Habitat fragmentation reduces nest survival in an Afrotropical bird community in a biodiversity hotspot

Abstract

Ecologists have long hypothesized that fragmentation of tropical landscapes reduces avian nest success. However, this hypothesis has not been rigorously assessed because of the difficulty of finding large numbers of well-hidden nests in tropical forests. Here we report that in the East Usambara Mountains in Tanzania, which are part of the Eastern Arc Mountains, a global biodiversity hotspot, that daily nest survival rate and nest success for seven of eight common understory bird species that we examined over a single breeding season were significantly lower in fragmented than in continuous forest, with the odds of nest failure for these seven species ranging from 1.9 to 196.8 times higher in fragmented than continuous forest. Cup-shaped nests were particularly vulnerable in fragments. We then examined over six breeding seasons and 14 study sites in a multivariable survival analysis the influence of landscape structure and nest location on daily nest survival for 13 common species representing 1,272 nests and four nest types (plate, cup, dome, and pouch). Across species and nest types, area, distance of nest to edge, and nest height had a dominant influence on daily nest survival, with area being positively related to nest survival and distance of nest to edge and nest height being both positively and negatively associated with daily nest survival. Our results indicate that multiple environmental factors contribute to reduce nest survival within a tropical understory bird community in a fragmented landscape and that maintaining large continuous forest is important for enhancing nest survival for Afrotropical understory birds.

Fig. 1.

Comparison between fragmented and continuous forest of (A) mean (±1 SE) daily survival rate of nests for eight understory bird species; (B) mean nest success for the same species. AB, African broadbill; LG, little greenbul; CG, Cabanis's greenbul; OGT, orange ground thrush; WTCF, white-tailed crested flycatcher; FB, forest batis; CS, collared sunbird; OS, olive sunbird. An asterisk indicates that the 95% confidence interval for a fragmentation effect on daily survival rate for the species does not overlap zero.

Fig. 2.

Comparison between fragmented and continuous forest of mean (±1 SE) daily survival rate of nests among nest types (cup, pouch, and dome).

Fig. 3.

Relation between daily survival rate (DSR) of nests and area for nine understory bird species. Solid lines represent the estimated DSR obtained by backtransforming the appropriate logit-linear model with all covariates except area set to their mean value. Dashed lines represent upper and lower 95% confidence intervals.

Fig. 4.

Relation between daily survival rate (DSR) of nests and distance of nest to edge for 11 understory bird species. Solid lines represent the estimated DSR obtained by backtransforming the appropriate logit-linear model with all covariates except distance of nest to edge set to their mean value. Dashed lines represent upper and lower 95% confidence intervals.

Fig. 5.

Relation between daily survival rate (DSR) of nests and nest height for eight understory bird species. Solid lines represent the estimated DSR obtained by backtransforming the appropriate logit-linear model with all covariates except nest height set to their mean value. Dashed lines represent upper and lower 95% confidence intervals.