In order to investigate the avifauna, we used an appropriate method: the mist-netting method. According to this method, an inconspicuous mesh net is erected vertically on poles and deployed in areas of high activity to intercept birds as they go about their normal daily routines (Fig. 1).

We used dark-coloured nylon nets and smaller mesh for smaller species. Our mist nets were fixed with the mounting poles which were chosen carefully and the choice of an appropriate mist-netting site was important for the capture success (Fig. 1). To ensure the capture success, we mainly identified their preferred flight paths, feeding areas, roosting and shaded sites and that along the hill from the base to tip.

We began our capture very early in the morning (5: 00 AM) and we finished very late in the evening (sometimes 6: 30 PM). We used the same eight mist nets in our different field study and we did nine field studies during eight months.

Fig. (1) Basic set-up of a mist net placed on a side of the Abobo-Etetak hill.

In order to calculate the relative Abundance, we considered to use the Statistical Analysis System [16 SAS Institute. SAS/STAT guide for personal computers. 6^th ed. Cary, NC 1985.]. Otherwise, we used a software program Excel to obtain our histograms and curves [17Microsoft Excel. Microsoft® Office Excel 2010.].

In order to calculate the occurrence, we used the same software, the Statistical Analysis System [16 SAS Institute. SAS/STAT guide for personal computers. 6^th ed. Cary, NC 1985.]. The obtained results are frequencies as well as graphics.

311 individuals were captured which belong to 39 genera (Table 2). Within the Abobo-Etetak avifauna, the representative genera are Ploceus (16.72%), Euplectes (10.29%) and Quelea (9.32%) which are members of the Ploceidae family (Table 2 and Fig. (3)). These genera are followed by Cyanomitra (6.43%) (Nectariniidae family), Colius (6.43%) (Coliidae family) and Eurillas (4.82%) (Pycnonotidae family) (Table 2 and Fig. (3)). The less represented genera (relative abundance < 0.50%) are mainly the non-passerine members (Centropus, Chrysococcyx, Campethera, Sasia) (Table 2).

Table 2
Avifaunal abundance of the genera on the Abobo-Etetak hill.

The majority of the genera (52.63%) captured on the Abobo-Etetak hill were rare (25% > FO ≥ 10%) (Table 3). However within the most representative family, the Ploceidae family, the genus Ploceus appeared very frequent on the Abobo-Etetak hill with a frequency of occurrence of 100% (Table 3). According to our results, except the genus Ploceus, two other genera have been frequent on the Abobo-Etetak hill, Cyanomitra (FO = 100%) and Colius (FO = 88.89) (Table 3).

Table 3
Frequency of occurrence of the diverse birds’ genera captured on the Abobo-Etetak hill from July 2015 to February 2016.

Fig. (3) Histogram of the relative abundances of the genera captured in the Abobo-Etetak hill and species number curve.

According to our occurrence results, only 5.88% of the birds captured are relatively common on the Abobo-Etetak hill, including the passerine members Cyanomitra verticalis (FO = 100%) and Ploceus nigricollis (FO = 88.88%), and the non-passerine member Colius striatus (FO = 88.88%) (Table 4). Species (21.57%) less commonly encountered included Platysteira cyanea (FO = 44.44%), Eurillas virens (FO = 44.44%), Eurillas latirostris (FO = 33.33%), Chlorocichla falkensteini (FO = 44.44%), Dryoscopus senegalensis (FO = 33.33%), Estrilda nonnula (FO = 33.33%), Euplectes ardens (FO = 33.33%), Ochrospiza mozambica (FO = 33.33%), Passer griseus (FO = 33.33%), Quelea erythrops (FO = 33.33%) and Turdus pelios (FO = 33.33%) (Table 4). In return, our analyses show clearly that the major part of the species (62.74%) sampled on the Abobo-Etetak hill were rare (25% > FO ≥ 10%) (Table 4).

Table 4
Frequency of occurrence of the different birds’ species captured on the Abobo-Etetak hill.

The Shannon index was 2.24 at the familial level, 3.11 at the generic level and 3.34 at the specific level (Table 5).

The Simpson index values were of 0.81 at the familial level, of 0.93 at the generic level and of 0.95 at the specific level (Table 5). As for the Shannon index, the general tendency is the same as the Simpson index with the obtained values which are also high (Table 5).

The equitability index values were 0.73 at the familial level, 0.85 at the generic level and 0.85 at the specific level (Table 5). As for the two first indexes, the obtained values also appear far from 0 (Table 5).

Table 5
Values of the different indexes of diversity obtained with the software PAST.

The survey accomplished on the Abobo-Etetak hill permits to highlight that Passerines birds were the most representative birds in this area with regard to Non-Passerines birds. Our study moves toward several surveys performed in Cameroon [1Louette M. The birds of Cameroon: An annoted check-list. Verhandeling Wetenschappen. Jaargang 1981; 43: 1-163., 6Good AI. The birds of French Cameroon. Mém IFAN, Centre du Cameroun 1953; 1(1): 269. Sci. Nat. 1952; 2: 1-203, 7Germain M, Dragesco J, Roux F, Garcin H. Centre ORSTOM de Yaoundé, Université du Cameroun. Paris: Muséum National d'Histoire Naturelle 1974; pp. 212-59.].

