Alien Birds in Sub-Saharan Africa: An Overview

1. Introduction

Dispersion, migration and colonization are ones of the fundamental features of living organisms. Therefore, changes in distribution and geographical ranges are natural, common and widespread, always and everywhere. Their intensity is related to environmental changes and to dispersal and reproductive abilities of living organisms. However, man can accelerated the changes on unprecedent scale, through high population growth and negative impact on natural environment.

According to the ‘tens rule’ (Williamson & Fitter, 1996) only about 10% of introduced species develop viable population. Out of these c. 90% achieve a status of naturalized, established populations, and only c. 10% of them assume the status of invasive species. Those species pose a serious ecological (e.g., disturbance of natural balance, dislocation of indigenous species), economic (e.g., agricultural pests) and social (e.g., human diseases) problems. In most countries in the world regions of the world, the negative impact of invasive species constituts the most important ecological problem, beside the habitat destruction.

Most avian introductions derive from the last three centuries, and most are associated with the British colonization. Today the major regions of avian introductions in the world include their major overseas settlements, i.e., Australia and New Zealand, Oceania (especially Hawaii), South Africa and USA. They deliberately tried to introduce exotic species mainly for hunting purpose, hence exotic birds are overrepresented today by game birds (Blackburn et al., 2009). The other motives for the introductions include pest control, aesthetic (bright colouration, pleasure songs), nostalgia and even conservation (e.g., Mandarin Duck Aix galericulata and Golden Pheasant Chrysolophus pictus) and religion (Buddhism and Taoism). Most of the recent introductions are however accidental and unintentional (Long, 1981; Downs & Hart, 2020).

Avian invasion has been well-studied, but subsequently summarized for only some parts of the world, such as USA (Johnston & Garrett, 1994; Cox, 1999; Avery & Tillman, 2005; Witmer et al., 2007; Garrett et al., 2018), Great Britain and Europe (Chiron et al., 2009; Kark et al., 2009; Hilton & Cuthbert, 2010), and Australia with New Zealand (Krull et al., 2014; Moulton & Cropper 2014; Vall-llosera & Cassey, 2017). For most tropical parts of the world, no comprehensive reviews are available on a continental (zoogeographical regions) scale. The presented paper attempts to summarize our knowledge on avian introduction in ones of such regions.

2. Materials and Methods

Most avian introductions in Africa began in the midlle of the 19th century with the European colonization (Blackburn et al., 2009). Appropiate literature since 1900 onwards (with a special emphasis on the last 50 years) was reviewed for information on avian introduction in Africa south of Sahara, including Madagascar and the following small islands and archipelagos: Cape Verde; São Tome e Principe, Bioko and Annobón of the Equatorial Guinea; Mascarenes, Seychelles, Comoros; Mafia, Zanzibar, Pemba off Tanzania; and Socotra.

Since definitions related to introductions differ between studies and authors, the following terms used in this article, require clarification. The definitions are based mainly on IUCN/WCU; IUCN 2015; Bern Convention; Genovesi & Shine (2004) and Hawkins et al. (2015). The following types of introduced species can be distinguished:

• Indigenous species (native, autochthonous): species living within its natural range.
• Alien species (introduced, non-native, non-indigenous, foreign, exotic): species introduced by man intentionally or accidentally.
• Invasive alien species (invader): introduced species which has been introduced, to areas not previously occupied, establishing viable breeding population, spreads and becomes a pest affecting ecosysstem, local biodiversity, economy and society (including human health).
• Non-invasive alien species: introduced species with developed viable population of low dispersal abilities and not affecting adversely ecosystems, economy and society in the conquered areas.
• Translocated species: accidental escapee from aviary or enclosure; may reproduce in wild, but has not develop viable population.
• Post-invasive alien species (established, naturalized): a species introduced long time ago (before 1900), well-established in the wild, but without expansion tendencies.
• Cryptogenic species: unknown origin (alien or indigenous), its expansion may be shaped by natural or anthropogenic factors.

The following types of introduction are recognized:

• Intentional introduction: purposeful relocation of a species beyond its natural range.
• Unintentional introduction: accidental relocation of a species beyond its natural range.
• Expansion: continuous natural enlargement of natural range by acquisition of the adjacent areas or natural occupation of new habitats within the original range
• Invasion (colonization): natural spread into new areas accompanied by a rapid and often explosive exponential population growth and changes in natural environment and human economy.

To characterize population of the introduced species the following terms are used:

• Population stages: not established, developing, viable, established (naturalized.
• Population growth (dynamic): stable, increasing, declining, locally extinct.
• The types of invaded ecosystems can be grouped as follow:
• Natural ecosystem: natural formations not altered and not disturbed by man, usually in the climax stage.
• Semi-natural: natural formation modified/altered by human; in successive or/and climax stages.
• Artificial ecosystem: artificial formations created by man, not in a climax stage.

