International Ornithologists' Union

Ornithology is a branch of zoology that concerns the "methodological study and consequent knowledge of birds with all that relates to them." Several aspects of ornithology differ from related disciplines, due partly to the high visibility and the aesthetic appeal of birds. It has also been an area with a large contribution made by amateurs in terms of time, resources, and financial support. Studies on birds have helped develop key concepts in biology including evolution, behaviour and ecology such as the definition of species, the process of speciation, instinct, learning, ecological niches, guilds, island biogeography, phylogeography, and conservation.

Austria, formally the Republic of Austria, is a landlocked country in the southern part of Central Europe, lying in the Eastern Alps. It is a federation of nine states, one of which is the capital, Vienna, the most populous city and state. Austria is bordered by Germany to the northwest, the Czech Republic to the north, Slovakia to the northeast, Hungary to the east, Slovenia and Italy to the south, and Switzerland and Liechtenstein to the west. The country occupies an area of 83,871 km2 (32,383 sq mi) and has a population of 9 million.

Budapest is the capital and most populous city of Hungary. It is the ninth-largest city in the European Union by population within city limits and the second-largest city on the Danube river; the city has an estimated population of 1,752,286 over a land area of about 525 square kilometres. Budapest, which is both a city and county, forms the centre of the Budapest metropolitan area, which has an area of 7,626 square kilometres and a population of 3,303,786; it is a primate city, constituting 33% of the population of Hungary.

Hungary is a landlocked country in Central Europe. Spanning 93,030 square kilometres (35,920 sq mi) of the Carpathian Basin, it is bordered by Slovakia to the north, Ukraine to the northeast, Romania to the east and southeast, Serbia to the south, Croatia and Slovenia to the southwest, and Austria to the west. Hungary has a population of 9.7 million, mostly ethnic Hungarians and a significant Romani minority. Hungarian, the official language, is the world's most widely spoken Uralic language and among the few non-Indo-European languages widely spoken in Europe. Budapest is the country's capital and largest city; other major urban areas include Debrecen, Szeged, Miskolc, Pécs, and Győr.

France, officially the French Republic, is a country located primarily in Western Europe. It also includes overseas regions and territories in the Americas and the Atlantic, Pacific and Indian Oceans, giving it one of the largest discontiguous exclusive economic zones in the world. Its metropolitan area extends from the Rhine to the Atlantic Ocean and from the Mediterranean Sea to the English Channel and the North Sea; overseas territories include French Guiana in South America, Saint Pierre and Miquelon in the North Atlantic, the French West Indies, and many islands in Oceania and the Indian Ocean. Its eighteen integral regions span a combined area of 643,801 km2 (248,573 sq mi) and had a total population of over 68 million as of January 2023. France is a unitary semi-presidential republic with its capital in Paris, the country's largest city and main cultural and commercial centre; other major urban areas include Marseille, Lyon, Toulouse, Lille, Bordeaux, and Nice.

London is the capital and largest city of England and the United Kingdom, with a population of just under 9 million. It stands on the River Thames in south-east England at the head of a 50-mile (80 km) estuary down to the North Sea, and has been a major settlement for two millennia. The City of London, its ancient core and financial centre, was founded by the Romans as Londinium and retains its medieval boundaries. The City of Westminster, to the west of the City of London, has for centuries hosted the national government and parliament. Since the 19th century, the name "London" has also referred to the metropolis around this core, historically split between the counties of Middlesex, Essex, Surrey, Kent, and Hertfordshire, which since 1965 has largely comprised Greater London, which is governed by 33 local authorities and the Greater London Authority.

Berlin is the capital and largest city of Germany by both area and population. Its 3.7 million inhabitants make it the European Union's most populous city, according to population within city limits. One of Germany's sixteen constituent states, Berlin is surrounded by the State of Brandenburg and contiguous with Potsdam, Brandenburg's capital. Berlin's urban area, which has a population of around 4.5 million, is the second most populous urban area in Germany after the Ruhr. The Berlin-Brandenburg capital region has around 6.2 million inhabitants and is Germany's third-largest metropolitan region after the Rhine-Ruhr and Rhine-Main regions.

Germany, officially the Federal Republic of Germany, is a country in Central Europe. It is the second-most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated between the Baltic and North seas to the north, and the Alps to the south; it covers an area of 357,022 square kilometres (137,847 sq mi), with a population of over 84 million within its 16 constituent states. Germany borders Denmark to the north, Poland and the Czech Republic to the east, Austria and Switzerland to the south, and France, Luxembourg, Belgium, and the Netherlands to the west. The nation's capital and most populous city is Berlin and its main financial centre is Frankfurt; the largest urban area is the Ruhr.

Copenhagen is the capital and most populous city of Denmark, with a population of around 1.4 million in the urban area, and more than 2 million in the wider Copenhagen metropolitan area. The city is on the islands of Zealand and Amager, separated from Malmö, Sweden, by the Øresund strait. The Øresund Bridge connects the two cities by rail and road.

Denmark is a Nordic constituent country in Northern Europe. It is the most populous and politically central constituent of the Kingdom of Denmark, a constitutionally unitary state that includes the autonomous territories of the Faroe Islands and Greenland in the North Atlantic Ocean. Metropolitan Denmark is the southernmost of the Scandinavian countries, lying south-west and south of Sweden, south of Norway, and north of Germany, with which it shares a short land border, its only land border.

Amsterdam is the capital and most populous city of the Netherlands, with The Hague being the seat of government. It has a population of 921,402 within the city proper, 1,457,018 in the urban area and 2,480,394 in the metropolitan area. Located in the Dutch province of North Holland, Amsterdam is colloquially referred to as the "Venice of the North", for its large number of canals, now designated a UNESCO World Heritage Site.

The Netherlands, informally Holland, is a country located in northwestern Europe with overseas territories in the Caribbean. It is the largest of four constituent countries of the Kingdom of the Netherlands. The Netherlands consists of twelve provinces; it borders Germany to the east, and Belgium to the south, with a North Sea coastline to the north and west. It shares maritime borders with the United Kingdom, Germany and Belgium in the North Sea. The country's official language is Dutch, with West Frisian as a secondary official language in the province of Friesland. Dutch, English and Papiamento are official in the Caribbean territories.