We have published two recent papers (led by Sam Andrew), that show how body size of adult birds appears to be shaped by the climate during development. The first of these, published in Auk (link), is an observational study of the house sparrow across Australia, showing that in hotter areas sparrows are smaller. This pattern, consistent with findings in North America and Europe, is better explained by hot summer temperatures, than cold winter temperatures, and we believe suggests that development might be constrained by growing in hot conditions.
This idea was then supported by experimental work (along with some field observations) from the zebra finch. In this study (published in Journal of Evolutionary Biology) (link) we showed that birds that were reared in temperature controlled rooms at 30ºC were smaller than their full siblings reared at 18ºC. This is one of the first experiments to show that the ambient temperature effects development and body size in a warm-blooded animal.