3. The Introduced Species

3.1. Invasive Alien Bird Species

Out of 150 alien species introduced in sub-Saharan Africa in the last 200 years, 49 (32.7%) have developed viable populations (this study), but only 7 (4.6%) became invasive (Liversidge 1985; Brooke et al. 1986; Dean 2000; Downs & Hart 2020; Shivambu et al. 2020), i.e., House Sparrow Passer domesticus, Common Starling Sturnus vulgaris, Common Myna Acridotheres tristis, Red-whiskered Bulbul Pycnonotus jocosus, House Crow Corvus splendens, Rock Pigeon Columba livia var. domestica, and Rose-ringed Parakeet Psittacula krameri (Table 1, 2; Figure 1A-F).

3.1.1. House Sparrow

Original range include the Middle East. In Africa, Passer domesticus indicus were introduced intentionally from India to Durban in the 1890’s, and Passer domesticus domesticus from Great Britain to East London, at the beginning of 20th century (Summers-Smith, 1963; Lever, 1987). It established stable population in Gauteng only by the year 1950 (Dean, 2005c). By the 1980s it invaded most of southern Africa, to southern DRC in the west and to southern Tanzania in the east (Vierke, 1970; Summers-Smith, 1988; Dean, 2005). It is, however, still absent in the southern interior, for example in 2013-2016, it was still absent in E Namibia and N Botswana (Table 3).

It has also invaded northern and eastern Madagascar and the surrounding islands. In Madagascar, it was first recorded in mid-1980s in Toamasina (Langrand & Sinclair, 1994; Goodman et al., 2017).

In the 1890’s, the House sparrow was introduced in Zanzibar, from where it spread to central Tanzania (Dean, 2005c). In the 1950s, it was introduced from South Africa (probably by ship) to Mombasa (Anderson, 2006), from where it spread (facilitated by trucks or trains) across Kenya along the Mombasa highway to Uganda (Schrey et al., 2014). In the 1970s, it was introduced from South Africa to Senegal, probably incidentally (Lever, 1987; Summers-Smith, 1988; 1992; Anderson, 2006) and spread in Senegal and coastal Mauretania (Dean 2005c).

There is a widespread and sharp decline of the House Sparrow in most European countries (Summers-Smith 2003), but in Africa the situation is revered. In most towns and cities in southern Africa, it belongs to the most common bird species today, even in those colonized quite recently in northern Namibia, i.e., in Opowo, Outapi, and Onguadiva, where it breeds in a density of 19-36 pairs per 10 ha (Table 3). It has been found especially abundant (comprising approximately a half of all breeding bird community) in impoverished rural parts (so called townships, locations) of towns and cities, e.g., Bloemfontein (Kopij, 1994, 2001a, 2015), Maseru (2000), coastal towns in Namibia (Kopij, 2018, 2022a, 2023b) and towns in most northern parts of the country (Kopij, 2018, 2019, 2021). While the House Sparrow is most abundant in the city centres (e.g., in Bloemfontein 25 pairs per 10 ha; Kopij, 1994), it tends to be much less common in residential areas (e.g., in Bloemfontein 2-3 pairs/10 ha; Kopij 2015), especially well-timbered ones, where the indigenous sparrow species (i.e., Cape Sparrow Passer melanurus) are often more common predominates (Table 3).

Today, the House Sparrow occurs in all other continents, except for Antarctica. Also introduced to islands, e.g., to Lesser and Greater Antilles, Macaronesia, Madagascar, New Guinea and New Zealand.

3.1.2. Common Starling

Native to the Palearctic Region. In the late 1890’s, Cecil Rhodes introduced 18 Common Starlings in Cape Town (Dean 2000). Soon they colonizing the Cape Peninsula, and spread northwards, crossing the Berg River in the year 1928, from where they expanding eastwards, reaching East London by the year 1966, Kei river by the year 1971 and Umtata in 1981 (Gebhardt, 1954, 1959; Winterbottom & Liversidge, 1954; Liversidge, 1962; Skead, 1995; Quickelberge, 1989). Today they are widestread and common all over Western Cape and East Cape provinces (Hockey et al., 1989).

The Common Starling was recorded for the first time in Durban 1973 (Cyrus & Robson, 1980). By 1993, it was established along southern coast of the KwaZulu-Natal province (Harrison et al., 1997). At the beginning of the 21st century it was estabished in Gauteng province. Occupation of the dry interior was limited, but by the year 1970 it has reached the Orange River (Brown, 1985). In the Free State Province, recorded for the first time on the southern border in 1986 (Earlé & Grobler, 1987). The first breeding was recorded in 1997/98 (Kopij, 2001b).

In Lesotho, the Common Starling was recorded for the first time in Maseru in 1988. In 1996-1998, 1-2 probably breeding pairs were recorded there (Kopij 2000). In Roma, first breeding was recorded in 1991; ten years later there were at least 15-20 breeding pairs in this area, including 10 pairs per 82 ha on the NUL campus (Kopij, 2001a). While in 1987-1997 only 1-2 pairs in two sites (Maseru, Roma) were known in Lesotho, 21-27 pairs in 4 sites (Maseru, Roma, St. Joseph and Masite) were recorded in 1998-2002 (Kopij 2014).

