An already bad year has now turned worse for Whooping Cranes. The cranes at Necedah NWR in Wisconsin have all abandoned their nests.
This spring, crane watchers were heartened by a record 12 nests in the 43,656-acre Necedah National Wildlife Refuge and surrounding areas in central Wisconsin.Seven eggs were recovered from the nests for artificial incubation.
But then adult cranes began leaving their egg-laden nests. The same thing happened in 2008.
Leading explanations: The cranes are suffering from an inadequate food supply or swarms of black flies are driving them from their nests.
Biologist Michael Putnam has been trapping minnows and scooping muck from the bottom of the refuge to see what kind of vegetation, snails and other animal life the cranes might be eating.
When he's lucky, he finds fresh feces and slips it into a plastic bag and freezes it for later analysis.
A lot of time is spent simply watching the movements of the birds and guessing what they're eating. Calculations are made based on crane movements, eating behavior and even their swallowing to guess the amount of calories they are burning.
The flock at Necedah is the eastern migratory flock created through a captive breeding and reintroduction program. These are the birds taught to migrate by an ultralight plane during their first fall and spring seasons. Establishing an eastern migratory flock is considered an important goal, both to preserve some degree of genetic diversity and to avoid the danger of the species becoming extinct through a single disaster. The other migratory flock, the only truly wild one, winters in Texas and breeds in Canada.
This flock and Florida's nonmigratory flock have been subject to a series of mishaps in recent years. Some setbacks are to be expected, of course, and in the meantime there are enough captive birds to continue producing chicks. But if the flock is to become wild it will need to start breeding successfully on its own. Hopefully the biologists on the scene will figure out what caused the cranes to abandon their nests and whether they can solve the problem.
Meanwhile, the western wild flock is not doing too well, either. They started off the year by losing 7 adults and 16 chicks, representing 8.5% of the flock. That makes it the worst winter for whooping crane mortality. In this case, the problem appears to be lack of food sources caused by low water levels on the rivers feeding the estuaries of Aransas NWR.
Lower flows have a direct effect upon the natural productivity of the bays and estuaries. Species such as blue crab spend much of their lives moving from one portion of the estuary to another, in large part following or seeking a preferred salinity level. Except during spawning, when the female migrates to saltier water, the ideal salinity for the blue crabs is less than 15 parts per thousand (ppt).Apparently the low levels this winter were linked to a drought and to human withdrawals of water from the Guadalupe and San Antonio rivers. It seems that in order to keep that crane flock stable, some minimum flow will need to be established and maintained. So far Texas has been unwilling to do this.
Salinity levels in the whooping crane wintering grounds were high this year. An April 7, 2009, report revealed measurements at Aransas National Wildlife Refuge of 29 ppt at the refuge boat ramp and 39 ppt in the adjacent marsh. The Gulf of Mexico typically is 32 ppt whereas the estuary is usually much lower.
It is also clear that the marshes typically used by the cranes were devoid of blue crabs. In good years, crabs make up 85 percent of the whooping crane diet. Yet in January through April of this year, field surveys showed that there were alarmingly few blue crabs in these marshes. There is a strong correlation between the increased salinity and the absence of blue crabs as well as between the absence of crabs and the cranes dying of malnutrition.