While walking round the Symphony Lake of the Singapore Botanic Gardens one evening, I came across one half of an eggshell on the ground. The white shell looked too nicely cut round the equator to be natural. So I threw it away.
A few days later Melinda Chan sent me an image of the eggshell of a Pink-necked Green Pigeon (Treron vernans) that she found lodged in the fork of a tree near where a pair of the birds was nesting (left). The white shell looked similar to the one I picked up earlier.
Both pieces must have been discarded by the parent birds immediately after hatching. This is usually the case as leaving the shells around the nest may attract predators. We earlier documented this during the nesting of the Peaceful Dove, also known as Zebra Dove (Geopelia striata) (below) and Little Tern (Sterna albifrons).
Now we come to the question of how come the break in the eggshell is so clean and how was the egg cracked nicely into two.
After the usual number of days of incubation, the developing bird embryo inside is ready to be hatched. Just before this, it develops an egg tooth, a short but pointed structure on the tip of the beak. Near the end of the incubation period the fully developed embryo rubs its egg tooth against the inner wall of the egg. By this time most of the calcium in the shell would have been reabsorbed by the developing embryo and the shell would be very much weakened.
Once the shell is punctured, the egg is said to be pipped. This first puncture is followed by a series of others encircling the blunt end of the egg. Eventually the shell gives way and the chick struggles free. Within a few days after hatching the egg tooth falls off or is reabsorbed by the growing chick.
Once the chick is hatched, one of the parent birds usually removes the shells and dump them some distance away.
(Images by Melinda Chan (top) and YC (bottom))