In Namibia, the common Starling was first recorded in Oranjemund in 1970 (Brown, 1985). At the beginning of 21st century, the Common Staring was recorded in southern Namibia (70 km south of Grünau, in Hohenfels, and in Lüderitz on the coast), indicating furhter northward expansion (Cunningham, 2016).

As the result of introduction, the Common Starling occurs today in all continents of the world, except Antarctica. It is today especially common in USA, although in 1966-2004, slight declined in numbers was recorded there (Avery & Tillman, 2005). Common and widespread also in SE Australia (Phair, 2015).

3.1.3. Indian Myna

The Common Myna original range includes SE Asia.

In sub-Saharan Africa, Common Myna Acridotheres tristis tristoides from Myanmar and Nepal has been introduced to Durban in c. 1900. From there it spread all over KwaZul-Natal and NE Free State, where for example, 3.5 pairs per 10 ha (with a dominance: 16.9%) were recorded in 1996 in Bethlehem town centre (55 ha) in this province, being also a dominant species in the residential and industry areas in the town (Kopij, 1997). Spreading northwards along the coast, it reached Ponta Malongane in Mozambique in 1997 (Craig, 2005). Whereas spreading southwards along the coast it has reached Porth Elizabeth and Cape Town at the beginning of the 21st century (Craig, 2005; Peacock, 2007).

The Common Myna Acridotheres tristis tristis from India and Sri Lanka has been introduced to Johannesburg in c. 1938, and Pretoria in 1955. It the later city it has developed viable population only in the 1980’s. Although it was recorded as breeding in Kimberley, North-West Province in the 1960’s, it failed to develop viable population in the town (Craig, 2005).

In Lesotho, the Common Myna was first recorded in the mid-1990’s in Maseru; in 1996-1998, 2-5 breeding pairs were recorded in the city (Kopij, 2000a), and 5-10 pairs in 1998-2002 (Kopij, 2014). The first record at Roma was made in 1999 (Kopij, 2001b). In 1998-2002, it was also recorded in Morija and Leribe (2 pairs in each town) (Kopij, 2014).

In Botswana, the Common Myna was recorded for the first time in Nata near Jwaneng in 2012 (Engell & Willard, 2017), and in Kasane on the Chobe River in 2014 (Tyler, 2015). In 2014, it was recorded also for the first time in Victoria Falls on Zimbabwe/Zambian border (Douthwaite, 2015), about 100 km E of Kasane.

It invades Namibia from the south and from the extreme north-east. In the south, it was recorded for the first time in Oranjamund in 2014 and in Rosh Pinah in 2017 (Thompson, 2017). In the north-east, it was first recorded in Katima Mulilo in 2016 (with first breeding in the town in 2017) and in Ngoma in 2016 (Engel & Willard, 2017), about 60 km W of Kasane. Therefore, the Common Myna has already crossed the Zambezi River and most probably now it spreads further north.

The Common Myna has been also introduced to Madagascar, Comoros, Mauritius, Mayotte, Seychelles, Reunion, Zambia and Zimbabwe. Beyond Africa, the Common Mynas has been introduced in North America, Middle East, Australia and New Zealand and numerous islands around the world.

3.1.4. House Crow

Naturally, the House Crow’s occurs in Iran, Afghanistan, Pakistan, India, Bangladesh, Burma, Nepal, Sri Lanka, Thailand and extreme south-western China (Meininger et al., 1980; Ali, 2002; Nyári et al., 2006; Ryall, 2010).

For the first time, it was introduced in sub-Saharan Africa, in Zanzibar in the 1890s. From there it spread along the coast (by ship) to Mombasa (1947), and from there along the railway to Kenyan interior (Ryall and Reid, 1987; Ryall, 1992). House Crows have become well-established in Mauritius.

In South Africa, the House Crow appeared on the Indian Ocean coast in the early 1970’s. It was first recorded in Durban, where it has developed a sizable population. In the early 1990’s it has also established a small population (several hundred individuals in the early 2000’s) in Cape Town (Dean, 2000, 2005b).

A control programme in Durban has significantly reduced numbers and range of the species. In 2019-2023 it occurred mainly in Durban coastal area from the Blue Lagoon in Durban North to Isipingo to the south (Mashao et al., 2023). Subsequently, the main centre of population became Richard’s Bay, 200 km north of Durban. In Cape Town, where population were markedly reduced, it now occurs mainly in Khayelitsha area (sabap2.adu.org.za). Isolated individuals was recorded in East London on 15 June 2010 (Demey, 2010; Ryall, 2016).

The House Crowvwas introduced unintentionally to various countries, often on ships as a ‘hitchhiker’ to Tanzania (Shimba and Jonah, 2016), and Socotra Island (Yemen). It was intentionally introduced to some countries to clean garbage, and to control caterpillars, and ticks on livestock (Brook et al., 2003; Fraser et al., 2015). In Zanzibar, they were reported to be introduced in the 1890s from where they spread to Kenya in 1947, especially along the coastal areas between Mombasa and Malindi, as well as hitchhiking on ships travelling from Asia to East Africa (Ryall & Reid, 1987; Ryall, 1992).

