Canadian Wildlife, October 2005

Some species of wildlife escape the long nights, harsh weather and food shortages of a Canadian winter by retreating into hibernation or dormancy, while others flee to more hospitable climes. Pileated woodpeckers do neither, staking their survival instead on planning and memory power.

It’s not often that a train prompts an important insight into wildlife behaviour, but that’s exactly what happened to biologist Rick Bonar, then a doctoral student at the University of Alberta, on a glorious fall day in 1993.

Bonar had been pursuing a pileated woodpecker he had named Saint. He’d fitted the bird with a radio transmitter backpack to learn more about this species’ habits and habitat in the Rocky Mountain foothills of west-central Alberta. The biologist’s brown eyes light up as he describes experiencing “a little bit of an epiphany” while following Saint on his rounds.

All afternoon, against a backdrop of brilliant blue skies and golden-leaved aspens, the woodpecker had been flitting from the base of one tree to another, never lingering long and eating only sparingly, despite the abundance of carpenter ant colonies Bonar found at the foraging sites afterwards. Saint’s lack of hunger seemed curiously at odds with his apparently keen interest in locating prey.

Many of the trees Saint visited that afternoon showed signs of previous woodpecker feeding activity and he diligently investigated these holes, peering and probing into the exposed ant galleries.

Now and then Bonar saw the bird extend his tongue and vibrate it rapidly against a tree trunk, producing a soft, barely audible rattle. Biologists rarely get to observe this subtle tongue-drumming, which they think may be the pileated woodpeckers’ way of inducing subsurface insects to move and betray their presence. Saint also drilled numerous small test holes, quickly penetrating the ants’ network of tunnels, eating briefly, then moving on.

As intriguing as Saint’s activity was to Bonar, he might not have paused to sort out its significance while it was still fresh in his mind if not for a chance interruption. Saint’s travels had brought both bird and biologist close to railway tracks running through the study area.

“I’m sitting on a log and he’s over there, 30 or 40 yards away, whacking on the base of a tree, and this train goes by,” recalls Bonar. Saint initially ignored the sound of the boxcars thundering past, but after a moment or two, flew off over the train and across the tracks. Unable to follow immediately, Bonar remained on his log, pondering the bird’s behaviour that day. Maybe, he speculated, the woodpecker was storing information with the intention of coming back to these sites in winter.

The theory Bonar formulated that day turned out to be correct, adding an invaluable piece of information about a bird whose habits and habitat use in Canada are much less well understood than its behaviour in the United States, where it has been studied extensively.

About half of the pileated woodpecker’s total North American range is in Canada. Until recently, however, the only published scientific information about its ecology came from research done south of the border. We understood very little about the woodpecker’s life in Canada’s boreal and western cordilleran regions. With forestry operations continuing to expand in these areas, it’s crucial to have local data on which to base management decisions.

As a biologist working for Weldwood of Canada in Hinton, Alta., Bonar was well aware of these knowledge gaps, so 12 years ago, he set out to fill some of them. Specifically, he wanted to look at how pileated woodpeckers would fare in northern forests where there are fewer tree species and smaller trees on average than in regions further south. He also planned to study their behaviour in the long, cold, snowy winters typical of their northern habitat. To do this, he fitted 32 birds with radio transmitters and followed them over a three-year period.

His research revealed that, like their relatives elsewhere, pileated woodpeckers living in the Alberta foothills vary their diet seasonally, but mainly depend on wood-dwelling ants, particularly carpenter ants. Summer is a time of plenty for the birds, who can easily pick off insects travelling on the surface of tree trunks or tear apart rotten logs and stumps to find more food. Besides carpenter and thatching ants, which they eat by the hundreds or thousands daily, pileated woodpeckers also consume beetle larvae and a variety of other insects. As wild fruits and berries ripen, they too are added to the menu.

Come winter, provisioning becomes more difficult. Most insects die or disappear; carpenter ants go into hibernation deep inside the bases of large live or recently dead trees. The only predator that can breach these fortifications is the pileated woodpecker — excavator extraordinaire.

With their sturdy, chisel-shaped bills, powerful neck muscles and reinforced skulls, pileated woodpeckers are superbly designed for chopping through even the hardest wood. But this is demanding work. During northern winters, when there may be only six or eight hours of daylight and temperatures may plunge to -30 or -40°C, woodpeckers cannot afford to waste time and energy on unproductive labours. They need to know their efforts will pay off.