We found that 84,55% of birds captured were passerines and 15,45% of birds were non-passerines (Tables 1 and Fig. (2)). Among these non-passerines, some families (Lybiidae, Meropidae, Alcedinidae, Columbidae, Cuculidae, Coliidae and Picidae) have been indicated in the relic Centre Region of Cameroon (Yaounde) but other passerine families (Pycnonotidae, Cisticolidae, Nectariniidae, Estrildidae, Ploceidae, Platysteiridae, Turdidae, Hirundinidae, Malaconotidae and Passeridae) have been already found in this Region [1Louette M. The birds of Cameroon: An annoted check-list. Verhandeling Wetenschappen. Jaargang 1981; 43: 1-163., 6Good AI. The birds of French Cameroon. Mém IFAN, Centre du Cameroun 1953; 1(1): 269. Sci. Nat. 1952; 2: 1-203, 7Germain M, Dragesco J, Roux F, Garcin H. Centre ORSTOM de Yaoundé, Université du Cameroun. Paris: Muséum National d'Histoire Naturelle 1974; pp. 212-59.].

Thus, this study permits to highlight new families in the relic Centre Region of Cameroon and particularly in the Abobo-Etetak hill. Comparatively to previous records, we newly recorded species representatives of the non-passerines families (Lybiidae) as well as the passerines families (Motacillidae, Nectariniidae, Hirundinidae, Nicatoridae, Ploceidae and Monarchidae) in this area (Table 1). Within the Abobo-Etetak avifauna, the most representative family is the Ploceidae family (Table 1 and Fig. 2). The Ploceidae family commonly called ‘weavers’ gets this name because of their meticulously woven nests; these small passerine birds are gregarious, breed colonially and occur in several habitats (woodland, forests, forest edge, lowland forests, wet habitats, gardens, open or semi-open habitats, human habitation, grassland, etc.) [19Hall BP, Moreau RE. An Atlas of speciation in African passerine birds: Trustees of the British Museum (Natural History). London 1970., 20Fry CH, Keith S. The birds of Africa. London: Christopher Helm, Vol. 7, 2004.]. Contrary to several other passerine families, their closest relatives still remain unknown and a future study in keeping with their phylogeny is long-awaited.

We found two new genera until now not encountered in this relic Centre Region, Cinnyris (the Nectariniidae family) and Terpsiphone (the Monarchidae family) (Table 2). Except for these genera, most genera we found have been already recorded in this Region [1Louette M. The birds of Cameroon: An annoted check-list. Verhandeling Wetenschappen. Jaargang 1981; 43: 1-163., 6Good AI. The birds of French Cameroon. Mém IFAN, Centre du Cameroun 1953; 1(1): 269. Sci. Nat. 1952; 2: 1-203, 7Germain M, Dragesco J, Roux F, Garcin H. Centre ORSTOM de Yaoundé, Université du Cameroun. Paris: Muséum National d'Histoire Naturelle 1974; pp. 212-59.]. Among the genera met in this relic area, the genera Ploceus and Cyanomitra have obtained an occurrence frequency of 100% (Table 2). The genus Ploceus is a representative of the Ploceidae family (see above) and the genus Cyanomitra is a member of the Nectariniidae family; African sunbirds are very small passerine birds which feed on nectar but in their youth feed on insects [19Hall BP, Moreau RE. An Atlas of speciation in African passerine birds: Trustees of the British Museum (Natural History). London 1970., 21Borrow N, Demey R. Field Guide to the Birds of Western Africa London: Christopher Helm 2004., 22Fry CH, Keith S, Urban EK, Eds. The birds of Africa. London: Academic Press, Vol. 6, 2000.].

Concerning species representatives, our survey brings several new elements compared with the general avifauna known of this relic Centre Region (Table 4). Thus, seven of the 21 families listed show new representatives at the species’ level (Tables 1 and 4). Before our survey, these species were not mentioned in this relic Region: Anthus brachyurus (Motacillidae), Cinnyris reichenowi (Nectariniidae), Euplectes gierowii (Ploceidae), Hirundo fuligala (Hirundinidae), Nicator vireo (Nicatoridae), Pogoniulus atroflavus and Pogoniulus subsulphureus (Lybiidae) and Terpsiphone rufocinerea (Monarchidae) (Table 4). Otherwise, two species (Chlorocichla simplex and Campethera cailliautii) had been mentioned only in a district of Yaounde (Nsimalen) [7Germain M, Dragesco J, Roux F, Garcin H. Centre ORSTOM de Yaoundé, Université du Cameroun. Paris: Muséum National d'Histoire Naturelle 1974; pp. 212-59.].

