Feeling Blue and Crabby

Feeling Blue and Crabby: Whooping Crane Winter Diet

**Yum Yum!! These aren't blue crabs, but crnnes eat fiddler crabs, too.**

How would you like to eat the same thing every day for 5 or 6 months every year of your life? That's what whooping cranes do on their wintering grounds, which now includes Aransas NWR in Texas and, for the new Eastern flock, Chassahowitzka WR in Florida. They can eat an assortment of foods that includes crabs, clams, eels, shrimp, crayfish, acorns, snails, mice, voles, grasshoppers, minnows, dead fish, marsh onions and snakes, but their clear favorite is blue crabs.

Journey North science writer Laura Erickson has gone to Aransas National Wildlife Refuge in Texas several times to observe cranes, and whenever she's seen them eating something she could identify, it has always been blue crabs. Tom Stehn of the Whooping Crane REcovery Team and Journey North's Crane Expert, studies and censuses Whooping Cranes at Aransas every winter. He notes that their breeding success each spring is closely related to the population of blue crabs. Even if there are plenty of alternative food sources, the cranes must have a large supply of blue crabs to build up their body resources or they have a poor chance of raising babies come spring.

Crab Nutrition
What's so great about blue crabs? One large blue crab has about 85 g of meat. This provides 87 calories, with 17 g of protein, 1.5 g of fat, and 0 g of carbohydrate. But it isn't just the protein and calories in blue crabs that is important for cranes. Each crab is also rich in calcium (necessary for strong bones and also for forming egg shells), iron, magnesium, phosphorus, zinc, copper, other minerals, and lots of vitamins. And this is just the actual meat in the crab. Cranes pick off the biggest claw and some of the hard parts of the shell on the largest crabs, but they swallow smaller crabs whole and eat most of the shell even on large crabs. The shell is extremely rich in calcium and other minerals. A crane would have to eat a LOT of acorns or worms to get the nourishing vitamins, minerals, and calories of just one blue crab!

Harder Than It Looks

Photo Operation Migration

Just because blue crabs are important in the crane winter diet doesn't mean that they're easy to eat. One February Laura Erickson watched a mother whooping crane feeding her baby bits of an enormous blue crab. Even though it was almost a year old, the baby was still having trouble pulling apart the crab to get the food inside.

Tom Stehn tells us that the parents have to teach the baby cranes how to eat crabs, and it isn't easy! From Aransas Tom writes, "Whooper chicks are fed most of the winter at Aransas by their parents, with feeding tapering off as the young get older and spring approaches. With vegetable matter such as wolfberries or acorns, the young cranes quickly feed on their own. When an adult catches a blue crab, junior runs over begging and looking for an easy meal. Small crabs are swallowed whole. With a big crab, the adult usually carries it to the edge of a pond and pulverizes it on the muddy marsh soil rather than in the open water ponds. Once stunned and on marsh soil, Junior is usually pecking at the crab and trying to eat underfoot of the adult crane. The legs are pulled off and often swallowed whole. When crabs get inactive from cold temperatures (below 18 degrees C), adults stand in one spot and probe the mud until they hit a crab. I assume the youngsters also do this. George Archibald has suggested that the beaks of 6-month old cranes are still growing and may not be strong enough to be smashing crabs, but Felipe Chavez has observed young whoopers catching crabs."

Felipe writes, "The cranes I observed catching crabs generally took the crab to the edge, an open spot to tear them up. Occasionally, particularly with larger crabs, the crane would break off one of the claws before it took the crab to the edge of the pond. A free claw is dangerous, it appears. I saw crabs cling to the cranes beak after the crane had let go of it. Once on the edge, the second remaining claw was generally broken off first. Then the crane generally flipped the crab over and proceeded to peck it repeatedly. I am not sure if it was trying to kill it, or break it up regardless of whether it was alive or not. I'm not sure whether cranes kill the crab first then tear it up or tear and kill along the way. This generally left only the carapace of larger crabs, since the undersides, inside, and all extremities were consumed except for very large claws.

Introducing Crabs to the Whoopers at Chassahowitzka
Would the whoopers wintering at Chassahowitzka National Wildlife Refuge in Florida learn how to eat blue crabs? Without experienced crane parents to teach them, the cranes that arrived in December, 2001, had a hard time figuring it out. Early on, Heather Ray of Operation Migration wrote, "Sessions to encourage birds to eat blue crabs took place on four days between Dec. 8th - 13th. Although there was some interest, the crabs were too large for the birds to swallow whole and they did not know how to break them up. They showed little interest in broken pieces of crab that did not move. On Dec. 12th Richard caught a smaller crab (species unknown, intermediate in size) and offered it to the birds. They were very interested and #4 seized and ate this live crab."