Developmental stressors often have long-term fitness consequences, but linking offspring traits to fitness prospects has remained a challenge. Telomere length predicts mortality in adult birds, and may provide a link between developmental conditions and fitness prospects. Here, we examine the effects of manipulated brood size on growth, telomere dynamics and post-fledging survival in free-living jackdaws. Nestlings in enlarged broods achieved lower mass and lost 21% more telomere repeats relative to nestlings in reduced broods, showing that developmental stress accelerates telomere shortening. Adult telomere length was positively correlated with their telomere length as nestling (r ¼ 0.83). Thus, an advantage of long telomeres in nestlings is carried through to adulthood. Nestling telomere shortening predicted post-fledging survival and recruitment independent of manipulation and fledgling mass. This effect was strong, with a threefold difference in recruitment probability over the telomere shortening range. By contrast, absolute telomere length was neither affected by brood size manipulation nor related to survival. We conclude that telomere loss, but not absolute telomere length, links developmental conditions to subsequent survival and suggest that telomere shortening may provide a key to unravelling the physiological causes of developmental effects on fitness.

Evidence accumulates that telomere shortening reflects lifestyle and predicts remaining lifespan, but little is known of telomere dynamics and their relation to survival under natural conditions. We present longitudinal telomere data in free-living jackdaws (Corvus monedula) and test hypotheses on telomere shortening and survival. Telomeres in erythrocytes were measured using pulsed-field gel electrophoresis. Telomere shortening rates within individuals were twice as high as the population level slope, demonstrating that individuals with short telomeres are less likely to survive. Further analysis showed that shortening rate in particular predicted survival, because telomere shortening was much accelerated during a bird's last year in the colony. Telomere shortening was also faster early in life, even after growth was completed. It was previously shown that the lengths of the shortest telomeres best predict cellular senescence, suggesting that shorter telomeres should be better protected. We test the latter hypothesis and show that, within individuals, long telomeres shorten faster than short telomeres in adults and nestlings, a result not previously shown in vivo. Moreover, survival selection in adults was most conspicuous on relatively long telomeres. In conclusion, our longitudinal data indicate that the shortening rate of long telomeres may be a measure of 'life stress' and hence holds promise as a biomarker of remaining lifespan.

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4099Fluctuations in food supply are the rule in nature, and animals have evolved numerous adaptations to cope with such fluctuations. Knowledge of physiological or metabolic adjustments to variation in foraging success is essential for making predictions about behavioural choices such as when to leave a site in which food availability decreases, or what levels of energy reserves should be maintained in different environments. The response to diminishing foraging conditions has many dimensions: resting metabolic rate may be altered, mass reduced, and consequently, flight and thermoregulation costs may change (Tiebout, 1991;Deerenberg et al., 1998;Bautista et al., 1998). Therefore daily energy expenditure (DEE), and thus requirements to remain in energy balance, are not simply proportional to the (foraging) activity.An approach that has frequently been used to study the physiological consequences of food stress is complete or partial caloric restriction (Daan et al., 1989;Cherel et al., 1994), which by definition results in a decrease in DEE (Fig.·1A). However, when foraging success decreases in the real world, animals that are not sit-and-wait predators have to spend more time and energy foraging for the same amount of food. Everything else remaining equal, DEE is expected to be an accelerating function of foraging costs per reward, because the extra energy spent foraging must also be acquired, which again increases foraging time and energy expenditure, and so on (Fig.·1B). Surprisingly, contrary to this simple prediction, experimental tests found that DEE decreased with increasing foraging costs per reward (Deerenberg et al., 1998;Bautista et al., 1998) Knowledge of the physiological consequences of variation in food availability may be essential for understanding behavioural and life history responses to such variation. To study the physiological consequences of food availability animals are generally subjected to caloric restriction or starvation, thereby reducing the upper limit to the energy budget. The relevance of this approach to free-living animals is questionable, however, because under natural conditions low food availability often results in higher foraging costs, and everything else remaining equal this results in a higher energy budget. We manipulated food availability by varying the foraging costs and studied effects on daily energy expenditure (DEE) and energy allocation of captive starlings Sturnus vulgaris. Birds in a closed economy earned their food by flying between two perches 5·m apart. The probability of a reward was set at three different levels, thereby creating a 'poor', 'intermediate' and 'rich' environment. Compared with the rich environment, birds flew 4 times more (2.3·h per day) in the poor environment, and increased DEE by 43% to 220·kJ·day -1 (3.7ϫ ϫBMR), within the range of freeliving parents rearing young. To our knowledge this is the first study to show an increase in DEE with decreasing food availability. Body mass, basal metabolic rate (BMR) and pectoral muscle size were redu...

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