National parks and other protected areas are playing an increasingly important role in the conservation of raptors (Liversidge 1984, Thiollay 2006). Not surprisingly, protected areas are critical refuges for most raptor species and they harbor the bulk of large raptor populations in Africa (Sorley and Andersen 1994, Herremans and Herremans-Tonnoeyr 2000, Thiollay 2006). Outside protected areas, raptors face increased threats associated with human activities including deforestation, overgrazing, increase in pesticide use, poisoning of predators, overhunting, and direct persecution (Thiollay 2006).

Raptors living within protected areas are assumed to be relatively free from these disturbances. However, substantial increases in human population have occurred in recent decades and protected areas are not immune to the pressures this has created on limited natural resources. A recent study showed that raptor numbers within 30 km of protected area boundaries were more than 40% lower than in the core, indicating a large edge effect on populations inside protected areas (Herremans and Herremans-Tonnoeyr 2000). In addition, some raptors, most notably vultures and large eagles, travel large distances in search of food and require extensive territories for breeding, making them more likely to range outside of protected areas to locations where they may become vulnerable to harmful human activities (Houston 1974, Pennycuick 1976, Herholdt et al. 1996).

The core of central Kenya's Laikipia District functions largely as a protected area despite being divided and privately owned. Almost half of the district's 966 600 ha consists of large-scale ranches and conservancies that are hospitable to wildlife and enable large-ranging birds like vultures and eagles to exist relatively free from habitat destruction and human interference. However, areas mostly outside of the core have seen considerable recent settlement by small-scale crop farmers and traditionally nomadic pastoralists. Approximately 35% of the district consists of settlements or small-scale urbanization (Frank et al. 2005).

To determine whether the abundance of raptors in central Kenya was changing, we monitored raptors every month for three years (2001–2003) in central Laikipia District.

Methods

Study Site

The study was conducted from January 2001–December 2003 at Mpala Research Centre (MRC), located in central Laikipia District in the highlands of central Kenya (Fig. 1). The habitat consists of wooded savanna, and the dominant tree species is Acacia drepanolobium. Large wildlife species include elephants (Loxodonta africana), reticulated giraffes (Giraffa camelopardalis), Grevy's zebras (Equus grevyi), elands (Taurotragus oryx), cape buffalos (Syncerus caffer), lions (Panthera leo), leopards (P. pardus), cheetahs (Acinonyx jubatus) and spotted hyenas (Crocuta crocuta).

Figure 1

Location of the Mpala Research Centre within Laikipia District in central Kenya.

The more common avian fauna in the study area comprises approximately 75 species (Ogada et al. 2008), of which nine are seasonal migrants. Common raptors include Rueppell's Vulture (Gyps rueppellii) and African White-backed Vulture (G. africanus), Black-winged Kites (Elanus caeruleus) and Eurasian Kestrels (Falco tinnunculus), which are seasonal migrants.

Results

Raptor Surveys

From January 2001–December 2003 we had 535 raptor sightings comprising 16 species, three of which were palearctic migrants. Of the species that were identified, the most common were Gyps vultures (69%) followed by Black-winged Kites (5%) and Tawny Eagles (4%).

The mean number of raptors observed each month declined significantly over time (P  =  0.05). Raptors declined 40% in the first year and 48% in the second, for an overall decline of 68% between 2001 and 2003. Scavenging birds accounted for most of the decline, although the correlation between year and mean monthly sightings of scavenging birds was not significant (P  =  0.40; Fig. 2). Scavenging birds declined approximately 50% per year and a total of 70% over the study period. Sightings for other raptor groups were too infrequent to permit analysis. The number of scavenging birds was not related to rainfall (P  =  0.28).

Figure 2

Annual monthly mean (+SE) number of raptor sightings during 2001–03, by raptor group. Species in each group are described in the text.

