The fierce stare of the Metallic Starling isn't enough to keep predators from gathering below its nesting colony. Photo: Robert Rouse

Science

Under Metallic Starling Colonies, Wildlife Gather for Impromptu Feeding Frenzies

More than 40 species—including rare cockatoos and tree snakes—are drawn to these pop-up ecosystems during Australia's monsoon season.

Most scientists are lucky if they make a single discovery in their lifetimes. Daniel Natusch made his when he was only a teenager.

Natusch grew up in Cape York near Australia’s Lockerbie Scrub rainforest, which lies at the northernmost tip of the mainland and some 100 miles across the sea from Papua New Guinea. It’s not an easy place to grow up. Six feet of rain fall each year, most of it during monsoon season from December to April. The remainder of the year is bone dry, with fires burning up the grasslands and savannas surrounding the rainforest.

But the budding scientist made the most of it, and spent as much time as possible chasing snakes in the rainforest. When he was 14 years old, he stumbled upon a strange clearing one night. A poison-dart tree several feet in diameter loomed overhead, and hundreds of gourd-shaped bird nests hung from its wide branches. The ground below was a “barren moonscape,” Natusch says, where a half-dozen snakes lay in wait of any dropped food or fallen chicks. “The snakes were obviously there to eat the birds,” he says.

Over the next decade, he evolved from a snake chaser into a snake scientist, with the skills and tools to properly study the unusual scene from his childhood. So in 2013, as a graduate student at the University of Sydney, he returned home to figure out why so many snakes gathered beneath the poison-dart tree.

Soon he found that more than snakes gathered there. Over the course of two monsoon seasons, Natusch counted more than 100,000 animals of 42 species feasting beneath Metallic Starling colonies in 27 different trees. The gatherings form veritable, if short-lived, ecosystems for the four months of the breeding season. And, to Natusch’s delight, the hotspots appear to exist because of the relationship between the birds and the snakes. 

A lucky python made a rather gruesome meal of this Metallic Starling. Photo: Daniel Natusch

Let’s start with the birds. Each year, Metallic Starlings migrate from Papua New Guinea to Lockerbie Scrub and, by mid-November, hundreds or thousands of birds colonize each tree they find suitable for nesting. Scanning the ground with their beady red eyes, the starlings collect twigs to build gourd-shaped nests and lay their eggs.

After the eggs hatch, the iridescent black parents ferry fruits and seeds from nearby trees to feed their chicks. All this activity has three main effects. First, the birds regularly drop food, which attracts a variety of scavenging herbivores and insects. Second, bird poop smothers the ground below. “You’ve got this massive nutrient surge because of all the guano from the thousands of birds above,” Natusch says. The nutrient levels are high enough to be toxic, and they kill most of the surrounding vegetation. Some dropped seeds germinate in the filth, and those seedlings are quickly devoured by the gathering animals. Third, chicks, eggs, and bits of nest inevitably fall from the tree, inviting carnivores into the fold. Soon, the ground is a rich ecosystem unlike any other in the forest.

“Just because of the sheer number of starlings above, there are all these other animals that are coming in to eat the food that they have been eating,” Natusch says. “It’s bringing in all of these animals that would otherwise be reasonably sparsely distributed in the landscape.”

The four months the nesting season spans are essentially a feeding frenzy beneath the trees. Australian Brushturkeys and feral pigs, which root around for dropped food and seedlings, comprise a majority of the animals—more than 90 percent by Natusch’s count. But the rich food resource attracts rarer animals, too. Palm Cockatoos and Sulphur-crested Cockatoos—two-foot crested parrots in black and white varieties, respectively—show up for the loot, as do doves, fowl, wallabies, and a variety of rodents and frogs. Those animals then attract bigger predators. Rufous Owls and Grey Goshawks hunt starlings and rodents, and dingoes eat whatever they can catch.

And then there are the snakes. At night, after most of the birds and mammals turn in, tree snakes and pythons come out to play. “When the birds arrive, [the snakes] go straight to the trees and they essentially sit there for the four months the birds are nesting, waiting for food to drop,” Natusch says. Their persistent behavior is so unusual that he suspects that Metallic Starlings nest colonially in these big trees for the purpose of snake avoidance. The birds seems to choose wide trees with smooth bark that the snakes struggle to climb; during fieldwork, he regularly watched snakes try in vain to wind their way up the broad trunks before falling to the ground.

“Perhaps an individual [starling] looks for a tree that it looks like snakes can’t climb, and there’s only a limited number of those trees,” Natusch says. “If a number of individuals use the same tree, almost by default you have colonial breeding evolving.”

That such an unusual, loud, and stinky wildlife gathering has avoided discovery by now is almost unbelievable. However, Lockerbie Scrub is remote and hard to reach, especially during monsoon season when the birds nest; the region’s roads frequently flood, making navigation difficult. Additionally, the gathered animals are extremely sensitive to noise, Natusch says.

“Obviously people have come across these [gatherings] before, but they’re walking through the bush and probably making a lot of noise so all the animals have scattered,” he says. “By the time they get to the tree, it just looks like a large starling colony with a unique landscape around it. They probably have made notes and observations but haven’t thought too much of it.”

Natusch saw what others’ couldn’t thanks to his familiarity with the area and its harsh conditions—and with the aid of motion-activated cameras that let him silently and secretly track the animals without disturbing them. The fascinating ecosystem serves as a nice reminder that big discoveries remain out there waiting for those in the right place, at the right time, and with the right equipment.

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