Parental care and bird embryonic metabolism: A comparison between uniparentally incubating Silver‐throated Tits and biparentally incubating Black‐throated Tits

Abstract Studies have shown that organisms may adjust metabolic rates in response to thermal variability, highlighting metabolic plasticity as a key adaptive mechanism. Understanding the extent of metabolic plasticity of an organism is key to predicting its adaptation to climate change. In birds, the embryos from the uniparentally incubating species are more frequently exposed to intermittent cooling due to the parents' repeated absences from the nest than the embryos from the species with biparental incubation. Such pressure may favour them to evolve a better ability to cope with the physiologically suppressive effects of low temperatures and temperature fluctuations. We compared embryonic heart rate, a proxy for embryonic metabolic rate, and its response to egg temperature change between two closely related species, the biparentally incubating Black‐throated Tit ( Aegithalos concinnus ) and the uniparentally incubating Silver‐throated Tit ( Aegithalos glaucogularis ). We also conducted an interspecific egg‐swapping experiment to investigate the effect of incubation environment on embryonic metabolism and its consequences (i.e. incubation period length and hatching success). Consistent with the expectation, Silver‐throated Tit exhibited a trend (although nonsignificant) of higher embryonic heart rate than Black‐throated Tit. Also, when egg temperatures dropped, Silver‐throated Tit showed a significantly slower decrease in embryonic heart rate than Black‐throated Tit, suggesting that they may better cope with temperature drop. In the egg‐swapping experiment, embryonic heart rates did not differ significantly between fostered and not‐fostered eggs, but following fostering, heart rates tended to decrease in Silver‐throated Tit embryos and increase in Black‐throated Tit embryos, suggesting that embryos of both species show some responsiveness to new thermal environments. Egg‐fostering treatment did not affect hatching success in either species, but the fostered eggs of Silver‐throated Tits and Black‐throated Tits tended to shorten and lengthen their incubation period, respectively, implying that the relatively stable biparental incubation environment may facilitate faster embryonic development than the more variable uniparental incubation environment. The findings enhance our understanding of the developmental strategies and responsiveness to changed environments in avian embryos under different parental care modes and provide insights into their potential to respond to climate change. Read the free Plain Language Summary for this article on the Journal blog.