Effects of forest fragmentation on avian breeding activity

Abstract

Biodiversity declines and ecosystem decay follow forest fragmentation; initially, abundant species may become rare or be extirpated. Underlying mechanisms behind delayed extirpation of certain species following forest fragmentation are unknown. Species declines may be attributed to an inadequate number of breeding adults required to replace the population or decreased juvenile survival rate due to reduced recruitment or increased nest predation pressures. We used 10 years of avian banding data, 5 years before and 4 years after fragment isolation, from the Biological Dynamics of Forest Fragments Project, carried out near Manaus, Brazil, to investigate the breeding activity hypothesis that there is less breeding activity and fewer young after relative to before fragment isolation. We compared the capture rates of active breeding and young birds in 3 forest types (primary forest, fragment before isolation, and fragment after isolation) and the proportion of active breeding and young birds with all birds in each unique fragment type before and after isolation. We grouped all bird species by diet (insectivore or frugivore) and nesting strategy (open cup, cavity, or enclosed) to allow further comparisons among forest types. We found support for the breeding activity hypothesis in insectivorous and frugivorous birds (effect sizes 0.45 and 0.53, respectively) and in birds with open‐cup and enclosed nesting strategies (effect sizes 0.56 and 0.44, respectively) such that on average there were more breeding birds in fragments before isolation relative to after isolation. A larger proportion of birds in the community were actively breeding before fragment isolation (72%) than after fragment isolation (11%). Unexpectedly, there was no significant decrease in the number of young birds after fragment isolation, although sample sizes for young were small (n = 43). This may have been due to sustained immigration of young birds to fragments after isolation. Together, our results provide some of the strongest evidence to date that avian breeding activity decreases in response to fragment isolation, which could be a fundamental mechanism contributing to ecosystem decay.

Habitat use of Amazonian birds varies by age and foraging guild along a disturbance gradient

“…Previous work in this system showed diminished breeding activity in secondary forest and forest fragments compared with primary forest [33,34], which suggests that we were likely unable to detect differences in young birds across habitat types due to the low probability of capturing young birds, although we were able to detect fewer young insectivores in fragments. In addition, while the species at the BDFFP show asynchrony in breeding and many species seem to breed year-round [53], the timing of the mist net sampling often corresponded to the low point in breeding activity for many species [53], which might exert a downward bias for young bird captures.…”

“…Based on previous results that insectivorous birds in secondary forest and forest fragments exhibit less breeding activity than in primary forest [33,34], we predict that there will be fewer adult and young insectivorous birds in secondary forest and small forest fragments compared with primary forest due to the relatively low-quality habitat for breeding.…”

“…Based on these seemingly simple questions we have several predictions about how individuals in different foraging guilds and of different ages are distributed among the landscape: Based on previous results that insectivorous birds in secondary forest and forest fragments exhibit less breeding activity than in primary forest [33,34], we predict that there will be fewer adult and young insectivorous birds in secondary forest and small forest fragments compared with primary forest due to the relatively low-quality habitat for breeding.Based on previous results that frugivores and omnivores are more likely to move through a second growth matrix than insectivore birds [32], we predict that all age classes of frugivorous and omnivorous birds will be similarly found across habitat types given their lack of territoriality and movements in pursuit of food or reproductive opportunities at leks.If adolescent insectivore birds are waiting for territorial vacancies in supposedly higher quality habitats, adolescents will be in the same habitats as the adults. Alternatively, if adolescent birds use lower quality habitat or get pushed to lower quality habitats by older birds, there should be more adolescents in lower quality habitats compared with higher quality ones.…”

“…For example, understory insectivore birds are less likely to occupy smaller forest fragments and secondary forest than frugivore, omnivore or nectarivore species [26,31,32]. In addition, fewer insectivore bird species show breeding activity in forest fragments and secondary forest compared with primary forest [33], while many frugivorous species breed in forest fragments and secondary forest [34]. Frugivores tend to move longer distances and exhibit fewer territorial behaviours than insectivores [35][36][37].…”