Suboptimal temperatures may exert negative effects on altricial nestlings, especially during their first days of life, when thermoregulation is not yet fully developed. We experimentally lowered nest temperatures by a mean of 4.5 °C during early development of great tit Parus major nestlings from the Mediterranean region. The thermal treatment only affected nestling size, as cooled nestlings had smaller tarsi by day 15 than controls. Female brooding constancy remained unaltered and female body condition was not negatively affected, so females did not incur additional energetic costs to ameliorate thermal conditions for nestlings. In conclusion, we found that colder nest microclimates may impair nestling growth, which may have negative consequences on future survival.