Chill out: Environmentally relevant cooling challenge does not increase telomere loss during early life

•Stress responses early in life can promote or mitigate cellular damage.•Previous work shows cooling initiates a stress response in altricial nestlings.•Experimental cooling did not affect telomere shortening of bluebird nestlings.•Larger broods and faster growth were associated with shorter telomeres.•Parental care may optimize stress responses to maximize fitness of young.