The House Sparrow is introduced also to the following countries: Benin (Cotonou, 2010), Djibouti, Egypt, Eritrea, Madagascar (Toamasina, 2014), Mauritius, Morocco, Mozambique (Maputo 1976, Pemba, Nacala, Quelimane, Beira, and Matola), Namibia (Walvis Bay, 2011), Seychelles, Socotra Isl. (Hadibu, 1994: 1 ind., 1996: 3 ind., 2003: 26 ind.), Somalia, South Africa, Sudan and Tanzania. A survey by Shimba (2011) in Dar es Salaam city, Tanzania, estimated that there were 800,000 House Crows in empoverished parts of the city.

As a result of introduction, the House Crow occurs today: southern Asia from Turkey to Vietnam, Japan and Australia; southern parts of U.S.A.; in Central America; South America (Chile); in many countries in Europe (Ryall, 2002; Brook et al., 2003; Ottens & Ryall, 2003; Ottens, 2003; Suliman et al., 2011; Kamel, 2014; Shimba & Jonah, 2016; Nxele & Shivambu, 2018)

3.1.5. Rock Pigeon

The Rock Pigeon occurs natually in the Mediterranean Europe, northern Africa, Middle East, and it extends to south-east Asia (Johnston, 1992; Stern and Dickinson, 2010). As a result of introduction, it occurs today in all continents, except for Antarcica (Stern and Dickinson, 2010). Outside its native range, it occurs in at least in 64 countries (Johnston 1992, Sacchi et al., 2002; Rose et al., 2006; Przybylska et al., 2012; BirdLife International, 2016) and it is still expanding.

It was introduced to South Africa during the Angolo-Boer War (1899-1902), to enable effcient communication (Lincoln, 1927), and then became invasive. At present, it occurs all over southern Africa, reaching northern Angola in the west and northern Kenya in the east. It is, however, still absent in DRC and in the East African Rift Valley (Dean 2005a, Johnston 1992). In Namibia, it reached high densities in towns such as Walvis Bay, Swakopmund or Katima Mulilo, but is virtually absent in Hentjs Bay, Ondguadiva or Tsumeb (Table 3). No geographical of ecological trend is apparent in this country.

In southern African cities/towns, population densities of the Rock Pigeon varied from 0 to 15 pairs/10 ha (Table 3). It may greatly vary even within the same city. For instance in the city centre of Bloemfontein, the population density was 15.0 pars/10 ha, but in the neighbouring residential suburbs, it was only 2.1 pairs per 10 ha. Similarly in Bethlehem town centre it was 7.3 pairs/10 ha but none was recorded in industry area (Table 3). In Lesotho, the Rock Pigeon was virtually absent in rural and urban areas, except for the capital Maseru, where it is common in the city centre (Kopij 2000).

3.1.6. Rose-ringed Parakeet

The Ring-ringed Parakeet occurs naturally in the Sahel zone of Africa and SE Asia (Pakistan, India, Bangladesh Sri Lanka, Nepal and Myanmar). In Soth Africa, itt has a recorded expansion/ invasion into five regions, i.e., Cape Town, Steytlerville (NW of Porth Elizabeth), Shelley Beach (S of Port Shepstone), Bethal and Hazyview. It occurs no longer in five regions, where it was recorded in 1987-1991, viz. East London, Ladysmith (KwaZulu-Natal), St Lucia (KZN), Mkuze (KZN) and Leeudoringstad (North West province) (Symes, 2014). The species develop viable population only in Gauteng (mainly Johannesburg and Pretoria areas), Durban and Pietermaritzburg area.

First recorded as breeding in South Africa in the 1850’s in Cape Town, but failed to develop viable population (Symes, 2014). Again seen there in the 1980’s, the 1990’s and in 2010, but still has not develop a viable population there.

The Rosy-ringed Parakeet was found to breed at Sodwana Bay in the late 1960s (Vincent, 1972), Mkuze Game Reserve in the early 1970s, in Mandini, Zululand, in July 1979, and St Lucia in July 1982, but everywhere it failed to establish a viable population. In Durban, it was introduced in the 1970’s and soon began to expand (Pereen & Cowgill 2005; Shivambu et al. 2021). In 2000, 700-1000 individuals were counted, in 2018-2019: 1783 +_505 ind./750 km² (Shivambu et al., 2021) and it is still expanding.

In Gauteng, the first record of 5-8 ind. was made in 1985 in central Johannesburg. By mid-1990 the population increased to c. 200 ind. (Perrin & Cowgill, 2005; Simes, 2014); in 2013: at least 300-500 ind. in Johannesburg and at least 400 ind. in Pretoria were counted.

In recent years, it is expanding in South Africa and Mauritius (Jones, 1980; Hart & Downs, 2014; Symes, 2014). In November 2018, the Rosy-ringed Parakeet was recorded in the Nhangaan Forest near Nhangau, Sofala Province, and in Maputo City in July 2021 and October 2021 (Trezza et al., 2023).

