Here’s the thing about birds that nest in tree cavities: not just any hole will do. Many species depend on certain plants for shelter, breeding, and roosting. If the host trees vanish, so could the birds. That’s already happened in some familiar cases, such as the extinction of the Ivory-billed Woodpecker, which primarily nested in giant cypresses in Louisiana, Arkansas, and Florida. Once those trees were cut down by loggers, the Ivory-billed Woodpecker disappeared with them.
To help other threatened woodpeckers avoid the same fate, the logging industry should require loggers to leave behind the necessary numbers and types of trees the birds need to thrive. Should being the key word there. Current practices merely involve skipping over the large, dead trees, called "snags," that woodpeckers can use for their nests.
Science has shown that selective logging does more than just save woodpeckers: Protecting their spaces also offers home security to other animals. Woodpeckers typically abandon their nests after a time, leaving them available for use by other birds, often for decades. This is known as the “nest web”—interactions between species that utilize each other’s nests, kind of like an extensive network of bird timeshares.
At least, that’s the way it works in North America. But research in Argentina has revealed that nest webs there may operate differently, and new modeling based on this research has found that these differences in nesting preferences could have important implications for the country's forest management.
“Most non-excavator species here do not use woodpecker-made cavities,” says Román Ruggera, a researcher with Fundación CEBio in Jujuy, Argentina. He and his colleagues examined cavities over a 10-year period and discovered that the region’s parrots, owls, woodcreepers, and more rely on natural crannies in living, decaying trees. Unlike in North America, these birds recycle woodpecker nests only nine percent of the time.
When Ruggera and his team began their research, they didn't set out to disprove the long-held cavity-sharing behavior; they were interested in seeing how the nest web worked in the few remaining areas of untouched, subtropical piedmont forest in northwest Argentina. The vast majority of the low-lying, deciduous forests in the region, he says, have already been converted to agriculture, pasture, and urban areas, so protecting what's left is essential for preserving the region’s biodiversity.
But once the team discovered this surprising information, it decided to investigate the potential impacts further. Published in the journal of Forest Ecology & Management earlier this summer, Ruggera and his coauthors' most recent research examined the behavior of 15 different cavity-nesting bird species in 11 types of trees. Starting with four local types of woodpecker—Cream-backed Woodpecker, Golden-olive Woodpecker, Dot-fronted Woodpecker, and White-barred Piculet—Ruggera and his team performed a series of simulations to see what would happen if the three most important tree species in the area—Castelo boxwood, vilca, and cedro orán—disappeared. All three kinds of wood are heavily valued by loggers, with a 2013 study calling cedro orán severely exploited. The scientists also modeled a second and third scenario to include non-excavator birds, such as owls and woodcreepers.
Again, the researchers uncovered some major differences from the North American pattern. In their simulations, the woodpeckers and owls persisted after the loss of the trees because they depended on many kinds of wood. Seven other bird species, however, were likely to go locally extinct if too many of the live trees were felled: the Streaked Flycatcher, Turquoise-fronted Parrot, Great Rufous Woodcreeper, Scaly-headed Parrot, Black-banded Woodcreeper, Hoy’s Screech Owl, and a local variety of forest falcon.
“If a given bird species only uses cavities from a given tree species, and that tree species goes extinct, that bird species goes also extinct,” Ruggera says. At least, according to the model. It’s possible that the birds could find alternatives if their preferred trees vanish, a possibility his ongoing research at other sites aims to answer.
Woodpecker researcher and Virginia Tech biology professor Jeff Walters, who was not involved with the research, says the work out of Argentina is important. “Nest webs are a fairly new area of study,” he says. Ruggera's findings have new implications for conservation beyond saving old dead trees for excavators, Walters says. “Now we realize it's much broader. You need to understand the underlying mechanism for tree hole creation.”
U.S. Forest Service technician Gina Tarbill, who has studied the woodpecker nest web in the Sierra Nevada region, says this means forestry managers in Argentina may need to go beyond preserving snags. "Woodpeckers are ecosystem engineers in many coniferous forests, so snag recommendations have primarily focused on meeting their needs,” she says. “The authors were wise to point out that this may not be the case in the deciduous forests of South America, and that the conservation of the cavity-dependent community will require retaining both snags and live trees.”
Time is not on the birds’ side. A 2015 report from the United Nations Food and Agriculture Office found that Argentina lost 22 percent of its forests over the previous quarter of a century and that it continues to lose another one percent per year. Ruggera says the logging industry needs to adopt new management policies soon to avoid “a fast and drastic biodiversity decline in these forest systems.”
But there might be a straightforward solution in this case. The researchers say Castelo boxwood, the most essential of the three tree types, should be considered a priority for conservation, especially when landowners are making plans about what to cut down or preserve on their properties. Ruggera, for one, is hopeful that loggers will welcome this new information and work with conservationists to save the boxwood—and all the birds that call it home.