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.

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.

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.

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.

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.

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!

 

 

What happened to the Bahama Nuthatch?

On January 6-10, CCSD scientist Leighton Reid joined Bert Harris, Kelly Farrell, and David Wilcove on a search for what has become one of the rarest bird species in the western hemisphere.

Grand Bahama Island, only known home of the Bahama Nuthatch.

The Bahama Nuthatch (Sitta insularis) is or was a bird found nowhere except on Grand Bahama Island, a thin, 153-km long piece of weathered limestone lying 84 km east of Palm Beach, Florida.

 

The Bahama Nuthatch differs from a widespread southeastern US species, the Brown-headed Nuthatch (S. pusilla) in having a longer bill and a distinctive, high-pitched warbling call. It is a denizen of the Caribbean pine (Pinus caribaea) forests that cover about 60,000 ha of Grand Bahama Island. Perhaps always rare, the species was a lot more common in the 1960s than 30 years later in the early 1990s. Ten years ago, a nearly island-wide survey found only 14 individuals in a single tract of forest east of Freeport, the island’s largest settlement. A local nature guide, Erika Gates, regularly found one to three individuals of the species in this area through June 2016, but in early October 2016, Hurricane Matthew (Category 5) blew across the island, causing significant damage. The Bahama Nuthatch has not been detected since June 2016 despite Ms. Gates and others searching in its previous locations. It is considered “endangered” by the IUCN.

A postage stamp sheet commemorating the Bahama Nuthatch (Sitta insularis), an extremely rare species known only from a single island in the Bahamas.

Bert Harris plays the distinctive warbling call of the Bahama Nuthatch through a speaker into a very quiet Caribbean pine forest.

For six days in early January, four of us intensively searched the area around the two most recent sightings, the ones from May and June 2016. We focused on the core area at first and gradually expanded outwards as it became clear that we were finding no individuals at the former sites. We estimated that we searched an area of roughly 4600 hectares of pine forest over a period of 26 hours (88 person-hours). We travelled approximately 92 km of roads and trails, both driving and walking. Many of these were old logging roads, which crisscross the entire island. While driving, we stopped every 0.4 km (0.25 mi) and played a recording of the nuthatch’s distinctive call. While walking, we played the call more frequently.

We did not find any Bahama Nuthatches. We think that our group, the first to search for multiple days for this species since 2016, was also the first to fail to find it. Perhaps the species’ conservation status should be changed from endangered to critically endangered. Ideally, some Bahamian ornithologist will be able to survey again for the species during the coming breeding season, and if the species is rediscovered, its remaining habitat will be protected, restored, and expanded.

 

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In addition to the Bahama Nuthatch, we noted that several birds which breed exclusively in the pine forests were also very rare or absent. Bahama Yellowthroats (Geothlypis rostrata), Bahama Warblers (Setophaga flavescens), and Olive-capped Warblers (S. pityophila) were relatively abundant during surveys in 1968 and 2007, but Bahama Yellowthroats were totally absent from our search, and we found only a handful of Bahama Warblers and Olive-capped Warblers, making them even rarer in our survey than the nuthatch was in 2007 (though we searched during some relatively cold weather and during the non-breeding season). We also failed to detect a single Bahama Swallow (Tachycineta cyaneoviridis). Looking through historical records on eBird we noted that the West Indian Woodpecker (Melanerpes superciliaris) was formerly abundant across the island but has not been recently seen.

The causes of decline for the Bahama Nuthatch and perhaps for other breeding birds of the pine forests are mysterious. Grand Bahama Island has been extensively logged, initially for large diameter timber (prior to 1900) and later (1940s-1970s) for pulpwood. The expansion of the city of Freeport and tree-killing inundation by seawater over large areas have both reduced the potential habitat area. Feral cats, introduced raccoons, and corn snakes introduced in the 1990s could be predating native birds. We saw at least nine raccoons during our short time on Grand Bahama. Altered fire frequency and the increased frequency of Atlantic hurricanes may also be impacting the species, possibly by removing snags that are required for nesting.

As recently as 2011, 65 Caribbean ornithologists were able to view the Bahama Nuthatch simultaneously and within three meters of a tour van. Photo by Erika Gates.

Addendum: After six weeks and 400 km of searching a team in Grand Bahama has located at least five extant individuals of the Bahama Nuthatch!