The impact of human presence and disturbances on birds has been well documented in the literature. As interpreted by Fernández-Juricic et al. (2001a), birds in general perceive human presence as a predation risk and tend to flee or show avoidance behavior. In the context of nature-based tourism, earlier studies have reported low abundance and species richness of birds under higher pedestrian or vehicle traffic in wildlife refuges (Klein et al. 1995; Fernández-Juricic 2000; Bautista et al. 2004; Burger et al. 2004; Marcum 2005). However, these relationships have been observed for avifauna in temperate regions and mostly for aquatic birds (Steven et al. 2011). Focusing on the avifaunal community in this tropical lowland rainforest in Sri Lanka, findings of this study further support the negative relationships found between the abundance of birds and increasing human recreational disturbances, elaborated by previous research.
Our results further suggest that not all tropical bird species are equally affected by human recreational disturbances. As in the case of temperate birds (Marcum 2005; Gill 2007), certain tropical bird species are capable of tolerating greater degrees of human recreational disturbances. The sensitivity of individual bird species to visitor recreational disturbances seems to vary with the stratum/layer of the rain forest usually occupied by bird species. Ashy-headed Laughing-thrush, Dark-fronted Babbler, Spot-winged Thrush and Tickell’s Blue Flycatcher, found to be particularly sensitive to recreational disturbances, are all forest birds that usually occupy the understory and edges of jungle trails (Kotagama and Goodale 2004; Goodale and Kotagama 2005; Harrison and Worfolk 2011; Warakagoda et al. 2012). As the intensity of human recreational disturbances increased, their presence near the trail declined significantly. This suggests that birds generally occupying the ground and understory are likely to be more affected by visitors. The response of birds to human recreational disturbances can vary by the distance to the disturbances (Fernández-Juricic et al. 2001a, b; Borgmann 2010). In the studied segment of our nature trail, only hiking is allowed and the distance between recreationalists using the nature trail and birds occupying the lower strata of the forest is smaller. Past studies have also shown that forms of recreation, such as hiking, can cause significant negative effects on a wide range of bird species (Buckley 2004; Pease et al. 2005).
Black-naped Monarch, Brown-breasted Flycatcher, Brown-headed Barbet, Greater Flameback, Malabar Trogon, Orange-billed Babbler, Sri Lanka Blue Magpie, Sri Lanka Scimitar Babbler and Yellow-browed Bulbul commonly occupy the sub-canopy layer of the forest. Out of nine sub-canopy occupying bird species, Black-naped Monarch, Brown-headed Barbet and Sri Lanka Blue Magpie showed higher tolerance to visitor recreational disturbances. The Brown-headed Barbet is not exclusively a forest bird and a common resident in a variety of habitats, including landscapes modified anthropologically (Harrison and Worfolk 2011). Their greater adaptability to modified habitats may explain why this species is not substantially avoiding edges of our trail under higher intensities of recreational disturbances. Although a forest bird, the Sri Lanka Blue Magpie did not show noticeable signs of avoidance behavior under human presence. Instead, they seem to be attracted towards small to medium sized visitor groups. This was evident by the high number of Sri Lanka Blue Magpies being recorded under low and moderate levels of recreational disturbances and their numbers showing a positive correlation with disturbance level, although the relationship was not statistically significant. This suggests a possible habituation of the Sri Lanka Blue Magpie population ranging around the Kudawa nature trail to low and moderate levels of recreational disturbances or human presence. In fact, during our field studies, it was often observed that a group of Sri Lanka Blue Magpies perching near the trail in anticipation of food, when visitor groups were present. Similar behavioral observations were made on ground occupying Sri Lanka Junglefowl. Visitors feeding blue magpies and junglefowls were a common observation during field studies. Such visitor behavior along with exposure to recurring recreational disturbances can alter the normal behavior of birds and induce habituation to human presence (Marcum 2005; Peters and Otis 2006; Baudains and Lloyd 2007; Smith-Castro and Rodewald 2010).
In general, recreational disturbances had less effect on birds occupying canopy and emergent layers. Layard’s Parakeet and Sri Lanka Crested Drongo commonly occupy the canopy or the upper parts of the sub-canopy (Kotagama and Goodale 2004; Goodale and Kotagama 2005; Harrison and Worfolk 2011; Warakagoda et al. 2012). As indicated by the results of our ANOVAs in Appendix 2, their abundance was found to be independent from the intensity of recreational disturbances at various distances from the trail. Among the bird species occupying the emergent layer of the forest, the Sri Lanka Myna—the only species with sufficient data for analysis in this study, also showed no statistically significant variation in its abundance under different intensities of recreational disturbances at various distances from the trail. Since these species commonly forage and occupy the canopy above 25 m from the ground, they may be less inclined to perceive human presence as a predation risk (Fernández-Juricic et al. 2001b) and thus less concerned about leaving the edges of this nature trail. However, despite being canopy occupants, the abundance of the Red-faced Malkoha and Scarlet Minivet was negatively correlated with the intensity of recreational disturbances at the trail, indicating their comparatively smaller tolerance to human presence.
