Declining insectivorous birds in Brazilian Amazonia
This post was originally written for the Neotropical Birding magazine: NBC Conservation Awards update.
The year 2019 will mark the 40th anniversary of the establishment of one of the most important research sites in tropical ecology – the Biological Dynamics of Forest Fragments Project (BDFFP) near Manaus, Brazil. Ever since 1979, ornithologists have captured and banded understory birds to track their response to forest fragmentation. This effort primarily focused on mist-netting birds in forest patches of various sizes, before and after isolation, but also included sustained effort in control sites placed within undisturbed, continuous forest. Over the years, it became clear that carving up the rainforest into cookie-cutter pieces is bad for most birds, and many species – most strikingly, terrestrial insectivores –disappeared from isolated patches. Several reasons have been proposed to explain these observed declines, including that terrestrial insectivores may be sensitive to microclimate changes that accompany forest clearing: fragments tend to be hotter, drier, and brighter than these species’ typical haunts in the deep forest interior. This idea has come to be known as the “microclimate hypothesis”.
Phil Stouffer, Cameron Rutt, and I have recently turned our focus away from fragments, instead looking at bird capture data from control sites over the past four decades. These sites are large, pristine tracts of forest away from areas where chainsaws and machetes did their damage. Much to our surprise, we found a disturbing pattern – there were many fewer terrestrial insectivores caught during the last decade than in the early 1980s. Capture rates for species like the Wing-banded Antbird (Myrmornis torquata) and the Black-tailed Leaftosser (Sclerurus caudacutus) have fallen by more than 90%. Nowadays it is a genuine rarity for a birder to stumble across one of these species, but that apparently didn't use to be the case at our study site. The apparent disappearance of terrestrial insectivores even in continuous forest is very alarming, and we need to understand what’s going on.
That’s where the NBC Conservation Award comes in, providing critical funds for the next phase of our research. Unambiguous support for the microclimate hypothesis is currently lacking, but it could explain why terrestrial insectivores are disappearing from seemingly untouched rainforest. Under climate change, the Amazon appears to be getting hotter and drier. If we find that, in contrast to non-vulnerable species, declining birds actively seek cool and wet “refugia”, we might just see a link between the loss of terrestrial insectivores and climate change.
Thanks in part to the generous support of the NBC, we outfitted >80 terrestrial insectivores with backpacks carrying sensors that collect environmental data every few minutes for at least a year. As you might imagine, it can be incredibly difficult to catch a rare species in the first place. A case in point, last year my trusty field hand, Jairo Lopes, and I traversed over 165 km of forest trying to try and capture Black-tailed Leaftossers. But after 66 hours of continuous trawling with playback, we had caught only a single bird. Those familiar with the less-than-musical song of this species can hopefully sympathize with us. Although we now have five tagged leaftossers, this feat took a heavy toll on our ears and psyche.
If generally catching terrestrial insectivores in the first place is difficult, then recapturing the same individual a year later to download the collected data is like looking for a musical needle in the rainforest. It appears that these birds have a long memory when it comes to their capture and handling and shy away from nets and playback the next time, or they simply move to a new location. For instance, one of the leaftossers we recaptured in 2018 was 2.4 km from its original capture location. Draw a circle with that radius and that is a lot of forest to comb through without trails. Despite these obstacles, this season we recovered 50% of tags that were deployed last year, which only took five months of nearly continuous effort, often using two teams.
The research is still ongoing and, although we have lots of data, we have few concrete results to report at this juncture. However, while skimming through the millions of data points we currently have, I can already say that Neotropical birds are doing some interesting things when no one is watching. Let’s hope these data provide a clue as to why some of the rarest and most sensitive birds seem to be disappearing from undisturbed primary forest. Although bittersweet, it would be a fitting way to mark the occasion of the project’s 40th anniversary.
Cabo Frio, Biological Dynamics of Forest Fragments Project, Amazonas, Brazil, July 2017. The domain of the terrestrial insectivores is the relatively cool forest floor, a place reached by less than 5% of sunlight.