Discussion

In our surveys, raptors declined by 68% over three years. Scavenging birds were primarily responsible for this decline (Fig. 2). Scavenging birds, especially vultures, are declining worldwide due to poisoning, persecution, food shortages, and land-use changes (Pain et al. 2003, Thiollay 2006). Precipitous declines have occurred in West Africa (Thiollay 2006) and recent catastrophic population crashes have garnered attention in South Asia (Oaks et al. 2004, Prakash et al. 2003, Green et al. 2004). Our results suggest that this pattern is occurring in East Africa as well.

Because we sampled only a small area, the declines we observed may have resulted from a change in space use by scavenging birds rather than a region-wide population decline. Dispersion in subtropical African raptors is usually related to rainfall (Brown 1971, Herremans and Herremans-Tonnoeyr 2000). Rainfall at the study site during the period of our observations was not unusual in magnitude or distribution, and we detected no significant effect of rainfall on scavenging bird abundance. However, we cannot rule out the possibility that scavenging birds in our study area relocated to other sites. Nevertheless, abundance at our site declined significantly, whether these declines represent local or region-wide declines.

One hypothesis to explain the rapid decline we observed at the study site is a lack of available food. Large vultures in particular feed mainly on carrion of large- and medium-sized mammals and are obligate scavengers (Ruxton and Houston 2004). However, during the study period, wild herbivores in Laikipia District increased by 9% and domestic herbivores, especially sheep and goats, increased by 37% (Georgiadis and Ojwang 2001, Georgiadis et al. 2003). These data suggest that low food availability was not the primary cause of the declines in scavenging birds.

Another potential explanation for raptor decline is mortality from infectious disease. Although this hypothesis could not be ruled out, it seemed unlikely in our study region, as there were no reports of scavenging birds, e.g., vultures, dying of natural causes at a number of colonies where monitoring is ongoing (M. Virani pers. comm.). Infectious disease does not appear to be a major cause of mortality in Gyps vulture populations in southern Africa (Benson 2000).

A third hypothesis is that scavenging birds were poisoned at carcasses. Vultures are particularly vulnerable to poisoning, because they can rapidly congregate in large numbers at a carcass. Thus, one poisoned carcass can affect a large number of birds (Newton 1979). Indeed, reports of mass vulture poisonings within Africa are numerous (Borello 1985, Allen 1989, Anderson 1994, Simmons 1995). Two groups of neurotoxic pesticides, the organophosphorous and carbamate compounds, tend to be very acutely toxic to birds and cases of mortality are frequently reported (e.g., Mineau et al. 1999, Mineau 2005). Because of their acute toxicity, carbofurans are frequently implicated in abuse cases where raptors or other vertebrates are targeted directly (Mineau et al. 1999). Carbofuran, sold under the trade name Furadan, is an agricultural pesticide that is legally sold throughout Kenya, and its use as a cheap, effective poison is well known among farming communities in Kenya (Ogada and Kibuthu 2008). In Laikipia District soil, water and plant samples have shown high environmental contamination with concentrations of carbofuran and its two toxic metabolites (Otieno et al. 2010). Recent studies have implicated carbofuran in the mortality of African White-backed Vultures near our study site (Otieno et al. in press a, Otieno et al. in press b). These reports also confirmed widespread involvement of carbofuran in vulture poisoning incidents throughout Kenya (Mijele 2009, Otieno et al. in press a, Otieno et al. in press b). Vultures, and perhaps other scavenging birds, may have become unintended victims of poisoning of predators that occurs following livestock depredation events (Otieno 2009). Whether poisoning explains the rapid decline in abundance of scavenging birds at our site is currently under investigation.

Scavenging birds provide some of the most important yet underappreciated ecosystem services of any avian group (Houston and Cooper 1975, DeVault et al. 2003, Sekercioglu 2006). Yet among birds, nearly 40% of vultures are extinction-prone, more than any other functional group (Sekercioglu et al. 2004). All the vulture species observed during our study are on the IUCN Red List of Threatened Species (IUCN 2009). The apparent decline of scavenging birds that we observed in central Kenya warrants population monitoring to understand whether declines are local or regional and to elucidate causes of population decreases.