behaviour

Female Eurasian blue tits (Cyanistes caeruleus) spent less time incubating when researchers experimentally warmed their nest boxes, and they also showed lower blood-parasite infection shortly after hatching. The study, published on 21 February 2026, raised nest-box temperature by about 3.3 °C during incubation and followed changes in behaviour, female health and breeding performance.

The experiment was carried out in 2022 in a montane Pyrenean oak forest at Valsaín in Segovia, central Spain. Forty-four nest boxes were paired by laying date and clutch size and randomly assigned to a heated group or a control group, with 22 nests in each treatment. Heat mats were installed from the first day of incubation until hatching, for roughly 12 days.

Warmer nests changed how females incubated

Heated nests averaged about 24 °C, compared with about 21 °C in control nests. Females in the warmer boxes spent 66.5% of the recorded time incubating, versus 74.5% in control nests. Their incubation sessions were also shorter, averaging about 11 minutes compared with 14 minutes, while they made more incubation sessions per hour.

Percentage of incubation time by female Eurasian blue tits in control and heated nest boxes
Females in heated boxes spent a smaller percentage of the recorded time incubating. Figure 2 from García-del Río et al. (2026), Behavioral Ecology, CC BY 4.0.

The treatment did not clearly change how often males fed incubating females. The authors interpret the reduced incubation effort in warmer boxes as a consequence of eggs cooling more slowly during female recesses, allowing females to spend more time away from the nest.

Short-term parasite differences appeared after hatching

Three days after hatching, fewer females from heated nests were infected with blood parasites, and fewer different parasite types were detected. Haemoproteus infection intensity was also lower in females from heated nests; Lankesterella showed the same direction, but the difference between treatments was weaker. In control nests, females that had spent longer periods incubating showed higher infection intensities of both Haemoproteus and Lankesterella.

Haemoproteus infection intensity in female Eurasian blue tits in control and heated nest boxes
On nestling day 3, Haemoproteus infection intensity was lower in females from heated nest boxes. Figure 3 from García-del Río et al. (2026), Behavioral Ecology, CC BY 4.0.

The researchers suggest that reduced incubation effort may have freed resources that could be allocated to immune function. By nestling day 13, the heated group still contained fewer infected females overall, but the difference in the number of parasite types was no longer detectable and most of the early parasite effects had faded. The two most common parasites did not follow identical trajectories: Haemoproteus prevalence changed little between the two sampling dates, whereas Lankesterella abundance increased by day 13 in both groups and particularly in females from heated nests. The treatment produced no detectable difference in female or nestling body mass, hatching success or fledging success.

The experiment covered one breeding season

The authors note that the experiment was conducted in a single year, while environmental conditions can vary among breeding seasons. They therefore frame the results as evidence for short-term effects of nest temperature on incubation behaviour and female health under the conditions tested.

Sources and image credit

  1. García-del Río, M., Merino, S., Castaño-Vázquez, F., Merino, Y., Fuertes-Recuero, M., & Cantarero, A. (2026). Impact of increased nest temperature on incubation behavior and female health in Eurasian Blue Tit. Behavioral Ecology, 37(2), arag016. https://doi.org/10.1093/beheco/arag016Primary peer-reviewed open-access article published 21 February 2026 under CC BY 4.0. Forty-four nest boxes were assigned to heated or control treatments; warming averaged 3.29 °C, reduced female incubation effort and was associated with lower short-term blood-parasite infection, while body mass and breeding success were unchanged.

Image: Daniel Schwen / Wikimedia Commons — blue tit (Cyanistes caeruleus) feeding its young, photographed 24 May 2007; representative breeding image, not a bird from the Valsaín experiment · CC BY-SA 3.0 — https://creativecommons.org/licenses/by-sa/3.0/ · Source

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