Mixed foraging flocks

Mixed foraging flocks (bird parties) were defined as feeding groups of birds of two or more species moving through the vegetation together as a loose flock. They may include a variety of species feeding in different ways and at different heights, or they may be small feeding only in one layer of the vegetation. There are several theories explaining the advantages for birds joining bird parties, e.g. they can better locate insect food because of the disturbance created or gain more protection against predators (Brosset 1969, Croxall 1976).

In total, 50 mixed feeding flocks were recorded in the Bossematié area. All of them were recorded within the borders of the Forêt Classée. Of these 50 flocks, 17 were recorded during standardised mapping on the 110 ha plot (see appendix D), another 33 were recorded extra, either on the plot or elsewhere in the Bossematié Forest. Most flocks were recorded in the control compartment (eleven during standardised mapping), and none were recorded in the plantation, but there was no overall significant difference in the frequency of occurrence between plantation, control, liberation thinning and swamp forest area (Chi-sq. test, F²3df= 3.33, p = 0.433).

The 50 flocks comprised a total of 350 identified individuals in 63 species from 23 families.

Most species (90%) were insectivorous species. Three nectarivores (partially insectivorous Nectariniidae), two frugivores (a barbet, Tricholaema hirsuta, and a hornbill, Tockus semifasciatus) and the granivorous Ahanta Francolin Francolinus ahantensis, were also recorded. Four species were encountered in more than 30 % of the flocks: two pycnonotids, White-bearded Greenbul Criniger calurus, and the Cameroon Sombre Greenbul Andropadus curvirostris, the platysteirid flycatcher Diaphorophyia castanea, and the forest weaver Malimbus rubricollis. Five more species were encountered in 20 - 30 % of the flocks: two sylviids, Grey Longbill Macrosphenus concolor and the Yellow-browed Camaroptera Camaroptera superciliaris, as well as White-throated Greenbul Phyllastrephus albigularis, Fraser’s Scarlet-tufted Sunbird Anthreptes fraseri and the Yellow-bill Ceuthmochares aereus, a non-parasitic cuculid. seventeen more species from eleven families were encountered in 10 - 20 % of the flocks, but 37 species (more than half of the 63 species) were only encountered in less than 10% of the flocks.

6.4.3.1 General composition

Most flocks (39 of 50, or 78%) comprised less than ten species, the mean being 6.9 (r4.6) species per flock. However, eleven flocks still consisted of ten or more species, the largest flock containing at least 24 species. The size of the flocks amounted up to 30 individuals, but most flocks were between five and fourteen individuals, giving a mean of 10.9 (r7.3). Species from all vertical strata were present, but especially midstorey species were represented by a higher number of species and in both understorey and canopy species, a lower number of species showed flocking behaviour (Chi-sq. test, F²3df= 8.41, p = 0.038). However, there was a statistically significant relationship between the species’ preferred foraging stratum and the

frequency of occurrence in flocks. The Gamma rank correlation coefficient J indicated a weak negative correlation between foraging height and encounter frequency (% flocks with respective species present). Whereas species of the lower strata tended to be more regular members of flocks, species of higher strata were less regularly encountered (J= -0.212, n = 63 spp., p = 0.0495). This relationship was only weak mainly because the most regularly encountered species were not true understorey, but lower midstorey species (White-bearded Greenbul Criniger calurus and Chestnut-Wattle-eye Diaphorophyia castanea).

Tab. 9: Distribution of species numbers and individuals encountered in 50 mixed bird parties in the Bossematié Forest, south-east Côte d’Ivoire.

a) Species <5 5-9 10-14 15-19 20-24

No. of flocks 16 23 8 1 2

% 32 46 16 2 4

mean (r s.d.) 6.9 (r4.6)

b) Individuals <5 5-9 10-14 15-19 >20

No. of flocks 8 16 13 8 5

% 16 32 26 16 1

mean (r s.d.) 10.9 (r7.3)

