Altricial bird species rely on their parents to bring food, defend the nest and to help them thermoregulate. Brood size may influence how well adults are able to provision their young. A low quality diet has been shown to have long-term developmental costs and usually results in elevated corticosterone levels, which may mediate these costs. Chicks from larger broods may experience costs from poor condition and elevated corticosterone levels. My thesis addresses the questions of how natural brood size in Tree Swallows (Tachycineta bicolor) affects parental effort, chick growth and corticosterone levels, and at what age Tree Swallow chicks respond to a handling stress. Parents increase their effort, but not enough to match brood size, and chicks from larger broods are smaller in mass, but did not exhibit elevated CORT levels. Chicks respond with elevations in CORT at all ages sampled, but younger chicks have lower stress levels of CORT.