Predation on Birds Caught in Mist-Nets in Upland Kenyan Forest FragmentsAuthor(s): Thomas BrooksSource: The Wilson Bulletin, Vol. 112, No. 2 (Jun., 2000), pp. 292-294Published by: Wilson Ornithological SocietyStable URL: http://www.jstor.org/stable/4164215 .

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292 THE WILSON BULLETIN * Vol. 112, No. 2, June 2000

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Wilson Bull., 112(2), 2000, pp. 292-294

Predation on Birds Caught in Mist-Nets in Upland Kenyan Forest Fragments

Thomas Brooks'-2

ABSTRACT.-I report a predation rate of 24 birds from 3707 total captures of birds caught in mist-nets in fragmented upland forests in Kenya. Predation was highly concentrated in specific forest fragments, with multiple birds simultaneously found killed in more than half of the predation events. Predators were prob- ably attracted to the mist-nets by the noise and activity of the netted birds because the capture rate of predated nets was more than three times the mean for the survey as a whole. The most common victims were Yellow- whiskered Greenbuls (Andropadus latirostris), and the most common predators were blue monkeys (Cerco- pithecus mitis). Although 20% of all our mist-net cap- tures were in Kakamega, a large forest, not a single bird was taken from our nets there, suggesting that predation rates on mist-netted birds are higher in small forest fragments. Received 5 May 1999, accepted 22 Dec. 1999.

Throughout 1996, we surveyed birds in three fragmented upland Kenyan forests (Brooks and Waiyaki 1996) as part of an as- sessment of the time-lag between deforesta- tion and local extinction of forest birds (Brooks et al. 1999). The three forests were Kakamega (1400-1600 m; April-May and September), the Taita Hills (1500-1800 m; July-August) and the southwestern flank of

Mt. Kenya (2000-2100 m; October-Novem- ber). These surveys involved a large amount of mist-netting, which we will summarize elsewhere. The purpose of this note is to re- port on some incidents of predation on birds caught in the mist-nets.

Throughout our fieldwork, we deliberately minimized the incidence of predation on mist- netted birds by checking nets regularly (nor- mally with a maximum of 1 hr between net- rounds) and actively scaring away predators, particularly blue monkeys (Cercopithecus mi- tis), when they were seen near the nets. All nets were about 20 m long and had four panels (ca 2 m high); we opened all nets at dawn, closed them during rain, and closed and furled them overnight. We operated a total of 300 net-hours in all large forest fragments (five in each forest) with two consecutive lines of 10 nets run perpendicular to the forest edge into the interior for 15 hours each. For each of these 200 m net-lines, we operated nets for one full day and early the following morning. In the main Kakamega Forest, we also ran 300 net-hours in this fashion in one subplot and 80 net-hours (as one line of 10 nets run for 4 hours from dawn on each of two mornings) in eight additional subplots (Oyugi 1998). For sites too small to accomnmodate 400 m of mist-net (three patches in the Taita Hills and four on Mount Kenya), we ran a single line of 5 nets for one day (50 net-hours each lo- cation).

In total, we caught 3091 individual birds,

I Dept. of Ornithology, National Museums of Ken- ya, P.O. Box 30658, Nairobi, Kenya; E-mail: kbirds@africaonline.co.ke

I Present address: Center for Applied Biodiversity Science, Conservation International, 2501 M St. NW, Suite 200, Washington DC 20037; E-mail: t.brooksC@conservation.org

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SHORT COMMUNICATIONS 293

TABLE 1. Predation on birds caught in mist-nets in three upland forests in Kenya. Species of prey is followed by species (or likely species) of predator- and number of predation incidents at that site. Systematic order of bird species in each site follows Zimmerman et al. (1996).

Site Bird species Predator species n

KAKAMEGA

lkuywa Andropadus latirostris small mammal 2 Cercopithecus mitis 2

unidentified Cercopithecus mitis 6 Malava West Phvllastrephus cabanisi Act ipiter tachiro I

TAITA HILLS

Chawia Nectarinia olivacea unidentified I Mbololo Pogonocichla stellatai safari ants I

MT KENYA

Chehe Pogonocichla stellata unidentified I Nectarinia olivacea Acc ipiter tachiro 1

Ragati Andropadus latirostri.s Cercopithecus mitis 1 Andropadus nigriceps Cercopithecus mitis I Bradypterus lopezi Cercopithecus mitis 1

Muthira Andropadus latirostris Cercopithecus mitis 3 Trochocercus albonotatus Cercopithecus mitis I Malaconotus nigrifrons Cercopithecus miti.s 1 Mandingoa nitidula Cercopithecus mitis 1

of which 24 (0.8%) were predated (Table 1). The overall predation rate is better expressed as 24 of 3707 (0.6%) birds, based on our total number of captures (including recaptured birds).

Predation events were not concentrated in any one of the three forests. The highest pre- dation rate was on Mt Kenya, with 1 1 of 1315 (0.8%) captures predated. The rate for Kaka- mega [11 of 1869 (0.6%) captures] was lower, and that for Taita Hills [2 of 523 (0.4%) cap- tures] was the lowest. However, the propor- tions of predation events per forest was not significantly different from those expected given the proportion of total captures per for- est (X2 = 0.86, 2 df, P > 0.05).

