The effect of ambient temperature on bird embryonic development: A comparison between uniparental incubating silver-throated tits and biparental incubating black-throated tits

The temperature experienced by avian embryos during development has important impacts on their growth and post-hatching phenotypes. Ambient temperature can directly affect avian nest temperature and indirectly affect it through its impact on parental incubation behaviours. Because the nests of uniparental incubators are usually left unattended more frequently than the nests of biparental incubators whose nests can be attended by another bird when one bird leaves the nest, we predict that the effect of ambient temperature on nest temperature and thus on embryonic development (specifically, incubation period length and hatching success) and post-hatching phenotype (i.e. potential carry-over effect on nestling body mass and condition) should be greater in uniparental incubators than in biparental incubators. To test this prediction, we studied two congeneric species, the biparental incubating black-throated tits (Aegithalos concinnus) and the uniparental incubating silver-throated tits (A. glaucogularis). We found that although the two species' embryos both developed faster (shorter incubation period length) when ambient temperature was higher, the slope was significantly greater for silver-throated tits than for black-throated tits, consistent with our prediction of a greater effect of ambient temperature on embryonic development in silver-throated tits. However, the result was not due to a greater effect of ambient temperature on nest temperature because nest temperatures of the two species had positive relationships with ambient temperature in a similar way. Therefore, it implies a greater response of silver-throated tit embryos to temperature change. In addition, ambient temperature during the incubation stage did not affect either hatching success or nestling body mass and condition in both species. Although our predictions were not fully supported, our findings highlight the different responses of embryonic development to environmental changes between a uniparental incubator and a biparental incubator and suggest further research to explore the mechanisms.