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Telomeres, the repetitive DNA sequences that cap eukaryotic chromosomes, are thought to play an important role in linking life conditions and senescence. In vertebrate somatic cells, telomeres shorten at each cell division, and the rate at which they do so has been linked to cellular and organismal senescence. Although telomeres generally shorten with age in vertebrates, in most species studied there is considerable variation between same age individuals. In this thesis, I examined the telomere dynamics of various avian species, investigating both the causes of variation in telomere length among individuals and what effect this variation has on attributes such as survival rates. Previous studies have shown that most telomere loss occurs in young individuals and it thus makes sense that early life conditions are responsible for much of the inter-individual variation in telomere length. I investigated this idea by studying chick telomere dynamics in a wild population of lesser black-backed gulls Larus fuscus. There was considerable variation in hatching telomere length among individuals and much of this variation was related to circumstances during embryonic growth. Larger hatchlings had shorter telomere lengths, suggesting that embryonic growth rate could have affected telomere attrition. Independent of this trend, males had longer telomeres at hatching than females. Although telomere length did decrease with age post-hatching, these initial variations remained consistent during the initial post-hatching period. The relationship between early life conditions and telomere length was investigated further with a longitudinal study of telomere length in chicks of the European shag Phalacrocorax aritotelis. A previous study on this population of birds had shown that telomere length declines with age within individuals over a period of several years. However no change in telomere length was detected over a period of 11-13 days during the chick period. Body size had no effect on telomere length, but males did have longer telomere than females. These initial chapters investigate telomere length in chicks; however there are very few studies that investigate telomere length over the entire lifespan of long-lived species. I thus next examined the telomere dynamics of two species of long-lived seabird, the northern and southern giant petrels (Macronectes spp.). In both giant petrel species, telomeres were shorter in adults than chicks, but there was no trend for adult telomere length to decrease with age. In southern giant petrels, there was a significant relationship (independent of age and sex) between an individuals telomere length and whether it was still alive 8 years after it was initially sampled. This relationship was not present in northern giant petrels, possibly due to a smaller sample size. The results thus support both the idea that most telomere loss occurs in young individuals and that telomere length may be an indicator of life expectancy. Various methods exist to measure telomeres. As the number of taxa whose telomere dynamics are being studied increases, it becomes increasingly important to know which methods are the best to use and to what extent these methods are applicable across species. These questions were investigated in relation to work conducted on the telomere dynamics of the blue-footed booby Sula nebouxxi. Both the TRF and qPCR techniques were used to measure booby telomeres, but problems arose with both methods. It is possible that these problems occurred because blue-footed boobies have a particularly large amount of interstitial telomeric DNA, although a more detailed analysis of booby telomeres would be necessary to determine this. These findings suggest that standardised methods to measure telomeres cannot necessarily be applied to every new species whose telomere dynamics are studied. The evidence presented here suggests that the study of telomere dynamics can be a very powerful tool for behavioural ecologists. It now seems possible that telomeres might provide both a way of measuring the long-term costs of early life-conditions and a way to measure the quality of an individual. However, further research is still needed to fill in the considerable gaps in our knowledge and fully exploit the potential telomeres have for behavioural ecology.