Telomeres (the ends of chromosomes) become shorter with increasing age. There is increasing evidence that telomere shortening reflects senescence and thus functions as a biomarker of biological aging. We will study telomere shortening in great and blue tits on Vlieland, where sub-populations differ consistently in survival rates, but in opposite direction in the two species. Further, family trees are well known, yielding high power to detect heritable components in variation in telomere length and dynamics. We will test experimentally whether telomere shortening and senescence are accelerated when ?life stress? increases by letting the birds carry a small weight.