It has established viable breeding populations in many part of the world. In Europe, it developed viable populations in the UK, France, Germany Belgium, the Netherlands, Spain, Portugal, Italy, Greece, Turkey and. All European populations numbered more than 85 000 individuals in 2016 (Pârâu et al., 2016). It has invaded also the northern Africa, Middle East, Thailand, Indonesia and Japan. Recently introduced to the New World, i.e., Florida, California, Venezuela and Hawaii (Iqbal, 2017; Nebot, 1999).

3.1.7. Red-whiskered Bulbul

Its original range extends from Pakistan throughout SW China and India to Indochina. In Africa, itt is well-established in Mauritius, where it was introduced intentionally in 1892. From there, it was introduced unintentionally to Reunion (1978) and Assumption Island in Seychelles (1977) (Carié, 1910). In 1987, there were c. 200 breeding pairs in the Assumption Island (Roberts, 1988); and several thousand individuals in 2014. (Uranie, 2015). In Mayotte, it was introduced in 1985, but unsuccessfully (Cheke, 2010). Recorded also in Madagascar, South Africa and Zimbabwe, although it has not develop viable populations in these countries.

As a result of introduction, the Red-whiskered Bulbul occurs in Japan, Indonesia, southern Malaysia, Singapore, Taiwan, Australia, Borneo, Hong Kong (China), the United Arab Emirates, Oman, Saudi Arabia, USA, Hawaii, Juan de Nova Island, Spain, Canary Islands, Nicobar Islands (Leven & Corlett, 2004; Walther, 2011; Clements et al., 2018).

3.2. Non-Invasive But Established Alien Bird Species

Most bird species which were introduced to sub-Saharan Africa and developed viable populations (n = 49 species) did not became invasive (Liversidge, 1985; Brooke et al., 1986; Dean, 2000; Faulkner et al., 2007; Dyer et al., 2017; Downs & Hart, 2020; Shivambu et al., 2020). In most cases the established populations are small with very restricted ranges. Most species have developed viable populations in small islands, mainly around Madagascar on the Indian Ocean (Table 1). On these small oceanic islands, some of them may even pose a threat to the indigenous fauna, becoming locally harmful, therefore invasive.

The Chaffinch is an exception in this regard. At the end of 19th century, a flock was released in Cape Town, which developed a small population of 1500-2000 individuals at the beginning of 21st century in and around this city in pine plantations between Tokai and Rondebosch, and around Fish Hoek and Noordhoek (Dean, 2005c). A similar situation may persist in the Masked Lovebird introduced in Dar es Salaam and Nairobi, and in the Nyasa Lovebird introduced in Lundazi, Zambia.

The Budgerigar is a species also of special interest in this group. From 1947, c. 50 individuals were recorded throughout the years in Umbogintwini, KwaZulu-Natal, but today its status is unknown there. Apparently, it was established in the city of Swakopmund, on Atlantic Ocean in Namibia in 2001 (Hockey et al., 2005), but not recorded there in 2017-2020 (Kopij, 2018a), neither in the neighbouring towns, Walvis Bay (Kopij, 2023b) and Hentje’s Bay (Kopij, 2022a).

The Mallard is regarded as invasive species in most areas, where it was introduced. In Africa, although introduced long time ago, it has not established viable populations to date. It exists in urban parks and gardens, as a semi-domestic/feral poultry and does not affect adversely the biodiversity nor the human economy (Stephens, 2019). The Graylag Goose, Muscovy Duck, and American Black Duck are in a similar situation.

3.3. Not Established Alien Bird Species

Most species introduced to sub-Saharan Africa has failed to develop viable populations. The group includes 101 species. It is, however, important to point out that the real number might be much higher, as some introductions could passed unrecorded, especially in countries beyond southern Africa. Excluded are here all species which could have been vagrants from its main range in other parts of Africa or short-lived aviary escapee, evidently not established even for one breeding season. For example, more than 30 parrot species have been observed in South Africa alone, most of them as escappe from cages (Symes, 2014).

Most of the introduced species were present for some time in the wild, but were probably not breeding there and subsequently disappeared altogether. Some others did develop a viable population, but subsequently the population declined and become extinct in the introduced areas (e.g., Mute Swan). Some individuals may persist as feral individuals on farmyards.

Among interesting introduction is that of the Rook. In the late 1890’s, as many as 200 ind. were released near Cape Town, but they failed to establish breeding population (Hockey et al., 2005). The Blackbird was also released in Cape Twon in the same time, it even bred there for several years, but finally became extinct by the year 1923 (Hockey et al., 2005). Similarly, the Song Thrush, released in the same place and time, developed a viable breeding population, but subsequently declined and became extinct by the year 1947 (Hockey et al., 2005).

4. General Characteristics of the Avian Introductions in Sub-Saharan Africa

4.1. Species Representativness

At present, 212 species are successfully introduced in the world. Out of this total, only c.10% affect biodiversity adversely through predation (1%), competition with native species (3%), and other effects (5%) (Martin-Albarracin et al., 2015). Worldwide 64% of established introduced mammal species become invasive, while this proportion was only 34% for birds (Jeschke, 2008). In USA, 97 alien species developed ‘self-sustaining populations’ (Avery & Tillman, 2005), in Australia, out of 242 avian species introduced, 77 (32%) developed viable populations (Vall-Ilosera & Cassey, 2017).

