kw: natural history, birds, eggs, predation
We soon recognized that both were victims of predation by a bird. The yellow smears on the first egg, which are sticking some bits of grass to it, are also dried yolk. Robin eggs are frequently punctured and eaten by crows and jays, and several other birds that are secondary predators (primary predators are the raptors such as hawks and eagles). Mammal egg predators take the whole egg, run off with it and crunch it down whole.
It is sad but true that "cute little birds" can be as vicious as any tiger. Except that "vicious", a word derived from "vice", is a human term and applies to humans. Neither tigers nor crows are vicious in the human sense. They kill to eat. They must kill or die. There are birds that more closely fit the term "vicious", such as starlings, which puncture eggs in a nest and drive off the parent birds so they can take over the nest.
So how do you tell when an egg shell you find is from a hatched bird, not a "murdered" one? The hatching chick pecks a girdle most of the way around the shell, then pushes to finish the crack. When you find a half shell with peck marks most of the way around, that is from a normal hatchling. The parent birds usually carry each half in a different direction away from the nest, 20 or 30 yards, maybe farther. This misleads predators as to the location of the nest with its vulnerable hatchlings.