At the spatial scale of mist-netting sites and the temporal scale of days, predation was highly concentrated in space and time: a 'feeding-frenzy' effect. All predation events occurred in just 7 (23%) of our 31 mist-net- ting sites, significantly fewer than the 16 sites (mean of 5 simulations picking 24 captures randomly from the total of 3707 captures and assigning these to sites according to the num- ber of captures per site) expected (X2 = 10.4, 1 df, P = 0.001). Notably, no predation events occurred in the main Kakamega Forest, sig- nificantly fewer than the 4 expected based on

the fact that 680 of our 3707 total captures were there (X2 = 4.8, 1 df, P = 0.03). Fur- thermore, birds were killed on only 9 of 87 (10%) days in which mist-netting was carried out, making the predation rate on these days 24 of 698 captures, significantly more than the 5 of 698 predation events expected had pre- dation been random across days (X2 = 91.2, 1 df, P < 0.001). Multiple birds were found dead simultaneously in five cases, comprising 14 of 24 (58%) predation events, and in three of these cases, two birds were found simul- taneously killed in the same net, comprising 6 of 24 (25%) predation events. Indistinct peaks in predation occurred in the late morn- ing and mid afternoon, and most birds killed were trapped in the net's lower panels with none in the highest (fourth) panel.

There is no strong evidence that predators returned to the nets over multiple days (which would indicate "learning" the presence of the food source). The mean day of predation was day 2.6 out of 4 days netting in each site where predation occurred. Only two of the predation events could have involved preda- tors returning the following day: an unidenti- fied small mammal returning to our fourth net at Ikuywa, Kakamega, and a blue monkey re- tuming to our second net-line at the same site.

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294 THE WILSON BULLETIN - Vol. 112. No. 2, June 200()

Predators were probably attracted to nets by noise and activity of captured birds. While our overall capture rate was 0.64 birds per net- hour, the mean capture rate (# birds captured in net/time since previous net-round in hours) for nets from which birds were killed was more than three times greater at 2.1 1 birds per net-hour. This idea is supported by a strong bias in the prey taken towards Yellow-whis- kered Greenbuls (Andropadus latirostris), which are noisy and relatively large (means of 24.6 ? 1.5 g in Kakamega and 25.1 ? 1.0 g on Mt Kenya). Of the victims with identifiable remains, 8 of 18 (44%) were Yellow-whis- kered Greenbuls, significantly more than ex- pected given the 854 (23%) individuals of the species amongst the 3703 total birds captured

(X2 = 5.1, 1 df, P = 0.02). Six victims were unidentified because there were only bloody holes left in the mist-nets, and/or we observed blue monkeys carrying off bird corpses.

The blue monkey was the most common predator, accounting for 17 of 24 (71%) pre- dation events. It was also a common predator in Tanzania (Cordeiro 1994). The species oc- curs in the Taita Hills but did not take birds from our mist-nets there. We regularly mist- netted birds attendant on swarms of safari ants (tribe Dorylini), but lost only one bird to ants, a White-starred Forest-Robin (Pogonocichla stellata) in the Taita Hills. An unidentified small terrestrial mammal killed two birds in Kakamega. Two birds were killed by African Goshawks (Accipiter tachiro), both of which were captured alongside their victims. The goshawk at Chehe on Mt. Kenya was caught two other times at the site, and so it is possible that this same bird also killed the White- starred Forest-Robin predated at this site. The only predatory birds that we mist-netted with- out prey were a Red-chested Owlet (Glauci- dium tephronotum), which is mainly an insec- tivore although it will take vertebrates (Zim- merman et al. 1996) in Kakamega; a Black Goshawk (Accipiter melanoleucus) which was observed to bounce out of a net in the Taita Hills; and another African Goshawk that was caught twice at a different Mt. Kenya site.

Occasional predation of birds caught in mist-nets is well known but it seems that the

predation we observed was elevated in forest fragments. All of these predation events oc- curred in small forest fragments (Ikuywa 1450 ha, W. Malava 50 ha, Chawia 50 ha, Mbololo 200 ha, Chehe 240 ha, Ragati 30 ha, Muthira 35 ha) and not a single one in the main Kak- amega Forest (10,000 ha). Elevated predation rates in forest fragments are usually explained by edge effects that allow generalist predators to access the forest (Murcia 1995), but in our case all of the predators were forest species. Alternative explanations could be that preda- tors have been concentrated into the forest fragments by the deforestation of the sur- rounding areas or that predators in the frag- ments were more stressed or more habituated to human presence and thus more willing to feed opportunistically on birds in mist-nets. Whatever the reason, it is clearly important that ornithologists conducting mist-net sur- veys in tropical forest fragments pay particu- lar attention to the danger of predation of birds caught in their mist-nets.

ACKNOWLEDGMENTS

Many thanks to J. Barnes, R. Barnes, D. Gitau, T Imboma, S. Karimi, J. Kageche Kihuria, J. Oyugi, N. Sagita, J. Tobias and C. Wilder for help with fieldwork; to the Ornithology Department of the National Muse- ums of Kenya, in particular L. A. Bennun, C. Jackson. L. Lens and E. Waiyaki, for help with organization; and to N. Cordeiro. A. Valero and two anonymous referees for comments. The study was funded by Na- tional Geographic Society Research Award #5542-95 to S. L. Pimm of the University of Tennessee.

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CORDEIRO, N. J. 1994. Opportunistic killers: blue mon- keys feed on forest birds. Folia Primatol. 63:84- 87.

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