Signs of Spring: Report Your Observations!

**How long will this nest stay clean?**
Photo Jim Gilbert

Baby robins remain in their nest for about 13 days. Just about every time the nestlings gulp down some food, they poop. Let's see--that's 13 days x 4 babies x 356 insects and worms on average each day--that's a LOT of poop! How on earth do robins keep their nest clean?

Baby robins can't wear diapers of any kind, but they do put their poop in a strong "bag" so the parents can carry it away. This bag is made of thick, strong mucus that a parent robin can pick up in its sharp beak and carry without puncturing, and is called a "fecal sac." Fecal sacs are just like disposable diapers for birds!

Within seconds of feeding, baby robins back up and poop. This ensures that whichever parent brought the food will still be there to carry away the fecal sac.

**Fecal sac from a house wren**

For the first couple of days after hatching, parent robins actually eat many of their babies' fecal sacs. This sounds gross! But before the babies are a few days old, their intestines don't have much bacteria to help them digest their food. It's the bacteria that make droppings germy and smelly. Before the bacteria kick in, the droppings are rich in only partially-digested food items. Fortunately, at first the babies are still absorbing their yolk sac so they don't suffer from malnutrition even though so much of their food literally goes to waste. Their parents can take advantage of the food still in the droppings. Eating from the droppings allows the parents to give more of the worms and insects they find to their babies rather than eating this food themselves. And once the droppings start having more bacteria, no way wll the parents keep eating them!

Growing out of Diapers
To get rid of the fecal sacs of older babies, the parents carry them away. Robins and bluebirds fly 20-50 yards away, and drop the sacs in flight. Grackles almost always drop fecal sacs over water. Long ago, grackles usually nested near rivers, and the flowing water would carry the droppings far away. Now many grackles nest in conifers in people's back yards, often far from rivers. But the instinct to drop those fecal sacs over water is still strong, so grackles often drop them in swimming pools and bird baths.

Making fecal sacs takes protein. It's worth the cost when nest sanitation is at risk, but as soon as baby robins leave the nest, they stop producing fecal sacs. Now instead of bouncing, their droppings spatter.

Not Just Poop
Besides the outer casing on nestling droppings, bird droppings have two parts--the brown fecal matter, which is the food waste from their intestines--and the white urine. Urine is produced by the kidneys as they filter the blood, and much of the waste is poisonous nitrogen-based molecules. The nitrogen in our urine is in the form of urea, which is clear and yellowish, so toxic that it must be diluted with a lot of water. The nitrogen in bird (and most reptile) urine is in the form of uric acid, which is white. Uric acid is toxic, too, but if it is very concentrated, it turns into a solid, or precipitates, becoming chalky. Producing uric acid is one way that bird bodies conserve water.

Try This! Discussion or Journaling

1. Some baby birds never produce fecal sacs. Ducklings, grouse and pheasants, raptors, and some seabirds that nest on cliffs are species that don't. How do these birds ensure that their nests will stay clean? What makes birds that don't produce fecal sacs different from robins and other songbirds? Write your thoughts in a journal, or discuss as a class. Then see what our Journey North science writer has to say about it here.

2. Fecal sacs aren't the only things parent birds carry away from nests to keep the nests clean. Can you think of some other things parent birds have to carry off? List as many as you can. Then see if you thought of some that our Journey North science writer didn't think of by clicking here.

Try This! Be a Poop Counter
If you know the whereabouts of a nest with nestlings--wrens, bluebirds, robins, jays, or any other species--find a place to sit close enough to see everything going on with binoculars without bothering the parents, and watch them for 15 minutes. How many times do the parents come to the nest with food? How many times do they carry away a fecal sac? Based on your 15-minute count, calculate the number of poops per hour (pph). If the parents didn't carry it all away, how long do you think it would take for the nest to get filled to the top with poop?