Ross, Howard A. 1980. Growth of nestling Ipswich Sparrows in relation to season, habitat, brood size, and parental age. The Auk, 97: 721-732.
Summary (AGH): The nestling phase is an important part of a songbird’s life — this is when the organ systems it will use throughout its life develop and determines how healthy and energetic the bird will be when it leaves the nest to fend for itself — a costly, risky period that most individuals don’t survive. This study measured two aspects of nestling growth: tarsus* length (a measure of overall body size) and weight (a measure of current energy resources). Growth in both measures was better in heath vegetation than in relatively open marram grass habitat, and better for offspring of adults that were breeding in their first year, likely because older adults spread their efforts so they could raise more broods in each breeding season. The more nestlings in the nest (i.e., the bigger the brood size), the lighter each nestling was, probably because of competition among nestlings for food from the parents. However, the larger the brood, the bigger each nestling was, as measured by its tarsus length, probably because huddled nestmates can allocate more energy from staying warm to growing, instead. The results are of particular interest for showing the benefits of dense habitat for this bird’s reproductive output, and for showing how younger and older adults differ in what accounts for successful breeding.
*Tarsus length is a measure of a bone – the tarsometatarsus − that is only found in the lower leg of birds and some non-avian dinosaurs, and runs from the foot up to the next bend in the leg.