Temperature-dependent ontogeny of bird thermoregulation and growth

Abstract

This thesis investigates the ontogeny of thermoregulation and growth in birds, to improve the understanding of how prevailing environmental temperature affects physiology and morphology, and to examine whether body size influences thermoregulation. My focus has been on temperature effects emerging after hatching, which are poorly understood compared to effects of variation in embryonic developmental temperature. In addition, there is limited empirical evidence for how morphology actually influences thermoregulatory capacity, despite such links being a popular explanation for extant size declines in birds.
First, I studied short- and long-term effects of cold or warm post-hatch developmental temperatures on thermoregulatory traits (metabolic rate, evaporative water loss, and cold- and heat tolerance; Papers I and II). The results showed that birds that grew up in high developmental temperatures had lower metabolic heat production but increased evaporative water loss, rendering them better able to tolerate both submaximal and maximal warm temperatures. Cold developmental temperatures, on the other hand, increased the cold tolerance limit and maximal cold-induced metabolic rate, but did not alter thermoregulation at submaximal temperatures. When the birds were measured again after a period without thermal challenges, no lasting effects were observed. In the second part of the thesis, I investigated how body size affected thermoregulation (Papers III and IV). I found that body size had limited effects on the thermoregulatory capacity, such that only large deviations in body mass and appendage length exceeding allometric expectations altered patterns of heat loss. Lastly, I tested theory arising from the captive studies in nature, where animals may struggle more to balance the costs of thermal challenges on account of resource limitations. Specifically, I investigated whether experimentally increased nest temperatures would limit growth and alter thermoregulatory responses in blue tits nestlings (Paper V). The results showed that high temperature during postnatal development caused a trade-off between thermoregulation and growth. Nestlings in heated nests incurred higher body temperature and decreased metabolic heat production as a compensatory response. However, this compensation was only partial, because body mass decreased in heated nests and more so the higher the nestling body temperature.
Taken together, this thesis shows that the effects of post-hatch developmental temperature on thermoregulatory traits may be costly in terms of growth and water balance but that they reflect phenotypic flexibility. In addition, decreasing body size has negligible effects on thermoregulation overall, but it may be meaningful under certain circumstances and for birds at the extreme ends of size distributions. While my work suggests that there might be potential to buffer effects of post-hatch heat exposure, birds developing under such conditions could still be maladapted if reversal of effects is slow, or if inheritance of thermoregulatory competence, leads to mismatches between phenotype and environment.

Abstract (Swedish)

This thesis investigates the ontogeny of thermoregulation and growth in birds, to improve the understanding of how prevailing environmental temperature affects physiology and morphology, and to examine whether body size influences thermoregulation. My focus has been on temperature effects emerging after hatching, which are poorly understood compared to effects of variation in embryonic developmental temperature. In addition, there is limited empirical evidence for how morphology actually influences thermoregulatory capacity, despite such links being a popular explanation for extant size declines in birds.
First, I studied short- and long-term effects of cold or warm post-hatch developmental temperatures on thermoregulatory traits (metabolic rate, evaporative water loss, and cold- and heat tolerance; Papers I and II). The results showed that birds that grew up in high developmental temperatures had lower metabolic heat production but increased evaporative water loss, rendering them better able to tolerate both submaximal and maximal warm temperatures. Cold developmental temperatures, on the other hand, increased the cold tolerance limit and maximal cold-induced metabolic rate, but did not alter thermoregulation at submaximal temperatures. When the birds were measured again after a period without thermal challenges, no lasting effects were observed. In the second part of the thesis, I investigated how body size affected thermoregulation (Papers III and IV). I found that body size had limited effects on the thermoregulatory capacity, such that only large deviations in body mass and appendage length exceeding allometric expectations altered patterns of heat loss. Lastly, I tested theory arising from the captive studies in nature, where animals may struggle more to balance the costs of thermal challenges on account of resource limitations. Specifically, I investigated whether experimentally increased nest temperatures would limit growth and alter thermoregulatory responses in blue tits nestlings (Paper V). The results showed that high temperature during postnatal development caused a trade-off between thermoregulation and growth. Nestlings in heated nests incurred higher body temperature and decreased metabolic heat production as a compensatory response. However, this compensation was only partial, because body mass decreased in heated nests and more so the higher the nestling body temperature.
Taken together, this thesis shows that the effects of post-hatch developmental temperature on thermoregulatory traits may be costly in terms of growth and water balance but that they reflect phenotypic flexibility. In addition, decreasing body size has negligible effects on thermoregulation overall, but it may be meaningful under certain circumstances and for birds at the extreme ends of size distributions. While my work suggests that there might be potential to buffer effects of post-hatch heat exposure, birds developing under such conditions could still be maladapted if reversal of effects is slow, or if inheritance of thermoregulatory competence, leads to mismatches between phenotype and environment.

• author: Persson, Elin
• supervisor: Andreas Nord; Lars Råberg
• opponent: Professor Ghalambor, Cameron, NTNU - Norwegian University of Science and Technology, Trondheim, Norway
• organization: Department of Biology; Ecological and Evolutionary Physiology (research group); Evolutionary Ecology and Infection Biology
• publishing date: 2026
• type: Thesis
• publication status: published
• subject: Biological Sciences
• keywords: Bird, Development, Thermoregulation, Growth, Heat tolerance, Cold tolerance, Metabolic rate, Evaporative cooling, Extreme weather event, Conservation physiology, Climate change, Life history, Bird, Development, Thermoregulation, Growth, Heat tolerance, Cold tolerance, Metabolic rate, Evaporative cooling, Extreme weather event, Conservation physiology, Climate change, Life history
• pages: 66 pages
• defense location: Blue hall, Ecology building
• defense date: 2026-06-05 09:00:00
• ISBN: 978-91-90202-02-9
• ISBN: 978-91-90202-03-6
• language: English
• LU publication?: yes
• id: 572d20a9-6f48-4b35-b303-baa7ba0be089
• date added to LUP: 2026-05-12 10:22:12
• date last changed: 2026-05-13 10:54:50