Typically the the kinds of models that I’m interested in assume that fledging rate is very dependent upon a phenological match between nestlings’ peak food requirements and the peak in caterpillar abundance. However I recently read a study by Cholewa and Wesołowski (2011; Acta Ornithologica) pointing out that, while literature shows that blue tits and marsh tits are indeed strongly dependent upon this food type, other tit species are more versatile and can successfully reproduce without relying upon it. Perhaps more troubling, the authors note that at the time of their writing, there was not a single study available that had data on changes in the nestling food composition with relation to climate warming.
Some of the reason for this must be related to a methodological difficulty in obtaining such data. Fortunately later work my Michalski et al. (2011; Acta Ornithologica) described some methods for analysing bird poop quite efficiently to determine their diet. It’d be interesting to see if diet shift - like habitat shift (Husek et al. 2014; Oecologia) or range shift - could be another way for birds to solve their phenological mismatch problem.