Heat stress in Alpine swifts — from nestlings to lifetime fitness

 

How do extreme weather events shape the lives of Alpine swifts, from incubation to old age — and across generations?

Human activities are driving more frequent and more intense extreme climatic events: heatwaves, cold spells, heavy precipitation and droughts. Endothermic vertebrates (birds and mammals) maintain their body temperature within a narrow optimal range, and extreme temperatures during early life can have lasting consequences on growth, physiology, behaviour and life history. Yet we still know surprisingly little about what thermal conditions actually stress wild nestlings, what the underlying physiological mechanisms are, and how these early-life experiences echo through adult life and across generations.

This project investigates these questions in a unique wild study system: the Alpine swift (Tachymarptis melba), an iconic aerial insectivore that breeds under the roofs of European buildings. Because nests are often in poorly insulated attics, swift nestlings are directly exposed to the urban heat-island effect, making them an ideal natural sentinel for the impacts of climate change on developing endotherms.

 

Background

Several lines of evidence point to climate change as a major driver of Alpine swift biology:

  • Heatwaves and cold spells have changed dramatically in frequency and intensity across Europe since the 1950s, with a steep increase in the number of heatwaves observed at our study colonies.
  • Nestling body size and growth rate have significantly accelerated between 1999 and the present, coinciding with rising June–July temperatures, and recent quantitative-genetic analyses suggest that these changes are not only phenotypic but also genetic.
  • Wildlife rescue centres routinely report sudden spikes in the number of swift nestlings admitted during heatwaves and other extreme events — an observation that motivated the Saving swifts, understanding climate Europe-wide survey.
  • Pilot data collected at our colonies in 2025 show that behavioural signs of heat stress in developing nestlings (panting, wing-spreading) start at nest temperatures as low as 27°C.
  • Preliminary unpublished analyses indicate adverse effects of heatwaves during incubation on hatching and fledging success, as well as on body size at 50 days of age. In adults, mitochondrial function decreases with increasing temperature, suggesting energy deficits that may impair thermoregulation, growth and survival.

Together, these observations frame an integrated research programme on the short-term, long-term and transgenerational consequences of extreme weather, combining field experiments in wild colonies, long-term data, ecophysiology, quantitative genetics and movement ecology.

 

Research aims

The project is organised around five complementary research lines:

  1. Causal short-term effects of extreme weather — define the causal consequences of cold spells and heatwaves on nestling development through in situ manipulation of nest microclimate during incubation and growth.
  2. Genetic basis and evolutionary potential of thermoregulatory traits — disentangle genetic from early-life environmental contributions to thermoregulation using partial cross-fostering combined with multi-generational quantitative-genetic (“animal model”) analyses.
  3. Lifelong and transgenerational effects — assess whether early-life thermal stress shapes adult physiology, age at first reproduction, lifetime reproductive success, lifespan, and the performance of subsequent generations (silver-spoon vs. environmental-matching hypotheses).
  4. Effects on parental behaviour — investigate how an individual’s own early-life thermal experience modulates its parental nest attendance, brooding behaviour and foraging effort under variable weather.
  5. Population-level consequences — integrate the previous findings into demographic models to quantify the contribution of early-life thermal stress to population dynamics under climate change.

 

Future perspectives

The framework is designed to be broadly scalable and transferable: the Alpine swift is a synanthropic species sharing buildings with humans across Europe, so insights from this system can be compared to and extended to other urban-dwelling vertebrates facing the same intensifying thermal stress. In the longer term, the project aims to integrate experimental, longitudinal and demographic evidence into mechanistic, transferable predictions of how endotherms will cope with — and adapt to — an increasingly volatile climate.

This work also connects directly to my Europe-wide rescue-centre survey, which leverages data from wildlife rescue centres to detect the population-level signature of heatwaves on swift fledglings at the continental scale.

 

Collaborators

Students on the project

  • 2026-27 Anna Kohler. MSc thesis — University of Zurich, Switzerland. “Effects of heatwaves on the behaviour and physiology of nestlings of an urban-breeding species, the Alpine swift (Tachymarptis melba)”. Ongoing.

  • 2024-25 Monti Spinas. MSc thesis — University of Zurich, Switzerland. “Extreme temperature effects on Alpine swift nestling survival and growth”. Outcomes: 2 articles in preparation; results presented at 2 conferences in 2025 (EOU Bangor and CIO Lecce).

 

This page summarises the integrative research line on heat stress in Alpine swifts that I am currently building. It is connected to the Saving swifts, understanding climate project and to the ongoing analyses of growth and body-size change developed during my Marie Skłodowska-Curie Global Fellowship.