Eggs vary substantially in shape, size and weight. Within any species egg shape tends to be relatively constant and similar to that in related species. Some generalizations as to shape include rather spherical eggs in owls and kingfishers, long eliptical eggs in the more aerial species (swallow, hummingbirds), and hyriform (top-shaped) eggs in many species with four-egg clutches (for compactness, e.g. waders) or nesting on bare ground (for stability against rolling, e.g. auks). Egg weights vary across orders of magnitude from a hummingbird's 0.3g to an Ostrich's 1,600g. Egg weight is closely correlated with the weight of the female, though there can be considerable differences in egg weight between different species groups. For example, the 4.5g egg of a 100g Cuckoo to the 21g egg of a 100g shearwater; in part this results from different modes of development.

Egg Size: is given as the average length and width (both in mm).

Egg Weight: is approximate fresh weight (from a calculation based on egg size). Shell weights are broadly correlated with egg weight but some species laying on bare rock e.g. Guillemot - produce eggs with shells unusually thick over some or all of their area. Several species near the top of their food chains (raptors, herons, etc.) have been shown to lay unusually thin-shelled eggs when contaminated with persistent agricultural organochlorines (such as DDT).

The data presented here are from Schönwetter & Miese (1960) and are generally for the nominate race of each species.

Schönwetter, M. & Miese, W. (1960-1992) Handbuch der Oologie. Academie Verlag, Berlin.