How birds fly determines the shape of their eggs

Sleek bird species adapted to flight lay more elliptical and asymmetric eggs, according to new research that overturns classic theories. Bird eggs can be squat spheres or elongated ovals, and can also have one pointy end or be perfectly symmetrical. Bird eggs have fascinated people for millennia, yet only now are biologists beginning to crack the mystery of what makes some eggs more 'egg-shaped' than others. Dr Joseph Tobias Over the years, many reasons for the spectrum of egg shapes have been proposed - such as diet, space available in the nest, or the tendency not to roll off cliff ledges - but new research suggests flying ability is the most important factor. The research team, led by Princeton University and including Dr Joseph Tobias from Imperial College London, measured the shapes of nearly 50,000 eggs of 1,400 bird species. The variation in these egg shapes was then analysed in the context of the bird family tree, alongside details about nest type, clutch size, diet and flight ability. The researchers discovered there was a correlation between strong fliers and more elliptical and asymmetric eggs.