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Male Blue Tit Endurance Rose 157% as Nestling Feeding Demands Increased

Eurasian blue tit at a nest box while feeding chicks
Research · Physiology

Male blue tits (Cyanistes caeruleus) increased their maximum exercise capacity as the demands of feeding growing chicks intensified, according to a study published on 31 March 2026. Researchers in southern Sweden measured the same breeding males early in the nestling period and again near the peak of chick provisioning, while also experimentally enlarging some broods.

The clearest change was in endurance. Between days 5 and 14 after hatching, mean endurance rose from 9.8 to 25.1 minutes, an increase of 157%. Maximum exercise-induced metabolic rate increased by 16% over the same period. Metabolic rate measured immediately before and after exercise also increased as the chicks grew.

Exercise capacity changed during the breeding period

The results show that maximum exercise capacity in free-living breeding birds is not fixed. The researchers interpret the increase as phenotypic flexibility associated with the greater amount of sustained work performed while older nestlings require more food.

Males feeding experimentally enlarged broods also showed evidence of greater exercise capacity than males with unmanipulated broods, but this effect depended on parental age. It was apparent in young birds and not in older males, indicating that age and experience influence how strongly parents can adjust to increased workload.

Parents did not simply work at a fixed physiological ceiling

The study addresses the idea that parental effort may be limited by a fixed maximum capacity for sustained aerobic work. Instead, the measurements indicate that this capacity itself can change during a single breeding attempt. The authors suggest that recent exercise, parental age and experience, behaviour and resource availability can all influence how much physiological capacity is expressed.

Body mass declined only slightly between the two measurements, by about 0.1 g on average, while the much larger changes in endurance and metabolic capacity occurred over the nine-day feeding interval.

Sources and image credit

  1. Engert, E. R., Nord, A., Andreasson, F., & Nilsson, J.-Å. (2026). Flexibility of exercise capacity during nestling feeding in blue tits. Journal of Experimental Biology, 229(7), jeb251043. https://doi.org/10.1242/jeb.251043Primary peer-reviewed open-access article published online 31 March 2026 under CC BY 4.0. Male blue tit endurance increased from 9.8 to 25.1 minutes and maximum exercise-induced metabolic rate increased by 16% between days 5 and 14 of nestling feeding.

Image: Frau1985 / Wikimedia Commons — Eurasian blue tit at a nest box while feeding chicks; representative breeding image, not one of the study birds. · CC BY-SA 3.0 · Source

About this content: This story was produced with AI assistance within an editorial workflow developed by Wildlife Vagabond. Editorial responsibility remains with Wildlife Vagabond.How AI is used

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