Among species collected in this area, we noted a lot of sedentary birds which are rarely dispersed more than a few kilometres from their natal sites (Eurillas latirostris, Camaroptera brachyura, Turdus pelios, etc.), seasonal birds which appear only seasonally in another part (Anthus brachyurus and Chrysococcyx caprius), intra-African migrant birds which breed in one part of Africa and spend the post-breeding season in a different area (Ceyx pictus) and vagrant birds which are outside their normal range (Hirundo fuligula) (Table 4). Among the captured species, we highlighted two species of the Monarchidae family, the genus Terpsiphone, which are not found in the Guide [21Borrow N, Demey R. Field Guide to the Birds of Western Africa London: Christopher Helm 2004.] and they are probably new species; for this, we will do a molecular study in the future to clarify their situation.

The Abobo-Etetak hill shows high avifaunistic diversity (see Table 5), with an absence of the supremacy of one species (H’ = 3.34; Table 5). According to some authors [2Amiet JL, Perret JL. Contribution à la faune de la région de Yaoundé (Cameroun). II Amphibiens Anoures. Ann Fac Sci Cameroun 1969; 3: 117-37., 23Heim De Balsac H. Contribution à la faune de la région de Yaoundé (Cameroun). I. Premier aperçu sur la faune des Soricidae (Mammifères Insectivores). Ann Fac Sci Ydé 1969; 2: 49-58.], the montane vegetation grows on hills near Yaounde (at elevations of only 800-900 metres) and these semi-montane biomes offer the same feeding favourable conditions for all birds visiting or living in this relic area. Otherwise, it is documented that the distribution of many tropical bird species is closely related to a particular vegetation type [1Louette M. The birds of Cameroon: An annoted check-list. Verhandeling Wetenschappen. Jaargang 1981; 43: 1-163.] and more generally a correlation between food areas and birds is well established [13Parsons H, Major RE, French K. Species interactions and habitat associations of birds inhabiting urban areas of Sydney, Australia. Austral Ecol 2006; 31: 217-27.
[http://dx.doi.org/10.1111/j.1442-9993.2006.01584.x] ]. The Abobo-Etetak hill appears with the same vegetation from base to tip (secondary forest with in some places farmland). The distribution of the Abobo-Etetak avifauna corroborates practically this principle with the obtained value of the Equitability index (J’= 0.85; Table 5) which establishes the equal distribution of the individuals inside species.

The Simpson diversity index of 0.95 (1 – D = 0.95; Table 5) shows that the diversity within the Abobo-Etetak avifauna is established and this result goes in the same direction that the absence of the supremacy of one species as well as the equal distribution of the individuals of the species. Thus, all obtained values show that the Abobo-Etetak avifauna appears diversified; the diversity of this avifauna could be explained by luxuriant vegetation in this relic area all year round and this would have been favoured by a lesser competition between the individuals living in this hill. Otherwise the transformation of this area by human activities (particularly farmland) would have had an impact on the diversity of this avifauna with some genera captured in this hill (Ploceus, Passer, Estrilda, Pycnonotus and Spermophaga).

Contrary to several African regions, this relic Centre Region shows the Montane district at very low altitude (an altitude of less of 800 to 900 metres) whereas generally it begins in other African regions at an altitude of 1600 to 1800 metres [1Louette M. The birds of Cameroon: An annoted check-list. Verhandeling Wetenschappen. Jaargang 1981; 43: 1-163., 2Amiet JL, Perret JL. Contribution à la faune de la région de Yaoundé (Cameroun). II Amphibiens Anoures. Ann Fac Sci Cameroun 1969; 3: 117-37., 24Nonnotte P. Etude volcano-tectonique de la zone de Divergence Nord Tanzanienne (Terminaison Sud du Rift Kenyan). Caractérisation pétrologique et géochimique du volcanisme récent (8 Ma-Actuel) et du manteau source. Contraintes de mise en place PhD Thesis. France: University of Western Bretagne 2007.-26Stuart SN. Conservation of Cameroon montane forests Report of the International Council for Bird Preservation. Cambridge, UK: ICBP 1986.]. Thus, several new and endemic species have been described [3Sharpe RB. On further collections of birds from the efulen district of cameroon, west africa, part II. Ibis 1904; 46(4): 591-638.
[http://dx.doi.org/10.1111/j.1474-919X.1904.tb00524.x] -5Bates GL. Anthreptes seimundi minor. Sanaga River, north of Yaounde, Cameroon. Bull Brit Orn Club 1926; 46: 107.] in this relic Region and certainly the two new taxa not found in the Guide [21Borrow N, Demey R. Field Guide to the Birds of Western Africa London: Christopher Helm 2004.] could be new species. Otherwise in spite of the urbanization as well as climatic changes, this Region preserves its originality in keeping with the fresh climate in an Equatorial area thus offering favourable conditions to birds all year around and receiving many European migrants’ birds during the winter.