Edge effects on nesting success of cavity-nesting birds in fragmented forests

Abstract

Forest fragmentation leads to the creation of isolated forest patches and habitat edges with subsequent impact on forest-interior bird species. Although the effects of fragmentation and edge on avian nesting success are well documented for open cup-nesting species in eastern deciduous forests in North America, it is unclear whether these effects are common for all birds nesting in predominantly forested landscapes. In particular, edge effects on nesting success of cavity-nesting birds are poorly understood. Using natural cavity nests, we examined nesting success of four species of cavity-nesting birds (two nonexcavators and two excavators), the yellow-rumped flycatcher (Ficedula zanthopygia), the great tit (Parus major), the great spotted woodpecker (Picoides major), and the grey-faced woodpecker (Picus canus) in relation to forest edges in Zuojia Nature Reserve, Jilin province, northeastern China. Our primary objective was to assess whether distance to the edge of agricultural lands was related to nesting success for cavity-nesting birds in fragmented forests. A total of 439 natural cavity nests of the four species were located and monitored during four breeding seasons. Probability of nest success was influenced by distance to forest edge for nonexcavators, but not for excavators. The rate of nesting success of the two nonexcavators, yellow-rumped flycatcher and the great tit, increased with distance from the edges. For all cavity nests, nesting success was 0.43 at 0–100 m, 0.56 at 101–200 m, 0.68 at 201–300 m, 0.61 at 301–400 m, 0.77 at 401–500 m from the edges. Nesting success ranged from 0.57 for the yellow-rumped flycatcher to 0.89 for the Grey-faced Woodpecker. Failed nests were often occupied by nest-site competitors (accounting for 68%). However, predation only accounted for 20% of all nest failures. Our results suggest that negative edge effects do exist for some cavity-nesting birds, especially for nonexcavator species.

Introduction

Forest fragmentation affects the distribution and abundance of organisms by reducing the amount and proximity of remnant patches of suitable habitat and increasing the amount of edges (Andrén, 1992, Boulinier et al., 1998, Ford et al., 2001, Maina and Jackson, 2003). Edges may affect the organisms by causing changes in the biotic and abiotic conditions some distance into the forest, such as increased amounts of sunlight, high wind speeds, and larger fluctuations in temperature and humidity (Saunders et al., 1991, Murica, 1995). Avian species may respond to one or a combination of these changes in the landscape as a result of different biological mechanisms (Robinson et al., 1995, Donovan et al., 1997). Species that require forest interior may avoid edges due to altered microclimate, vegetation structure, or high density of predators or brood parasites (Yahner and Scott, 1988, Malcolm, 1994, Marini et al., 1995, Pasitschniak-Arts et al., 1998, Stephens et al., 2003). On the other hand, although predators and brood parasites may be more abundant or active in the adjacent habitat and edge, some bird species were concentrated near to edge areas. This paradox of high bird abundance and richness but low nesting success near edges was termed an ecological trap by Gates and Gysel (1978).

Many studies have tested the occurrence of edge effects on nesting success of forest birds and concluded that high predation and brood parasitism rates near edges were main causes of nest failures (Hannon and Cotterill, 1998, Manolis et al., 2002). However, an edge effect has not been found in all studies (Angelstam, 1986, Santos and Telleria, 1992, Hanski et al., 1996, Morrison and Bolger, 2002). Early studies suggested that abrupt or permanent edges generally were thought to be associated with higher rates of predation and parasitism than gradual or regenerating edges (Suarez et al., 1997, Deng et al., 2003), but many contradictory results exist, particularly in forest landscapes (Yahner and Scott, 1988, Rodewald, 2002). These conflicting results may be explained in part by the mediation of edge effects by life history traits of organisms (e.g., open- vs. cavity-nesting bids) and landscape characteristics.

Most early studies were completed on open cup-nesting birds. However, nest predation and brood parasitism are unlikely to provide a general explanation for the edge effects on bird species that nest in more protected sites such as cavities (Matthysen and Adriaensen, 1998). Nesting success of cavity-nesting birds near edges may be influenced by other factors such as changes in competition for nest sites (Deng, 2001). Very few studies have assessed nesting success of cavity-nesting birds near edges in fragmented forests (Nour et al., 1993, Matthysen and Adriaensen, 1998). Many studies of effects on nesting success have used artificial nests (Wilcove, 1985, Picman and Schriml, 1994, Hartley and Hunter, 1998, Githiru et al., 2005). Such studies can identify general patterns of variation in nest predation intensity (Haskell, 1995, Sloan et al., 1998). However, predation and brood parasitism rates on artificial nests may be poorly or inconsistently correlated with predation and parasitism rates on real nests (Ortega et al., 1998, Moore and Robinson, 2004).

Here, we examine edge effects on nesting success for natural nests of four cavity-nesting bird species (two excavators and two nonexcavators) in a forested landscape in which edges were created by farmland reclamation. We also used data from these four species to determine whether life history characteristics (excavators vs. nonexcavators) affect the relationship between nest success and distances to edge.