Global climate is changing at an unprecedented rate. Adjustments to breeding phenology represent responses to current climate change, and some climatic effects have negatively affected population reproductive performances. Here we simulated the possibility that climate warming-induced changes in the timing of egg-laying may modify the phenotype composition (i.e. proportion of high- vs. low-quality phenotypes) of avian populations of single-brooded, long-lived species in northern countries (where asymmetric changes of weather conditions are more pronounced), therefore affecting the internal structure and long-term stability of populations. In northern countries, prelaying temperatures have risen and laying and hatching are expected to occur earlier. However, because post-hatching temperatures have remained stable, early chicks hatch under conditions of low temperature and great precipitation, and may face increased mortality. Because early breeders are generally high-quality individuals, their contribution to the future recruitment of the breeding population will decrease, engendering a doubly negative effect: (1) the number of offspring in a population will be lower than in the past because of higher mortality in the largest broods; and (2) the population will increasingly be composed of the offspring of low-quality individuals, which will consequently decrease fitness of the entire population.