Life-history trade-offs have been well-documented within the literature through correlational and experimental studies. However, the physiological mechanisms underlying these trade-offs are less understood. Currently, there is great interest in shared mechanisms, specifically endocrine mechanisms, that might underlie the variation in life-history traits. Insulin-like growth factor-1 (IGF-1) may be one shared mechanism that is particularly important. IGF-1 is a metabolic hormone that is part of a highly conserved insulin-signaling pathway known to influence multiple life-history traits including growth and longevity across taxa, however, little is known about these trade-offs outside of laboratory populations. This dissertation focuses on the role of IGF-1 as a hormonal mechanism underlying the life-history trade-off between growth and aging in wild birds. While the causes of aging are not fully understood, telomere dynamics (length and change in length) are a potentially important mechanism underlying lifespan. To investigate the role of IGF-1 as a hormonal mechanism underlying the life-history trade-off between growth and aging in Franklin’s gulls (Leucophaeus pipixcan) and house sparrows (Passer domesticus). In Franklin’s gulls, dietary restriction reduced growth rate and IGF-1 levels but did not impact telomere dynamics. However, there was a significant negative correlation between IGF-1 levels and telomere length at the end of the post-natal growth period. In house sparrows, we found that nestling growth rates varied with respect to year, but IGF-1 levels did not. Telomere dynamics were not related to growth rates or IGF-1 levels, suggesting that during post-natal growth nestlings may be able to mitigate or even delay costs to later life stages. Finally, when exogenous IGF-1 was administered to house sparrow nestlings during the post-natal growth period, nestling growth was impacted but only in some years. Exogenous IGF-1 increased growth and final mass in 2016 and final mass in 2018. There was a trend suggesting experimental birds had shorter telomeres in 2016. Similarly, in 2018, experimental birds had significantly shorter telomeres than control birds. These effects were not observed in 2017, suggesting that trade-offs between growth an aging might only be visible under certain environmental conditions, which may vary with respect to year.