Why are birds or eggs oval rather than spherical?

After a series of biostatistics analysis, the author found that: Results 1: The length of eggs is related to the adult's weight, the number of eggs in the nest, feeding habits and the development mode of young birds. These findings show that the size of eggs is controlled by life history parameters and the space size of nests at least to some extent. Results 2: The shape of eggs (asymmetry and ovality) has no significant relationship with the number of eggs in the nest, development mode, environmental factors or nest characteristics. Result 3: The shape of an egg is related to its size and HWI. The higher the HWI value or the larger the egg, the greater the ovality of the egg. Because the HWI index is related to flight efficiency, researchers infer that the characteristics of adaptive flight are the key driving factors for the diversity of bird eggs. Some adaptive characteristics with strong flying ability, such as small body size, small abdominal volume and no functional ovaries and fallopian tubes, affect the formation of ovals. In particular, these adaptations limit the width and maximum shape of fallopian tubes, resulting in more oval shapes. Given the circumference (widest part), a more oval egg can have a larger volume.