– At this time of the year, when summer is coming to an end in the Northern hemisphere, pairs of fish eagles (Pandion haliaetus) start to abandon their nests in the northeast USA to set on a long journey towards warmer territories in the Caribbean and South America. They are large birds reaching a wingspan of 2 meters and their life expectancy is of 27 years. They are given many names, from osprey, fish eagle and sea hawk in English to águia pescadora, gavião-caripira, guincho, and gavião-papa-peixe in the Portuguese language spoken in Brazil...Two to three weeks after their first flight, they are already capable of seeking out fish in distant places and returning to their nests.
In warmer areas they usually remain until the month of March when summer ends here in Brazil. They travel the heights between 200 and a thousand meters from the ground and at a speed between 30 and 40 Km/h. Some of them will be tracked by satellite by researchers from the University of North Carolina in Charlotte. The chosen ones carry a solar-powered transmitter that sends in information to the scientists on a daily basis.
One of the persons in charge of this project is biologist Rob Bierregaard, who came to Brazil to take part in an ornithology meeting and, just before that, gave a talk on the monitoring of these osprey eagles to researchers and post-graduate students in Manaus. The biologist lived in Brazil in the late 70s when he served as the first director of the Biological Dynamics of Forest Fragments Project and, at the time, he gave a good-humored talk in Portuguese, on the monitoring of eagles.
Studies have shown that the females of the species are the first to leave and the last to return. They are also the ones that fly to the farthest places. They reach further south than their male counterparts while these remain longer close to their nests to ensure their territory and ward off invaders. “Males usually return earlier to reclaim territory”, explains Rob Bierregaard. Next year, eagle couples will meet in the same place they were this year. They usually mate with the same partner year after year and in case of the death of one of them, the survivor will seek another mate and continue its life-cycle.
Thanks to the work of North Carolina researchers, migration routes of these birds along the east coast of the US, the Caribbean and the Amazon Rainforest are now well known, a route still unknown by young hawks. They already undertake regular flights over considerable distances. One monitored individual left an island on the Atlantic Coast and spent a season close to the Great Lakes. But the trip they now undertake towards the south is both unheard of and dangerous. Of every ten young hawks that migrate for the first time, only two return to their place of birth. According to information supplied by Bierregaard, 66% of these hawks die in their first year of life. In the second year, mortality falls to 22%.
The young ones will spend two winters in the south and only after 18 months will return, to occupy yet another new region in North America. If any osprey is seen in Brazil after the month of April, it must be a young one who will be spending the year here. After this one more season, it will probably return to its place of birth, a return which is of great importance for the young ones since this will be an opportunity to learn the safest route to be used for the nest migration. However, these hawks don’t traverse the exact same route every year. Actually, they follow the same general direction but from year to year, this heavenly path can deviate by hundreds of kilometers, although the final destination remains the same. “They head south, preferably over land. If that’s not possible, they go anyway”, says good-humored Bierregaard.
When they came, these inexperienced birds flew instinctively towards the south, not knowing anything and under the influence of wind currents, came up on vast ocean expanses spanning hundreds of kilometers ahead. Travelling at 30 or 40 Km/h, a common cruising speed for these species, they found few places to stop and rest. A path quite to the east of the one used by the more experienced birds that follow the shoreline and take advantage of the islands of Cuba and Hispaniola (Haiti and the Dominican Republic) to bridge the gap between continents.
These are solitary animals. They do not fly in flocks or even in pairs, but are quite wide awake when it comes to eating. “If they encounter another bird carrying fish that belongs to a school of fish, then they fly in that direction, if not, they ignore the fact and continue their previous path”, tells us Bierregaard. But just how do these ospreys differentiate between fish? This bit of information is yet unknown. But any fish of 20cm or larger that swims up to a meter or so below water can be taken completely by surprise by a swift and deadly dive of hawk claws.
On their way, ospreys feed when they can. There are reports of a fish eagle migrating over open sea and carrying fish. “As if carrying a sandwich to enjoy later”, jokes Bierregaard. Aside from the long journey, there are other dangers. The American biologist tells us that four ospreys where shot down in the island of Hispaniola. “People don’t know what they are and surmise they are a threat to their poultry and decide to take them down”, he laments.
Not all ospreys follow this path. This bird of prey, found on almost all continents except in the Antarctica, lives where it can find fish to feed on. The ones in Europe seek warmer areas in Africa while those of the American west, seek Central America. Those of the south of the US travel little and the ones in Australia don’t migrate at all.
After undergoing a steep decline in its population in the 50s and 60s due to use of DDT, the population of fish eagles is now making a comeback in the US. Each female can lay up to two eggs per reproduction period. But its average reproduction rate is of 1.2 to 1.3 offspring per couple which ensures its population growth.
There’s is much yet to known about these birds, for instance: why, in the Americas, do they only make their nests in the Northern Hemisphere? It is believed that this is because of the abundance of fish in the warmest period of the year. But if satellites have done much to tell us a great deal about migration, they have taught us little in the way of discovering more about this bird’s ecology.
To learn more about the fish eagle, simply follow these links:
http://www.osprey.com
http://www.peregrinefund.org/explore_raptors/osprey/osprey.html
