Environmental pollution and biodiversity

Ekaterina Batchvarova

Cover: Stork nests on the main highway of Sofia - The condition of storks is an indicator of the quality of the environment and of people's attitude toward nature. That is why the increase in the stork population is a sure indicator that the environment is less polluted here than elsewhere. Is this the case in Sofia? Hard to believe following the air pollution monitoring data, but who knows? May be we do not monitor what is important for the storks. Storks are not rare in Bulgaria, but nesting on a highway in a city with 1 million inhabitants is a curiosity. This made me search on the Internet for similar events. There I learned that storks came back in Britain after 600 years (I saw one there myself, but thought this is "normal" like it is in Bulgaria in the fields). Also, when looking at hundreds of them gathering for their overseatrip at the Black Sea side in the fields close to Burgas just before 20th of August, I had no idea that I will find out a week later that 200 storks died landing on a chemical dump in Israel. Here I present some information that is related to our studies - the effects of environmental pollution and other human activities.

WHITE STORK (Ciconia ciconia)

Description: White Storks are tall (1 m., 2.3-4.4 kg) long-necked wading birds with long bare red legs and a straight pointed red bill. The white plumage of the head, neck, and body contrasts with the black wing feathers highlighted with a sheen of purple and green iridescence. The contour feathers of the lower neck and chest are elongated to form a fluffy ruff that can be erected during courtship displays. A small patch of bare black skin surrounds their brown eyes. Sexes are similar in appearance, though males are slightly larger.

Though storks are considered to be largely silent birds, most species perform some variety of a bill-clattering display. This display reaches its most advanced form in the White Stork. They begin by throwing their heads straight back to create an amplifying resonance box in the gular pouch of the lower neck. As they clatter their upper and lower mandibles together rapidly they produce a loud machine-gun-like rattle that rises and falls in pace.

Distribution and Habitat: The European subspecies of the White Stork breeds in several discontinuous populations across much of Europe, the Middle East and west-central Asia. They are found in southern Portugal and western Spain; along the northern coast of Africa in Tunisia, Morocco, and Algeria; in The Netherlands and southern Denmark west to St. Petersburg and the Gulf of Finland, south to Turkey, northern Greece, the former Yugoslavia, and Azerbaijan. A very small population breeds in extreme southern Africa in Cape Province, South Africa.

Storks are large birds that rely heavily on energy efficient soaring flight during migration. Soaring requires the presence of thermal air currents that are not found over water. White Storks are therefore reluctant to fly across large bodies of water such as the Mediterranean Sea to reach their wintering grounds in tropical Africa. They solve this problem by having the bulk of the European population split into two distinct migratory routes. Western birds cross the Mediterranean at the Straits of Gibraltar, while most of the eastern birds cross the Bosporus and circle around the Mediterranean through the Middle East. Migration is highly synchronized and flocks contain as many as 11,000 individuals. Birds migrating from Denmark to South Africa and back again may cover a total distance of 20,000 km. Small numbers of birds cross the Mediterranean directly by flying south from the southern tips of Italy and Greece. Some western European White Storks join the Asiatic subspecies C. ciconia asiatica to winter in India.

Wintering birds may congregate in large numbers as they utilize a locally abundant food source such as locust or grasshopper swarms. One hundred thousand white storks were reported in one 25 km² area in Tanzania during a severe infestation of army-worms. These wintering birds occupy dry savannahs and open grasslands but tend to congregate around lakes, ponds, and rivers.

Breeding: White Storks prefer lowland open habitats of wet pastures, flooded meadows, and shallow lakes and marshes with scattered trees for roosting and nesting. They have adapted to nest on man-made structures and forage in freshly plowed fields.

Diet: White Storks are highly opportunistic feeders who will consume a wide variety of prey items including insects, frogs, toads, tadpoles, fish, rodents, snakes, lizards, earthworms, mollusks, crustaceans, and, rarely, the chicks or eggs of ground-nesting birds. Foraging storks search for prey visually while walking deliberately with bill pointed toward the ground. When prey is spotted, they cock their necks back, then jab the bill forward to grasp their victim. Wintering birds in Africa will congregate around the edges of grass-fires to capture small prey fleeing the flames.

Reproduction: White Storks form loose informal colonies while breeding. Several pairs may nest closely together within sight and sound of one another while appearing completely oblivious to their neighbors. Nine pairs have shared one rooftop in Spain. Though storks form monogamous pairs for the duration of the breeding season, they do not migrate or over-winter together. If the same pair reforms in successive years it is largely due to their strong attachment to their nest site.

Males usually arrive at the nest-site first. A male will greet a newly arriving female with the Head-Shaking Crouch display, as he lowers himself on the nest into the incubating posture, erects his neck ruff and shakes his head from side to side. If the male accepts the new arrival as his mate they will cement their pair bond with an Up-Down display. In this display the birds hold their wings away from their sides and pump their heads up and down. This is often accompanied by bill-clattering. Shorter courtships may indicate that the male and female were paired in previous years.

Nests are huge, bulky affairs constructed of branches and sticks and lined with twigs, grasses, sod, rags, and paper. Though they may be reused year after year, breeding birds will add to the structure each season. Particularly old nests have grown to over 2 m in diameter and nearly 3 m in depth. Some nests have been in continuous use for hundreds of years. Both sexes participate in nest construction with the male bringing most of the material. Completion of the structure is often signaled by the addition of one leafy branch to the edge of the nest.

European Storks have been building their nests on man-made structures since the Middle Ages. They can be found on rooftops, towers, chimneys, telephone-poles, walls, haystacks, and specially constructed nest towers. Many homeowners will add embellishments such as wooden wagon wheels to old chimneys to encourage storks to nest on their houses. Nests can also be found in trees, on cliff-ledges, or occasionally on the ground. (The street lamps of Sofia seem to be attractive as well, see cover.)

The female usually lays 3-5 eggs, more rarely up to seven. Parents share incubation duties for 33-34 days. Young chicks are covered with white down and have black bills. Both parents feed the young on the nest until they fledge at 8-9 weeks of age. Fledglings may continue to return to the nest site each evening to beg for food from their parents. Young birds reach sexual maturity in their fourth year. Banding records indicate that wild birds can live and reproduce successfully past 30 years of age.

Conservation: The overall population of White Storks has declined steadily over the last half century. The decline in Western Europe has been the most pronounced. Pollution, pesticides and wetlands drainage have severely reduced suitable foraging habitat across the breeding range. Storks no longer breed in southern Sweden, Switzerland, western France, Belgium or southern Greece. In The Netherlands the number of breeding pairs has declined from 500 in 1910 to 5 in 1985. Denmark was home to 4000 pairs in 1890, but only 12 in 1989. Captive propagation and reintroduction efforts have been hampered by their tendency to produce overly tame birds, which over-winter in Europe without migrating normally.