The size and number of eggs are important components of reproductive strategies, with implications for offspring and parental fitness. Birds typically produce larger clutches in more poleward latitudes, in response to the strong seasonal peak of food availability required to raise young. However, changes in egg size impact the interpretation of this pattern, and latitudinal variation in egg size remains poorly explored. Here we comparatively investigate this issue at both interspecific and intraspecific levels, using datasets from 298 populations of 154 passerine species breeding in mainland China. Clutch size was larger toward the north, but egg size was relatively constant and therefore total clutch volume (clutch size 9 egg size) increased. No negative correlation was evident between egg size and clutch size along latitudinal gradients, suggesting the lack of a direct trade-off between the two traits. This may be explained by latitudinally increased food availability and concentrated annual reproductive investment due to seasonal time constraints on raising more broods, arising from the later onset of reproduction at higher latitudes. In addition, the latitudinal patterns of egg size and clutch size appear to be independent of parental investment in terms of the lengths of incubation and nestling periods, which were similar for lower-and higher-latitude birds. Our results shed light on the evolution of avian life histories across latitudinal gradients.

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