Flocks of wood storks, or Brazilian storks (Mycteria americana) show up in the Pantanal (swampy land in Center West region of Brazil) every year, from June onwards. For a few weeks, these waders that are up to 1 meter in height gather together in colonies to reproduce, forming from 50 to 5,000 nests. Flocks like these are scattered over other regions of the Americas, from the southeast of the United States to the north of Argentina, passing through the Brazilian Center-West.
It used to be supposed that the populations of wood storks in North America and South America, being far apart, were genetically distinct. However, a study by the Federal University of São Carlos (UFSCar) has revealed a high level of genetic similarity – around 99% – between the wood storks from Brazil and those of the United States. “From the point of view of genetics, we could even say that we are dealing with a single population”, comments Sílvia Nassif Del Lama, who coordinated the work.
How to explain the fact that the populations are genetically homogeneous, despite occupying geographical regions so far from each other? The hypothesis of the group from UFSCar is that, at intermediate points between Florida and the Pantanal, birds from North America interbreed with birds from South America. The result of this crossbreeding would be offspring with the genetic characteristics of the two regions – which biologists call genic flow.
From 1997 to 2000, the team from São Carlos made three expeditions to the Pantanal, where they handled about 500 wood stork nestlings. All of them had colored metal rings fixed to their legs. These rings follow international standards for identification, and their use makes it possible to discover the migratory routes. Should the ringed birds be found by researchers from other regions or countries, their route can be discovered, after the reproductive cycle. If they are identified amongst the flocks, they may help to clarify afterwards in low long, and in what proportion, males and females go back – if they do go back – to the place where they were born. Sílvia reveals that she never found in Brazil a wood stork that had been ringed – identified by a colored ring on the leg – in the United States.
However, birds ringed in the United States have already been sighted in flocks in Mexico, which reinforces the idea of gene flow. “Central America must be the meeting point between the populations of North and South America”, says the researcher. The group tried to do a comparative analysis with two studies made in the United States in the nineties, on the genetic structure of the populations of wood storks. The analyses, carried out in the same regions of the molecules of DNA analyzed by the Americans, led to the same results. Studies with the two kinds of genetic markers point in the same direction. That is to say, the populations are indeed very similar genetically.
In one of the eight flocks studied, Sílvia founded a higher proportion of females among the offspring. This deviation happened in an area affected by mercury waste, coming from the prospecting for gold in the Poconé region, in the north of the Pantanal. This was the place where female offspring predominate, in the proportion of two for each male. “With the birds, it is the females that determine the sex of the offspring, differently from what happens with human beings”, comments the researcher.
When more female are born, the structure of the population of a flock is altered. “The female wood storks”, says Sílvia, “seem to disperse themselves more than the males, returning to the flock in which they were born to a lesser extent. Three years later, females born in this flock will be forming others in other places. Whereas the males will go back to the place where they were born – it is the so-called philopatry”.
As Poconé does not offer good conditions for reproduction, the females favor the production of more females, the more they scatter and less they depend on this place for their reproduction. The researcher believes that the deviation observed in the sexual distribution in this group of nests may therefore be signaling that the colony is in the process of extinction.
The conclusions of the group from the Department of Genetics of UFSCar may help to change the strategies for preserving the populations of this bird. The research found that the Pantanal populations of wood storks are doing very well, with growing levels of reproduction. A very different picture from the populations in the United States – where the species has been on the list of those threatened with extinction since 1984. In the Everglades region, in the south of Florida, for example, the population fell from 20,000 couples to a little over 5,000.
To restore the affected populations and to avoid crossbreeding between related birds, some American researchers have proposed the introduction of individuals from abundant populations, like those in Brazil, in areas such as the Everglades. Studies by UFSCar, however, show that this strategy would be of no avail, since the Brazilian birds are very similar to those in the Everglades, and the objective, the introduction of “new blood”, would not be achieved with this procedure.
The occurrence of genic flow leads to new strategies for the conservation of the species. “Perhaps it would be the case of giving priority to the maintenance of the general conditions that guarantee genic flow, in stead of preserving one or other site where the birds gather to feed or to reproduce”, Sílvia ponders. “If the flow is real, the conservationist actions cannot be planned in isolation. It has, for example, to be understood that the impact in the Everglades did not affect just the North American populations, but all those present on the American continents. And if losses have happened in that place, they have now been limited by the flow of individual migrants, which, without being perceived, have restored the genetic variability of the impacted population. That means that to keep the populations genetically healthy, there has to be joint action in thethree Americas, with a view to monitoring and to protecting the areas that are essential for these birds to reproduce and to feed themselves.” The research project corrects mistakes, clarifies a little more about the behavior of this bird, and proposes a conservation plan, but, for Sílvia, there are still some intriguing questions to be answered. It remains to be known, for example, where they migrate to when they leave their reproductive cycle in the Pantanal: “Do they really go to Central America? Which are the regions where they blend most?”. They are questions that the team from UFSCar intends to clear up in a forthcoming project.
Fast and long-living
The wood stork has the fastest reflexes ever recorded among vertebrates: it only needs 25 thousands of a second to close its beak and swallow a fish. And, to favor its preservation, the glutton also has a long life: the Mycteria americana lives around 40 years and is sexually mature from its third year onwards. The couples are apparently monogamous during each breeding season, and the reproductive phase begins with the formation of couples in the flocks. The wood storks form their flocks amid other water birds like the Great Egret (Ardea alba) and the Roseate Spoonbill (Platalea ajaja).
These large nuclei, which may house from 200 to as many as 4,000 couples, attract predators like the Crested Caracara (Caracara plancus), the Black Vulture (Coragyps atratus) and the Green Anaconda (Eunectes murinus). The eggs are laid at intervals of from one to two days. The eggs are incubated for one month and are brooded 24 hours a day by the male or by the female, which take turns in this task and in fishing.
The noise of outboard motors, the presence of cattle and human beings circulating in the area of the flock cause disturbances that often lead the couples to abandon their nests. The wood storks prefer to eat fish, but they do not turn down insects, mollusks, crustaceans, and even small amphibians and reptiles. They fish intensively, day and night, in waters that are no deeper than 50 centimeters.
Structure of the Populations of Mycteria americana of the Flocks from the Pantanal of Mato Grosso (nº 98/06160-8); Modality Regular line of research benefits; Coordinator Sílvia Nassif Del Lama – Department of Genetics of the Federal University of São Carlos; Investment R$ 30,935.96 plus US$ 18,693.00