The abundance of Ashy-headed Laughing-thrush, Brown-breasted Flycatcher, Greater Flameback, Dark-fronted Babbler, Malabar Trogon, Orange-billed Babbler, Spot-Winged Thrush, Sri Lanka Scimitar Babbler and Yellow-browed Bulbul was higher along the jungle trail when no humans were present. These species generally prefer relatively open spaces inside the forest (Harrison and Worfolk 2011; Warakagoda et al. 2012). Under moderate and high recreational disturbances, the abundance of these species increased at the 75 m distance level from the trail. These fluctuations in abundance provide possible evidence for differing fleeing distances of bird species. The Ashy-headed Laughing-thrush, Dark-fronted Babbler, Orange-billed Babbler and Sri Lanka Scimitar Babbler are characteristically weak flyers (Siriwardhane 2007; Harrison and Worfolk 2011; Warakagoda et al, 2012) and do not flush far from the jungle trail. A statistically significant dependence of abundance on the intensity of recreational disturbances was observed for Malabar Trogons, indicating that they flush beyond 50 m under increasing visitor disturbances. However, other species such as the Brown-breasted Flycatcher, Greater Flameback and Yellow-browed Bulbul with good flight abilities did not seem to flush beyond 50 m from the trail. Hence there is only weak evidence to support the hypothesis that strong flyers flush deeper into the forest.
Territory size and territoriality may be important factors that determine how far birds would move away from a disturbance. Species, defending territories, are more likely to have reduced flight initiation distances and are less likely to move out of their territories under disturbances, although such responses are not always universal (Stankowich and Blumstein 2005). The Spot-winged Thrush is a highly territorial bird that seldom abandons its territory under disturbances (Weerakoon 2007). Instead, such species rely on camouflage in the vegetation. The abundance of Tickell’s Blue Flycatcher also did not increase substantially under disturbances in the inner parts of the forest, indicating its territorial behavior (Rasmussen and Anderton 2012).
Another possible factor that can affect the response of different bird species to recreational disturbance in this forest is the predominance of mixed-species bird flocks. Many bird species recorded in this study, such as Orange-billed Babbler, Ashy-headed Laughing-thrush, Dark-fronted Babbler, Malabar Trogon, Red-faced Malkoha, Scarlet Minivet, and Yellow-browed Bulbul are known to participate quite regularly in mixed-species bird flocks (Kotagama and Goodale 2004; Goodale and Kotagama 2005). It was observed that the abundance of these species including the Orange-billed Babbler, the nuclear species of the flock system, were negatively correlated with increasing intensities of recreational disturbances. As such, mixed-species bird flocks may be negatively affected by visitor groups and under such circumstances, a flock can avoid habitat edges and travel quite quickly to the interior of the forest. The increase in abundance of the Malabar Trogon and Black-naped Monarch at 75 m level with the intensity of recreational disturbances may be explained by this phenomenon. Although a bird with a small territory, the abundance of the Dark-fronted Babbler was positively correlated with intensity of recreational disturbance at the 75 m distance level. They are known to join mixed-species foraging bird flocks usually within their territory (Shermila and Wikramasinghe 2013). Moreover, other research suggest that flush distances of certain bird species can be further associated with habitat structure represented by features such as vegetation cover, shrub and tree height (Fernández-Juricic et al. 2001b).
Some limitations of the study should be acknowledged. First, the structure of the forest near this nature trail is somewhat open and disturbed and differs from the forest interior. As such, the ability to detect birds is comparatively higher near the trail than inside the forest. Although efforts were made during the study design to avoid significantly open areas near the trail, higher abundance of birds near the trail may still be affected by greater visibility. All the same, there were several edge-specialists among the bird species recorded. It is well documented that mixed-species bird flocks of the Sinharaja forest tend to use habitat edges such as cut-over areas near the trails quite heavily (Kotagama and Goodale 2004; Goodale and Kotagama 2005). Secondly, the bird activity is relatively high in the early morning, apparently a time of low visitor activity. In general, larger visitor groups were more frequent after 10:00 a.m. while smaller visitor groups including birdwatchers entered the reserve during the early hours of the day, i.e., between 6:00 and 8:00 a.m. The study design accounted for this factor by avoiding bird census for this period, yet the effect of this compounding variable on bird abundance cannot be completely ignored.
Also, it is worth studying the movements of individually marked birds to determine their actual distance of displacement, minimum approaching distance, habitats where they seek refuge when disturbed and how they habituate due to their regular contact with visitors. Such information will be vital in recreational planning and management to minimize the disturbances to avifauna.
Implications for management
Recent visitor studies conducted at the Sinharaja World Heritage Forest have revealed that most visitors to this forest are “picnickers”, defined as the visitors who are visiting the destination with the primary motive of spending free time with family/friends with nature observation/appreciation a secondary motive (Perera et al. 2012). Such visitors often show a type of behavior that is potentially disturbing to wildlife. If human recreational activities in a forest reserve are displacing birds from their preferred habitats, the primary conservation objectives of the forest reserve are not met.
Findings of this research stress the importance of introducing sound visitor management strategies. Many birds can tolerate low intensity recreational disturbances, i.e., disturbances due to the presence of smaller and less noisy visitor groups (Remacha et al. 2011). Hence, visitor management strategies should be ideally focused on limiting the group size while encouraging appropriate visitor behavior. Strengthening park services such as nature interpretation, visitor education and awareness play a key role in this regard. Only small visitor groups should be allowed to more sensitive areas of the forest. Jungle trails that run through sensitive areas/habitats of the forest should be allowed for low intensity recreational uses under close supervision of management.
Owing to the inherent nature of the study design, the findings of this study may have local applicability. Similar studies should be repeated in other tropical forest settings elsewhere to improve our understanding of how avian communities respond to various levels of recreational disturbances. Furthermore, the methodology used in this study to develop recreational disturbance profiles is based on relative noise levels (a crude measures of noise) and group size. Possible visual disturbances for birds due to the color of clothes that visitors are wearing were not considered. Simulations studies could be done to understand such scenarios better. Nonetheless, reserve managers can directly incorporate the findings of this study into adaptive management cycles based on visual observations of the distribution of bird assemblages and recreational uses of nature trails in tropical lowland forest settings without engaging in time-consuming and expensive site-specific studies.