Having only one ovary, a female American robin lays one egg per day.  She does not begin incubating her unborn nestlings until she has laid the last egg, ensuring the entire clutch hatches at approximately the same time- about twelve days.

A baby robin emerges from its blue egg twenty four hours after breaking the shell open with its egg tooth.  The small hook like protuberance will fall off it’s beak a few weeks later.  The featherless nestling’s bulging eyes will not be able to see for five days.  Its empty stomach is not one of the organs visible though it’s transparent skin.  Hungry and helpless, the newborn is completely reliant upon its parents.

The mother and father robins share the overwhelming job of keeping the nestlings fed.  Each newly hatched bird requires about forty feedings of soft invertebrates, fruit and plants daily.  Its diet will consist of beetle larva and earthworm parts regurgitated (recycled) by its parents until it leaves the nest in about two weeks.

As well as the tiring responsibility of feeding, the adult robins are challenged with housekeeping.  Each newborn poops within seconds of feeding.  Four baby birds defecating forty times a day is a big mess.  Although nestlings don’t come with diapers, Mother Nature understands the health risks of a nest littered with droppings and created the next best thing.

Each of the approximate one hundred sixty daily droppings comes neatly enclosed in a white, translucent sac known as a fecal sac.  During its first few days of life the baby robin doesn’t have much bacteria in its intestines and the majority of the feces is parts of the regurgitated meal.  The fecal sacks just contain partially digested food. 

This eliminates a nest painted white from droppings; but now the parents have to get rid of the bouncing sacs.  They are able to rid the nest of the bagged poop, conviently deposited right after feeding, before they leave to find more food.  Since the fecal sacs are devoid of bacteria, the adult robins eat (recycle) them.

The parents continue feeding by regurgitation (recycle- regurgitating regurgitated fecal matter) and the nestlings keep depositing fecal sacs.  When the nestlings are older and their fecal sacs contain bacteria, the parents will carry the mucus bags away in their beaks for disposal.  They will drop them from flight several yards away so predators cannot easily follow the scent back to the nest.

When the baby robins are feathered and outgrow the nest, they usually depart within two days of each other.  The complying parents continue to feed the still flightless fledglings on the ground.  Their bodies no longer producing fecal sacs, the young robins excrete the familiar white droppings that splatter on just washed windshields.  Relieved of their housecleaning duties the parents protect their offspring and teach them to forage on their own.

In two weeks the fledglings will be able to fly, find and eat their own food.  Although they won’t begin recycling until they have their own clutch; they are already regurgitating the seeds of the fruit they eat.