In sub-Saharan Africa, at least 150 bird species were introduced, but only 49 of them (32.7%) have developed viable populations, and only 7 (4.6%) became invasive (Table 1 and 3). Accoriding to Faulkner et al. (2017), 87 bird species were introduced to Africa, (including 67 species from Mauritius); the number is certainly underestimated. Dyer et al. (2017), on the other hand, listed 218 species. There is, however, a number of doubtful and inaccurate records in this list. For many species no information of the place and year of the introduction is given, some records include de facto introductions or reintroductions of indigenous species to their native ranges, some other may represent records of vagrant from other parts of Africa, or ephemeral escapee from aviaries. In a similar way overestimated seems to be the number of introduced species (n=192) compiled by Downs & Hart (2020).

Most avian introductions in sub-Saharan Africa took place in Southern Africa (mainly Cape Town, Durban and Johannesburg areas), in Madagascar and surrounding islands (mostly Mauritius, Reunion and Seychelles) and in the coast of Tanzanya and Kenya (icluding islands: Pemba, Zanzibar and Mafia).

Over the last 200 years, two introduction peaks took place in sub-Saharan Africa: the first one at the end of 19th century, and the second one at the end of 20th and at the beginning of 21st century (Figure 1). The former resulted from the activity of the former British Acclimatization Society, which promoted introductions of alien species in the end of 19th century in South Africa and other British colonies in Africa and beyond, for both utilitarian (hunting) and ‘aesthetic’ reasons (‘enrichment’ of local flora and fauna).

In the latter period, the peak is a result of increased commerce, inernational contacts and increased interest of bird-lovers in keeping ornamental birds in aviaries and cages.

Worldwide, Psittacidae (131 species) and Anatidae (92 species) are the most speciose groups of invasive bird species (Dyer et al., 2017). Other families with high number of invasive species include Phasianidae, Passeridae, Fringillidae, and Columbidae (Blackburn & Duncan 2001, Vall-Ilosera & Cassey, 2017). In sub-Saharan Africa, the most specious groups were parrots (Psittaciformes) comprising 33.1% (including Pisittacidae: 13.9%, and Psittaculidae: 15.9%), Anatidae: 11.3%, Phasianidae: 11.3%, and Passeriformes: 29.1% (including Estrildidae: 6.0% and Ploceidae: 5.3%, as the most speciose passerine families) (Table 4).

Most introduced species which have developed viable populations originate from Afrotropical, Oriental and Palearctic regions (altogether 78%), with only 2% from the New World. The proportion among introduced species which have note not developed viable populations are quite different: 30% are from the New World, and 56% from the Afrotropical, Oriental and Palearctic regions (Figure 3).

4.2. Factors Affecting Successful Introduction

There are several factors that determine the effective establishment of introduced species, viz. environmental and landscape-level changes, food resources, climate (House Sparrow, Rock Pigeon; could be Common Starling and Common Myna), ecological and physiological characteristics (e.g., behaviour, breeding ecology, diet composition, juvenile development, migratory status, body size and genetic variability), and introduction circumstances (e.g., time of release, the number of individuals released and the number of release events) (Dean, 2000; Sol & Lefebvre, 2000; Kolar & Lodge, 2001; Butler, 2003; Hayes & Barry, 2008; Shwartz et al., 2009).

Urbanization results in homogenization, with most introduced species present in urban areas (van Rensburg et al., 2009). This is because in urban areas introduced species expoloit rosources (food and nesting sites) better than native species (Sol et al., 2012). In sub-Saharan Africa all invasive species are strictly associated with urban environment, with some affinities to rural habitats.

4.3. Ecological Characteristic of Successful Species

There are several factors which determine success of introduced species:

-the pioneer group is large enough to prevent stochastic extinction within the first few years;

- a commensal with man; strongly associated with man-made structures (including impoundments), cultivations, lawns and gardens (Rock Pigeon, House Crow, House Sparrow);

- eurobiont (wide range in habitat tolerance);

- an generalist feeder of invertebrates (e.g., pest species on monocultures), seeds and scraps of human-processed foods (Common Myna, House Sparrow, House Crow, Rock Pigeon);

- communal roosting although single breeder (House Sparrow, Common Starling, Common Myna, Rock Pigeon, Rosy-ringed Parrot);

- group foraging (same species as above);

- climate tolerance;

- ‘r’-reproductive strategy (high clutch size and breeding success, few clutches per year) (Common Starling, House Sparrow);

-flexible in nest site selection, including man-made structures and alien trees and shrubs (Common Staling, Rock Pigeon);

-aggressive in the competition for food and nesting site (Common Starling, Common Myna).

