Bird populations are declining. They orient themselves with the sun, landmarks, river basins, coasts, or mountains; nocturnal birds recognize the direction in which they are flying thanks to the constellations. This year's World Migratory Bird Day is celebrated on Saturday, October 9.

There are approximately 10,100 species of birds in the world and, according to each region, 25 to 30 percent are migratory; that is, they move from one place to another during certain times of the year. However, their population numbers have been reduced in just a few years.

"Migration, as a process, is in danger in the sense that the loss or alteration of habitats and climate change have caused us to have increasingly smaller populations of migratory birds in places where they used to be very abundant," says Adolfo Gerardo Navarro Sigüenza, coordinator of the Postgraduate Program in Biological Sciences at UNAM.

He is also a member of the Department of Evolutionary Biology of the Faculty of Sciences and says that the transformation of ecosystems has caused the extinction of some species, such as the Eskimo curlew (Numenius borealis). Mexico is the center of distribution of numerous migratory bird species from North America -some spend more than half of the year in our territory and only go to reproduce in Canada or the north of the United States-, so it is important that habitats are preserved to sustain populations, warns the expert.

On the occasion of World Migratory Bird Day, to be celebrated this year on October 9, the scientist emphasizes that one of the important migratory events in our country is the so-called "river of raptors", which is registered when entire populations of these animals fly from the United States and Canada; it is a phenomenon of great importance, even for tourism, where hundreds of thousands of these specimens pass through Veracruz on their way to the south.

It is also a great attraction to receive enormous quantities of aquatic birds in winter; "all the lakes are filled with ducks, geese, white pelicans, seagulls, etcetera. On the coasts the change is also spectacular: there are lots of species that feed in the estuaries and coastal lagoons". Even in cities, such as the country's capital, both in wooded areas and in wetland sites, such as the Xochimilco ecological park, they can be observed. "I have seen flocks of white pelicans pass over my car on the Periferico," he says.

The number changes from region to region; for example, in the US and Canada there are approximately 650 species of birds, of which 350 are migratory; in Mexico, we have about 1,100 species of birds, 25 percent of which move to other latitudes and an important number of them are those that come from the north in winter. World Migratory Bird Day, which is celebrated worldwide on two days each year -the second Saturday in May and the second Saturday in October-, is the only international program for awareness and education on the subject.

Evolution

Navarro Sigüenza explains that migration is an evolutionary phenomenon that only some groups of animals develop. In addition to whales, bats, and monarch butterflies, there are birds, whose movements could have begun in times of strong climatic changes, such as glaciations, several hundred thousand years ago.

By moving to where there are resources, food, and places to reproduce, they ensure their survival. "They look for the optimal place to raise their broods, and this is repeated within a temporal cycle that generally lasts a year. In the different groups of birds, there are migratory species, explains the specialist; for example, ducks, plovers, and tildillos join the passerines (songbirds), etc. The variability is enormous. The variability is enormous.

There are those that travel during the day and others at night; in some, the males go first and then the females. Those that travel by day can use the sun, marks on the Earth, river basins, coasts, or mountains to orient themselves. Meanwhile, nocturnal birds know how to recognize the direction in which they are flying thanks to the constellations.

Recently it has been found that through a pigment in their retinas called cryptochrome they can observe the direction of the Earth's magnetic field; in addition, they have magnetite crystals in their beaks that allow them to perceive its intensity. They also see ultraviolet color. Although this phenomenon has been studied, little is known about the orientation mechanisms they use, "What we do know is that they don't get lost".

There are species that make their journey in a single flight, without stopping to eat, so they must accumulate too much fat before leaving. There are others, on the other hand, that make stops to feed and recharge their batteries, says Adolfo Navarro. Most of the migration takes place from north to south and Mexico is part of several main routes, such as the one that comes from the Rocky Mountains, in Canada and the United States, and arrives in our country where "we have wonderful migration shows".

Migratory birds, says the specialist, are part of the ecosystems and play a very important role in their functioning. There are places, especially in the north of our territory, in arid and dry areas, such as Sonora and Chihuahua, where the most important component of the avifauna are precisely migratory birds.

Further south, migratory birds coexist with resident birds and their proportion is smaller, but in the end, they play an important role in each ecosystem, says the scientist. This has been a good reason for nations to share study and preservation programs, such as the North American Bird Conservation Initiative (NABCI). Thus, there are monitoring programs common to all three countries, such as winter survival and censuses.

Threats on their journey

Species that move from one region to another face threats at the sites where they breed and overwinter, as well as in the places affected by human activities through which they pass. Ecosystems deteriorate and as forests are transformed into agricultural areas or cities, the resources available to wildlife are reduced. "Habitat destruction is definitely the main cause of many declines in the numbers of migratory populations around the world."

In addition, decades ago, the problem caused by urban areas was detected because, when migrating, especially nocturnal migrants, they encounter lighted buildings that attract their attention and crash. This is worrying: hundreds and thousands of birds die, especially in the USA and Canada. In other parts of the world, such as the Balkans, people catch them to eat them; "hunting can also be a problem".

Also, cats (feral cats, which are no longer domestic, and those that are domestic, but are allowed to roam freely) are a problem for birds in general; "they are tremendous predators". And to that is added climate change that alters the cycles of nature and the use of plastics, because a high percentage die from eating them, as a result of irresponsible human behavior.

"The peak of the migration in Mexico will be in November and in December we will have it in effervescence", abounds the university student who refers that the best way to preserve species is to know them, to go to the field, to the park and to see the diversity of feathered animals that arrive at this time. "With the beginning of autumn we are already beginning to receive the first migratory birds, such as Wilson's warbler; we will appreciate this diversity, and we will want to conserve it," concludes Navarro.