Nestling body mass is a reliable measure of condition and future survival
This hypothesis has received some support in some studies, which seems to have led to the concensus opinion that it is a reliable assumption. It had better be at least somewhat reliable considering how much research depends on it being a generalizable truth. However, most radio telemetry studies directly observing fledgling survival from the nest to well after independence from adult care have found that juvenile songbird survival is not related to nestling body mass. Nearly all such studies show that fledgling survival is strongly related to nest site, post-fledging habitat use, and/or predator abundance. Logic dictates that even the strongests, fastest, most agile, most massive fledgling songbird has little chance against even the slowest hawk, snake, or mammal. Therefore, if most fledgling mortality is attributable to predation, and a substantial portion of first-year mortality occurs during the fledgling stage, then nestling condition should not be expected to predict juvenile survival or recruitment into the breeding population.
In our studies of fledgling Ovenbirds and Golden-winged Warblers, we found no relationship between nestling mass and fledgling survival before or after independence from adult care. We also made the simple observation that the mass of a nestling varies substantially depending on the contents of it's digestive system. In Golden-winged Warblers, a nestling could be one of the heaviest in the population, then poop and become one of the lightest, and then be fed by an adult and instantly become one of the heaviest again. This continuous variation in individual nestling mass suggests body mass is not a reliable measure of body condition, let alone a predictor of potential survival.