BIOLOGISTS from the University of the Philippines Diliman-College of Science (UPD-CS) have released the results of a study with various environmentalists that analyzed tree cavities, or tree holes, at the Subic Watershed Forest Reserve in Zambales.

The research — which sought to obtain information that would help improve habitat management and protect threatened species — was conducted in established and off-trail routes in Subic from February to June 2022.

Many birds rely on tree cavities for nesting and raising their young. Woodpeckers, known as primary excavators, can create their own cavities.

But non-excavator birds — also called secondary cavity nesters — depend on naturally occurring or previously excavated cavities for reproduction. These birds face challenges, such as scarcity of suitable cavities, which can limit their population growth.

To protect them, it is important to understand the specific nesting needs of each bird species and how they interact with one another.

In the Philippines, where forests are vulnerable to change, there is still limited information on how these birds nest and how they are connected.

Second study of its kind in Asia

“This study is the second of its kind in Asia — and the first in the Philippines — that examines nest web interactions at the community level,” the team of biologists said in a statement.

Nest web interactions refer to the way different animals, especially birds, are connected through their use of tree cavities for nesting.

“Unlike in temperate Europe and subtropical South America, where woodpeckers are not considered keystone species — or animals that have a particularly large impact on their environment — this study provides evidence of their keystone role in tropical forests, similar to findings in India and Canada,” the biologists explained.

The research team was composed of Vince Angelo Gicaraya from the Institute of Environmental Science and Meteorology (IESM) and the Institute of Biology (IB) at UPD-CS; Dr. Carmela Española from UPD-CS’s IB and the Natural Sciences Research Institute (NSRI); biologists from the Biodiversity Management Bureau of the Department of Environment and Natural Resources (DENR) and from Pampanga; and local guides and volunteers.

A white-bellied woodpecker inside its nest. PHOTOS BY VINCE ANGELO GICARAYA

The team found nests of 10 species of cavity-nesting birds in both living and dead native trees, primarily belonging to kupang trees (Parkia timoriana) and white lauan trees (Shorea contorta).

Using multinomial logistic regression modeling, the group discovered that cavity characteristics — such as depth, volume and entrance area — play an important role in how bird species choose their nesting sites.

The single rope climbing technique was used to access nests in trees.

The research, aside from showing the importance of conserving and planting native trees to support the nesting opportunities of cavity-nesting species, also provides baseline data (measurements) for developing conservation tools (e.g., species-specific nest boxes) for the Philippine Red List’s Critically Endangered and Vulnerable species Blue-naped Parrot (Tanygnathus lucionensis) and Luzon Hornbill (Penelopides manillae), respectively.

Made by woodpeckers

The study also showed that 63 percent of nests used by non-excavator birds were made by woodpeckers.

“These results highlight the urgent need to prioritize research and conservation efforts for primary excavators — birds like woodpeckers that create their own nesting holes — in the Philippines, where both these species and those that rely on their nesting sites remain largely understudied,” the researchers said.

The study, titled “Cavity characteristics explain the differences in realized nest niches among tree cavity-nesting birds in a lowland tropical forest in Luzon Island, Philippines,” was published in Annales Zoologici Fennici, a Finland-based, peer-reviewed journal which shares original research reports, in-depth reviews, and commentary on all aspects of animal ecology, evolution and related fields.

It was funded by NSRI and the DENR-Foreign-Assisted and Special Projects Service, in collaboration with the Biodiversity Management Bureau (BMB) and IB.