Tawi-Tawi is a group of islands in the Sulu Sea at the very bottom of the Philippine archipelago, nearer Borneo than Manila. The beaches are still quiet enough at dawn to hear individual birds moving thru the undergrowth, and the forest almost reaches the waterline. It is precisely the kind of location where an age-old animal tactic—burying eggs in warm sand and then moving on—has continued to be effective for longer than anyone has been able to document. Until today, it seems.
There isn’t much casual interest in the Philippine Scrubfowl. It lacks the characteristics of a kingfisher or hornbill. It is a low-lying, stocky, brownish bird that field guide writers tactfully refer to as “unassuming.” Its reproductive method is what truly sets it apart. The scrubfowl is one of the very few birds in the megapode family that does not require body heat to nurture its eggs. Instead, they bury them, depending on outside heat sources such as volcanic soil, decaying flora, or sand that has been heated by the sun to complete the task that would otherwise be completed by sitting on a nest. It is a mechanism that has functioned in dozens of island ecosystems across the Indo-Pacific for millions of years. To establish and maintain a nesting site, it needs the proper substrate, the proper ground temperature, and—most importantly—enough undisturbed space.
In the past, Tawi-Tawi’s coastline beaches have offered just those circumstances. Scrubfowl eggs require a stable thermal environment to grow properly, which is created by dark, fine sand that absorbs and retains heat throughout the day. However, over the past 20 years, those same beaches have been under increasing strain.
As coastal settlements have increased, human egg harvesting, a long-standing tradition in the area where megapode eggs have long been taken as a food source, has become more intense. When the birds normally go to and from nesting locations during low visibility hours, nighttime light from communities and fishing activities disturbs them. Additionally, sea level rise is flooding low-lying sand burrows with seawater during high tides, freezing eggs below the temperatures required for development and, in some cases, completely washing nests away. This is happening more slowly than the other forces but with greater tenacity.
The birds seem to be responding by moving inland, away from the beaches, and into the understory of the forest. There, scrubfowl have been seen building what scientists refer to as “compost mounds,” which are massive piles of collected decomposing twigs, leaf detritus, and soil that can have a diameter of several meters. These mounds are not heated by the sun. It originates from microbial decomposition, which is the same biological process that warms a garden compost pile. The birds actively control the temperature of the mound by adding or withdrawing material in a behavioral display that is surprisingly accurate for a creature not typically thot to have complex intellect.
This form of adaptation—a change in substrate type in response to environmental pressure, too gradual to be documented in any written record—may have occurred earlier in the species’ history. Megapodes are outdated. The family’s adaptability to heat sources may have contributed to their ability to withstand major environmental changes throughout the course of their extended existence. However, the current change is taking place against a backdrop of habitat loss that did not exist in earlier millennia. The birds are migrating toward a habitat that is also diminishing because Tawi-Tawi’s inland forests are under pressure from logging and agricultural conversion.
What’s going on on these islands is a narrow, particular tale. However, it relates to a broader issue on how species reacts to coastal change—not always by going extinct, but sometimes by relocating, adapting, or choosing the next best alternative. Researchers are still figuring out whether Tawi-Tawi’s forest interiors have enough unaltered room to handle the nesting demand being pushed off the beaches. Without waiting for a response, the scrubfowl are performing their calculations in real time.
