Crop milk

It is extremely high in protein and fat, containing higher levels than cow or human milk.

[11] Pigeon's milk begins to be produced a couple of days before the eggs are due to hatch.

[12] Research suggests that a pair of breeding pigeons cannot produce enough crop milk to feed three squabs adequately, which explains why clutches are limited to two.

[1][2][3][4] In flamingos, crop milk produced in the first weeks is a bright red, relatively thin liquid.

[5] After the first few weeks, the color fades gradually; flamingos can produce crop milk for up to the first 6 months of their young's life.