What does the Black-faced Antthrush tell us about tropical forest restoration?

Anna Spiers (University of Colorado Boulder) describes a recent field experiment done with Emma Singer (Hamlin College) and Leighton Reid (CCSD) during an Organization for Tropical Studies Field Ecology Course in Costa Rica.

Bird diversity and forest restoration are synergistic. Birds facilitate forest regeneration through seed dispersal, pest control, and pollination. Forest restoration replenishes lost bird habitat by providing food, protection from predators, and suitable territory for breeding and nesting. Monitoring bird communities in a regenerating forest is an effective strategy to gauge the success of restoration.

While some birds are flexible regarding the quality of their habitat, others require a narrower set of conditions to survive. One such bird is the Black-faced Antthrush (Formicarius analis), a medium-sized, ground-dwelling insect-eater, easily distinguished by its plaintive song and chicken-like strut. The bird spends its days flipping over leaves and sticks with its bill to expose tasty ants, beetles, and other arthropods (and sometimes small vertebrates). A member of a bird family highly threatened by forest fragmentation (Formicariidae), the Black-faced Antthrush is known to disappear from small forest fragments and to struggle crossing even narrow strips of open space. Finding such sensitive birds in a regenerating forest is a positive signal that forest restoration is increasing habitat for forest-dependent species.

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Black-faced Antthrush (Formicarius analis) strutting across the rainforest floor. Image: Luke Seitz/Macaulay Library at the Cornell Lab of Ornithology (ML54054261).

Earlier this month, we did an experiment to find out how different forest restoration strategies affect the Black-faced Antthrush. Specifically, we tested whether the bird exhibited a stronger territorial response in tree plantations, naturally-regenerating secondary forests, or areas where patches of trees (tree islands) had been planted to stimulate forest recovery. We expected to find that birds would be more defensive of areas where trees had been planted, given that these areas had a more closed canopy and more leaf litter for the birds to pick through for arthropods.

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Leighton holds up a speaker to conduct a bird call playback. Unsurprisingly, there was no response in this scrubby, abandoned pasture (one of the control points in our experiment). Image: Martha Bonilla-Moheno.

To test the bird’s territorial response, we amplified a locally-recorded sound file of the bird’s vocalization and recorded its response. We noted how long it took for the bird to respond, how many notes it sang in response, and how close it approached the speaker. For this species, a short call with 4 notes is a “hello”, but a long call with upwards of 12 notes is a warning to let the other birds know that this territory is taken.

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Our study area at Las Cruces Biological Station in southern Costa Rica. Each of the two restoration sites contained a tree plantation, a natural regeneration area, and a “tree island” area where patches of trees were planted to kick-start forest recovery. Image: Google Earth 2018.

Antthrushes defended restoration areas where trees were planted

As we expected, Black-faced Antthrushes responded more quickly and more forcefully when we taunted them with calls broadcast from tree plantations and tree island plantings – an indication that they were expending more energy to defend these areas. However, we only found this at one of the two restoration sites. The other site was a veritable antthrush desert with not a single response during any of our trials. Leighton’s collaborator Juan Abel Rosales often finds Black-faced Antthrushes at both sites, but this second site is near a road and dogs occasionally wander into the regenerating forest, possibly causing birds to temporarily abandon this area.

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Black-faced Antthrushes responded quickly and with many tooting notes when we played their song to them from tree islands, plantation, and mature forest, but they responded not at all in abandoned pastures or in natural regeneration. The data representing restoration treatments are from one site only – at the other site we recorded no birds during any trials.

Tree islands and plantation had a couple of habitat features that natural regeneration lacked. First, the understory was more open, providing ground-dwelling birds with greater visiblity. Second, planted areas also had deeper leaf litter, and leaf litter is essential for a bird that makes a living flipping leaves to find its dinner.

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Understory comparison between natural regeneration (left) and a tree plantation (right). Both have been recovering for 15 years. Natural regeneration vegetation is thick and still grassy from pasture days. A closing canopy in the tree plantation produced a thinner, more visible understory, with lots of nice leaf litter, full of delicious arthropods.

So what does the Black-faced Antthrush tell us about forest restoration?

 It may be telling us two things. First, restored forests growing up alongside remnant ones can be valuable habitat worth defending. When birds spend time calling, that is time that they do not spend foraging, and they can pay a price with their energy budget. Second, tree planting may create habitat for these birds faster than natural forest regeneration – although natural regeneration is highly variable from site to site, and we only found a pattern at one site right next to an old-growth forest. Promisingly, we did not see a difference between tree islands and the tree plantation, which suggests that we could plant fewer trees and still see the return of a forest-dependent bird species within about 15 years.

For more information about the Islas Project (with the tree islands) see previous NHER posts here, here, and here. Thanks to Bert Harris for some of the ideas that we used in this project!

 

 

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