In wild animals, telomere attrition during early development has been linked with several fitness disadvantages throughout life. Telomerase enzyme can elongate telomeres, but it is generally assumed that its activity is suppressed in most somatic tissues upon birth. However, recent evidence suggests that this may not be the case for long-lived bird species. We have therefore investigated whether telomerase activity is maintained during the postnatal growth period in a wild yellow-legged gull (Larus michahellis) population. Our results indicate that telomerase activity is not negligible in the blood cells, but activity levels sharply decline from hatching to fledging following a similar pattern to the reduction observed in telomere length. Our results further suggest that the observed variation in telomere length may be the result of a negative effect of fast growth on telomerase activity, thus providing a new mechanism through which growth rates may affect telomere dynamics and potentially life-history trajectories.