6.4.3.2 Interspecific associations

It has been shown that obligate members of mixed flocks are especially vulnerable to logging (Thiollay 1992, 1994a). If logging in the Bossematié Forest affected the abundances of obligately flocking species, it can be assumed that flocks are more ill-defined associations of non-obligatory species. In the primary forest of Gola, Sierra Leone, the principal „core“

species of mixed species flocks are Icterine Greenbul Phyllastrephus icterinus, Shining Drongo Dicrurus atripennis and Fraser’s Scarlet Tufted Sunbird Anthreptes fraseri. In the Bossematié Forest, however, Icterine Greenbul and the Shining Drongo were only recorded once and five times respectively (all observations made in flocks). Somewhat more frequent, but still not common, was Fraser’s Scarlet tufted Sunbird, which was observed thirteen times in flocks (out of 19 total records). The species was one out of the eight most regularly encountered species (present in more than 20 % of flocks) whose associations (number of flocks shared) are shown in Fig. 18. From these associations, three principal categories of flocks can be separated:

1) Understorey flocks, mainly composed of greenbuls Pycnonotidae, with the “core” species Red-tailed Greenbul Criniger calurus, Cameroon-Sombre Greenbul Andropadus curvirostris, and White-throated Greenbul Phyllastrephus albigularis. One more pycnonotid, the Grey-headed Bristlebill Bleda canicapilla, essentially belongs to this group, as well as the Brown Illadopsis (Timaliidae). A special kind of understorey association is that of ant-following species. In the Bossematié Forest, these species, whose respective habits are already well known, are the thrushes Fire-crest Alethe Alethe diademata, Forest Robin Stiphrornis erythrothorax, and White-tailed Ant-thrush Neocossyphus poensis. The Red-tailed Bristlebill Bleda syndactyla was also observed in several occasions, but ant-following flocks were not encountered as regularly as the above mentioned association of not ant-following species.

2) Midstorey flocks, with flycatchers and small foliage gleaning species dominating. The

“core” species are Chestnut Wattle-eye Diaphorophyia castanea (Playtsteiridae) and Fraser’s Scarlet-tufted Sunbird Anthreptes fraseri (Nectariniidae). Other flocking midstorey species are the monarchids Terpsiphone rufiventer and Trochocercus nitens, and the more infrequent members, Finsch’s Flycatcher-thrush Neocossyphus finschii and the Red-billed Dwarf Hornbill Tockus camurus. Furthermore, there are species from both group (1) or (3) mixing in variable proportions with this group.

3) Midstorey/canopy flocks. This guild is both rich in species as heterogeneous in guilds. The

“core” species are small foliage-gleaners such as Yellow-browed Camaroptera Camaroptera superciliaris, Grey Longbill Macrosphenus concolor or Sharpe’s Apalis Apalis sharpii (Sylviidae), but a very regular member (in 37% of all flocks) is the ploceid Red-headed Malimbe Malimbus rubricollis. Less regular mixed flock members associated with canopy flocks are Yellow-bill Ceuthmochares aereus (Cuculidae), White-naped Weaver Ploceus albinucha, Western Black-headed Oriole Oriolus brachyrhynchus, Blue Cuckoo-shrike Coracina azurea, and the Drongo Dicrurus adsimilis, the three latter species being very vocal.

During the study, no associations between birds and mammals (Primates or Duikers) were observed.

% Similarity

8 5 8 0 7 5 7 0 6 5 6 0 5 5 5 0

C A M S U P

M A C C O N

M A L R U B

A N T F R A

P L A C A S

P H Y A L B

A N D C U R

C R I C A L

Fig. 18: Associations between the eight most frequent members of mixed flocks in a heavily degraded rainforest in south-east Côte d’Ivoire. Presence/absence data of 50 mixed flocks clustered using weighted pair group average clustering and % non-overlap as distance measure (converted to similarity on y axis).

Six-letter codes, from left: Camaroptera superciliaris, Macrosphenus concolor (Sylviidae), Malimbus rubricollis (Ploceidae), Anthreptes fraseri (Nectariniidae), Diaphorophyia castanea (syn.: Platysteira, Platysteridae), Phyllastrephus albigularis, Andropadus curvirostris, Criniger calurus (Pycnonotidae).