4.4. Factors Affecting Unsuccessful Introductions

In comparison to other parts of the world, most avian introduction if Africa were unsuccessful. The following factors affect this:

- too low number of introduced individuals;

- species does not benefit from urbanization or agriculture, not strongly dependent on human activity;

- the diet is mainly seeds/grain or insects;

- competition from indigenous species;

- preference for disturbed mesic habitats, but avoidance of arid areas and natural vegetation, especially dense forests;

- low breeding success due to egg and chick predation;

- predation on adults by a wide array of vertebrate predators;

4.5. Introduction Pathways

Globalization facilitates and accelerate the rate of species invasions (Meyerson & Mooney, 2007). In sub-Saharan Africa, birds have been intentionally introduced into new areas for the following reasons: 1) acclimatization (acclimatization societies, which were popular in the 19th century in European colonies; introduced plants and animals into regions in which the fauna was considered deficient, e.g., many bird species in and around Cape Town); 2) biological control (control of insect populations, rodents, carcasses etc.; e.g., Common Myna, House Crow, Red-whiskered Bulbul); 3) ornamental/aesthetic purposes (e.g., Chaffinch, Common Starling); 4) game birds, for hunting purposes (most waterfowl and gallinaceous birds); 5) intentional release of captive pet or ornamental birds (most parrots and finches). However, many bird species were introduced unintentionally, being often s escapee from aviary (most ornamental birds) (Brochier et al., 2017).

5. Management and Nature Conservation Implications

5.1. Impact Categories

Many introduced species cause apparently minimal or no damage, others assume status of persistent and destructive pests.

Introduction and naturalization of non-native species is one of the most important threats to global biodiversity. Seven types of impact on native ecosystems were evaluated: competition (Strubbe & Matthysen, 2009; Hernández-Brito et al., 2014; Grandi et al., 2018); predation (Mooney & Cleland, 2001); hybridization (Gaertner et al., 2016); spread of disease and parasites (Weber, 1979; Crowl et al., 2008); chemical, physical, and structural impact on ecosystem through grazing/ herbivory/ browsing. Most of these impacts are theoretical/potential/not evidenced (Craig & Edwards, 2024).

5.1.1. Hybridization

The Mallard may hybridize with other ducks. The following species in Africa may be affected: Yellow-billed Duck Anas undulata in South Africa, African Black Duck Anas sparsa in South Africa and Meller’s Duck Anas melleri in Madagascar. Mallards may pose threat specially in small oceanic slands.

5.1.2. Competition

The Common Staling competes for holes with other hole-nesting birds (e.g., Red-winged Staring Onychognathus morio. Little Swift Apus affinis, Olive Woodpecker Dendropicos griseocephalus). Usurping nests by contamination (as well as physical competition) is also a problem (e.g., native parrots use little, if any, bedding, whereas starlings will rapidly fill and contaminate tree hollows) (Joubert, 1945; van der Merwe, 1984; Kopij, 2009). The Rose-ringed Parakeet and the Common Starling, which breed prior to most other secondary cavity-nester species, may reduce nesting sites availability for bats and parrots.

5.1.3. Predation

The Common Myna competes Seychelles Magpie Robin Copsychus sechellarum by occupying nest boxes, and destroying eggs and chicks (Komdeur, 1996; Canning, 2011). The Seychelles Magpie Robin is an endangered species. Similar situation was recorded in the critically endangered Seychelles Flycatcher Terpsiphone corvina (Feare, 2010), vulnerable Seychelles White-eye Zosterops modestus (Payet, 2007) and sea birds (Feare et al. 2015). The House Crow destroys clutches or broods of other bird species, e.g., African Sacred Ibises Threskiornis aethiopicus, African Spoonbills Platalea alba and Black-headed Herons Ardea melanocephala (Dean, 2005b). The Red-whiskered Bulbul preys upon the endangered native golden orb weaving spiders (Nephila spp.) in Mauritus (Islam & Williams, 2000; Linnebjerg et al., 2010).

5.1.4. Disease Transmission

The House Sparrow is a vector of arthropod-borne viruses, such as western equine encephalitis virus, eastern equine encephalitis virus, Buggy Creek virus, St Louis encephalitis virus, and West Nile virus and. The Common Starling is a vector of psittacosis Chlamydophila psittaci, bacteria (Escherichia coli, Salmonella spp., Campylobacter, Mycobacterium) and avian malaria Plasmodium relictum. Also the Common Mynas is known as a vector of avian malaria, dermatitis,psittacosis, salmonellosis and arbovirusal infections (Clark et al., 2015). The Rock Pigeon can be a vector of more than 60 different pathogens including those recorded in the Common Starling. The Rosy-ringed Parakeet is a carrier of some zoonotic diseases such parrot fever Chlamydia psittaci (Pisanu et al., 2018) and H9N2 Influenza A viruses (Mase et al., 2001). The House Crow transmits the New Castle disease.

5.1.5. Parasitism

Virtually all birds have parasites which they transmit among themselves and some may transmit to human. The Rock Pigeon has especially large burden of the following ectoparasites: bugs (e.g., Cimex lectularius), fleas (e.g., Ceratophyllus gallinae), flies (e.g., Fannia canicularis), mites (e.g., Dermanyssus gallinae) and ticks (e.g., Argas reflexus) that are known to infest humans (Haag-Wackernagel, 2005).

