Following an almost two-decade (1951–1970) research of the

Danube Delta The Danube Delta ( ro, Delta Dunării, ; uk, Дельта Дунаю, Deľta Dunaju, ) is the second largest river delta in Europe, after the Volga Delta, and is the best preserved on the continent. The greater part of the Danube Delta lies in Ro ...

territory, 274 bird species were identified, of which 176 were

brood Brood may refer to: Nature * Brood, a collective term for offspring * Brooding, the incubation of bird eggs by their parents * Bee brood, the young of a beehive * Individual broods of North American Periodical Cicadas: ** Brood X, the largest bro ...

ing (

sedentary Sedentary lifestyle is a lifestyle type, in which one is physically inactive and does little or no physical movement and or exercise. A person living a sedentary lifestyle is often sitting or lying down while engaged in an activity like soci ...

and migratory) and 98 non-brooding ones (winter, passage, erratic, and

accident An accident is an unintended, normally unwanted event that was not directly caused by humans. The term ''accident'' implies that nobody should be blamed, but the event may have been caused by unrecognized or unaddressed risks. Most researcher ...

bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...

s).

Brooding birds

The species of brooding birds occurring in the Danube Delta were classified into 12 main

biotope A biotope is an area of uniform environmental conditions providing a living place for a specific assemblage of plants and animals. ''Biotope'' is almost synonymous with the term "habitat", which is more commonly used in English-speaking countrie ...

s, the classification criterion being represented by the nesting place where they show the greatest relative density. As to their dynamics, the brooding birds include: 44

species and 132 migratory ones. The great number of migratory species is explained by the optimal conditions of nesting and especially of feeding the

delta Delta commonly refers to: * Delta (letter) (Δ or δ), a letter of the Greek alphabet * River delta, at a river mouth * D (NATO phonetic alphabet: "Delta") * Delta Air Lines, US * Delta variant of SARS-CoV-2 that causes COVID-19 Delta may also re ...

ic biotopes are providing in the warm season of the year. This phenomenon is also supported by the presence – in different deltaic biotopes as against the common ones – of some species, which in the rest of their area cover other life environments.

Non-breeding birds

The species of non-brooding deltaic birds were classified into the

s they visit while resting on the Danube Delta's territory and they are connected to by the way these biotopes are providing their food. The ornithological biotopes of the Danube Delta belong to three well-individualized habitats, namely the aquatic, the

amphibian Amphibians are tetrapod, four-limbed and ectothermic vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terres ...

and the

terrestrial Terrestrial refers to things related to land or the planet Earth. Terrestrial may also refer to: * Terrestrial animal, an animal that lives on land opposed to living in water, or sometimes an animal that lives on or near the ground, as opposed to ...

ones. The non-brooding species include: 35 winter

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

visit the Danube Delta only in winter; 46 passage species which cross the delta's territory only in spring and in autumn; 6 erratic ones which irregularly show up in different biotopes and 11 accidental species. The number of biotopes increases from the aquatic to the terrestrial environment, while their dimensions follow an inverse proportionality, as they are smaller in the terrestrial and larger in the amphibian and aquatic environments.

Geographical origin

According to their geographical origin, the

birds contain the following types:

arctic The Arctic ( or ) is a polar regions of Earth, polar region located at the northernmost part of Earth. The Arctic consists of the Arctic Ocean, adjacent seas, and parts of Canada (Yukon, Northwest Territories, Nunavut), Danish Realm (Greenla ...

species (27);

Siberia Siberia ( ; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive geographical region, constituting all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has been a part of ...

n (32);

Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...

an (108);

Mediterranean The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the e ...

(29);

Mongolia Mongolia; Mongolian script: , , ; lit. "Mongol Nation" or "State of Mongolia" () is a landlocked country in East Asia, bordered by Russia to the north and China to the south. It covers an area of , with a population of just 3.3 million, ...

n (33);

Chinese Chinese can refer to: * Something related to China * Chinese people, people of Chinese nationality, citizenship, and/or ethnicity **''Zhonghua minzu'', the supra-ethnic concept of the Chinese nation ** List of ethnic groups in China, people of va ...

(2). The transpalearctic type contains 43 species. For

Romania Romania ( ; ro, România ) is a country located at the crossroads of Central Europe, Central, Eastern Europe, Eastern, and Southeast Europe, Southeastern Europe. It borders Bulgaria to the south, Ukraine to the north, Hungary to the west, S ...

's latitude – delta included – the north-originating species (Arctic and Siberian) appear as winter-and passage birds, while the south-originating species (European, Mediterranean and Mongolian) are met with as sedentary and summer species. Further comments upon the manner birds migrate - namely the causes, which start on migration - and upon the way they orient themselves during migration, resulted in the

hypothesis A hypothesis (plural hypotheses) is a proposed explanation for a phenomenon. For a hypothesis to be a scientific hypothesis, the scientific method requires that one can test it. Scientists generally base scientific hypotheses on previous obse ...

of the "''inner time''" and "''inner space''", which, along with their known senses, allow the migratory birds to perfect the migratory act in time and space. An interesting ethologic aspect appearing enhanced by the conditions of the Danube Delta (especially by the floods) is represented by egg laying in common nests. Thus, as a consequence of the general lack of egg-laying places, especially at flood time when a great part of the aquatic-species nests are flooded, the birds this way affected go on laying eggs in alien nests either belonging to the same species or to species of the same families. The following up of the tropic regime of the deltaic birds according to its origin had in view to know the food cycles, especially as final production represented by birds, the classification of these latter in categories with aquatic, mixed and respectively terrestrial nutritive regimes. In this respect, the delta birds appear as having a trophic regime of predominantly aquatic origin owing to the huge quantity the nutritive biomass of this milieu is produced in the Danube Delta in the warm season of the year. While the sedentary species generally close their trophic chains at the delta's level, the migratory, the passage, and the wintertime species annually carry away from the Danube Delta a great part of the nutritive biomass accumulated while resting in this territory. As far as the habitats are concerned, the species of the aquatic ones almost exclusivity feed from the aquatic millennium, while those living in the terrestrial habitat hardly have 50 per cent terrestrial food, the remainder having a mixed and aquatic origin. Many species owe their presence or existence in the delta – in rather high densities as compared to the rest of the area – just to this food richness. The contrast created between the great food abundance available as compared to the existence for some species of plenty of sites for nest location resulted in the simple and mixed form of colonial nesting. Such an observation and interpretation of the colonial nesting origin in the

may constitute an argument for properly explaining the colonial nesting phenomenon in different regions or the globe.

External links

Danube Delta: Photo Gallery

Danube Delta Biosphere Reserve

Nos Oiseaux Bird ringing migration
{{Protected areas of Romania Danube Delta