Data from: Parental care and bird embryonic metabolism: A comparison between uniparentally incubating silver-throated tits and biparentally incubating black-throated tits

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 (A. 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 slighter 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 unfostered 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.