5.1.6. Human-Wildlife Conflict

The Rock Pigeon, House Crow, House Sparrow, Common Staling and Common Myna cause damages to infrastructure, roof linings, buildings, and monuments. The Common Starling forms large flocks in non-breeding season and is renown from collisions with aircrafts. The House Crow attacking people to obtaain food (Ryall, 1992; Suliman et al., 2011; Shimba and Jonah, 2016), and kills chickens on poultry farms (Dodoma, Tanzania; Ngongoda, Mirimi 2024). The Rosy-ringed Parakeet can cause severe noise disturbance, as it roosts in the thousands at night and have very loud vocalizations (Parau et al. 2016).

5.1.7. Agricultural Pests

The Common Starling is notorious pest of fruits. It damages especially often cherries and grapes, but also figs, peaches, straw berries, blueberries. Also the Common Myna may consume grapes, pears, strawberries, apricots, apples, and other fruits.

5.1.8. Seed Dispersal

The Red-whiskered Bulbul disperses seeds invasive plants altering plant communities (Simberloff & Von Holle, 1999; Linnebjerg et al., 2009; Mo 2015). The Common Staling is known to disperse the invasive Western Coastal Wattle Acacia cyclops (Glyphis et al., 1981). The Rosy-ringed Parakeet is known to cause damage to vine grape, almonds Prunus dulcis, Citrus spp., guavas Psidium guajava, mangos Mangifera indica, and common sunflower Helianthus annuus heads (Ahmad et al., 2012; Mentil et al., 2018). The Rock Pigeon causes damage to maize, seeds, legumes (soybeans and chickpeas) and sunflower crops, while the House Crow damages fruits and crops such as cereal, maize or groundnuts (Suliman et al., 2011; Akram et al., 2013; Kamel, 2014; Khan et al., 2015).

5.2. Control Measures

Control measures to reduce detrimental alien bird populations include: prevention, early detection and rapid response, eradication and restoration. Some of eradication methods include shooting, limiting food resources, clutch/brood destruction,trapping, using toxic substances, bioacoustic, and sterilization (Brook et al., 2003; Yap & Sodhi, 2004; Iriarte, 2005; Feare, 2010; Geartner et al., 2016). A need for invasive species control, eradication methods used etc. varies among species (Veitch & Clout, 2001; Bremner & Park, 2007; Coates, 2007).

Feral and domestic cats are main predators of the House Crow and Common Myna. The Peregrine Falcon Falco peregrinus is an efficient predator of Rock Pigeons, while Accipiter-raptors may efficiently control communally roosting birds, such as House Sparrow, Common Starlings, Common Myna or parrots. Some species, like Common Starling can be controlled using sonic devices.

Physical and chemical methods have been used to control Rock Pigeons, including shooting, poisoning, contraceptives, nest destruction, and reduction of food availability. Eradication from small islands is feasible and achievable by trapping, nest destruction and shooting, poisoning, and mist-netting (Canning, 2011).

Examples of successful eradications:

- Common Myna in North Island, Seychelles, (>200 ha, estimated populations of myna: c. 750–1000 ind.), 4641 ind. were killed (Feare, 2021; Canning, 2011; Feare et al., 2016, 2021); Assumption Is (11.1 km²), Seychelles, 2004, 5270 ind. were killed (Millet et al., 2004; Cruz & Reynolds, 2019); Fregate Island (219 ha), Seychelles, 2010-2011, 745 ind. were killed (Canning, 2011); Socotra (Yemen et al., 2010).

- House Sparrow in Round Island (219 ha), Mauritius, 320 ind. were killed in 2008-2009; but not eradicated totally (Bednarczuk et al., 2011)

- Madagascar Fody in Assumption Island, Seychelles, over 3200 ind. were eliminated, with the last bird shot in January 2015 (Bunbury et al., 2019).

- House Crow in Socotra Island. Between 2002 and 2008, the numbers of adults were successfully controlled by clutch/brood destruction. In April 2009 two professional snipers shot all adults. A total of 242 young House Crows were collected and killed (Suleiman & Taleb, 2010; Suleiman et al., 2010); House Crow in Mauritius were poisoned with the starlicide (Puttoo & Archer, 2004), and α-chloralose Crows (Feare & Mungroo, 1990).

- Red-whiskered Bulbul in Mayotte, eradicated in the 1990s (Sinclair, 2013). In the Assumption Island (Seychelles), 5279 birds were eliminted in 2012-2014 (Bunbury et al., 2013; Uranie, 2015); Mascarene Islands, pesticides and bird lime were used to kill the mynas (these untargeted chemicals destroy also indigenous birds) (Clergeau & Mandon-Dalger, 2001); in Réunion Island, in addition to trapping, management practices were also used to mitigate the human-wildife conflict to protect agricultural crops (Clergeau et al., 2006; Amiot et al., 2007).

-Rosy-faced Parakeet in Mahé (155 km²), Seychelles, shooting by a team of professional hunters, 548 birds were shot from 2013 to 2017 (Bunbury et al., 2019).

-Ring-necked Parakeet in Mahe Island, Seychelles, 545 ind. were shot in 2012. The last known wild ring-necked parakeet was culled in the Morne Blanc region of Mahé in August 2017 (Bunbury et al., 2019).