Bonar’s observations of pileated woodpeckers in the northern part of their range suggest that they use what biologists call “search images” — mental pictures of desired items — to select sites that are most likely to yield food. They consistently bypass live, vigorous trees and focus their attention on those that have been damaged in some way and therefore have the potential to have been colonized by carpenter ants. Fungal conks (the shelf-like or knobby projections that appear on the exterior of fungus-infected trees), dead branches, scars and other signs of decay are like neon signs flashing the message “Eat Here.” When only one or two per cent of the trees in the forest harbour carpenter ant colonies, knowing what to look for is a great time saver.

Bonar says one reason other researchers have overlooked the species’ use of search images is that they have focused on what pileated woodpeckers eat, not how they find their food. Unlike some other researchers, Bonar was particularly interested in observing the woodpeckers first-hand. “I simply used the radio tag as a way to find the bird and establish contact with it, or re-establish contact if I lost contact, and then I watched the bird or listened to what it was doing,” he says. “The next natural question is ‘Why is it doing that?’”

As Bonar saw that afternoon he spent with Saint, pileated woodpeckers — at least in northern parts of their range — do not feed heavily when they find carpenter ant colonies during their exploratory foraging in summer and fall. “It’s like they think, ‘I’ll sample the deli and come back and shop later,’” he says. The important thing is knowing the exact location of the grocery store for future reference. When times are tough, they will rely less on the search images that guide them to probable food sources and more on memorized knowledge of guaranteed meals, enabling them to conserve vital energy during the coldest months.

“In winter they’ll come out of their roost trees in the morning and if it’s sunny, they may sit and warm up a little bit,” explains Bonar. “Then they’ll fly directly to one of these [carpenter ant] trees and start tanking up. They don’t spend any time searching at all. They go from known tree to known tree to known tree until they’re full. They don’t have any fat and they have very little in the way of thick plumage for insulation, so they’ve got to stoke the furnace; otherwise they’re not going to make it.”

Through his research, Bonar found that individual pileated woodpeckers return to the same foraging sites repeatedly throughout the winter, as well as year after year. Each time they visit, they remove more wood, digging deeper, wider and higher to gain access to untapped galleries and gradually creating the substantial foraging excavations for which this species is renowned. These distinctive rectangular or oblong holes may measure more than 30 centimetres in length and can be so deep that they will fell a tree as effectively as a woodsman’s axe.

These foraging sites are a crucial dependable source of food for the birds. With up to 15,000 carpenter ants living in the base of one tree, these colonies are a rich, long-lasting resource, but not so bountiful that woodpeckers can afford to share them indiscriminately. Indeed, it appears that the need to ensure exclusive access to this essential winter food supply is one of the main determinants of pileated woodpecker territory size, at least in northern forests.

Pileated woodpeckers mate for life and, unlike most North American woodpeckers, each pair defends its joint territory throughout the year. In Bonar’s study area, the average size of pair territories, based on cumulative year-round movements, was 2,157 hectares — more than five times the size of Vancouver’s Stanley Park — and more than four times larger than pileated woodpecker territories in other, more southerly areas.

During summer, when they were raising families, Bonar’s birds typically confined their activities to the vicinity of the nest tree, easily acquiring enough food for themselves and their offspring without travelling significantly further than pileated woodpeckers would elsewhere. Only in winter did they use the full extent of their territories, relying on their exceptional talent for recollection to find the precious food stores scattered about these vast domains.

J.M. Barrie, the Scottish playwright of Peter Pan fame, once said “God gave us memory so that we might have roses in December.” Had he been a Canadian biologist he might have mentioned that memory also grants pileated woodpeckers full stomachs in the depths of winter.

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SIDEBAR STORY: Condo Contractor

It’s rare to see northern flying squirrels by day, but Rick Bonar has a technique for improving your odds. Find a tree with a pileated woodpecker nest or roost hole — recognizable by an oval entrance the size of hefty man’s fist — and scratch the trunk with a stick. The first few times he demonstrated this trick for me, nothing happened. Then, at our fourth tree, a furry grey face appeared in the opening, my first glimpse ever of one of these elusive nocturnal creatures.

After briefly scrutinizing us with its beady black eyes, the squirrel retreated, apparently reassured that we posed no danger. Its confidence was well justified, for there are few bedrooms in the forest as secure — or as cozy — as a woodpecker cavity.

In Bonar’s study area near Jasper, Alta., pileated woodpeckers provide nesting, roosting and denning sites for 18 species of birds and mammals, including ducks, owls, squirrels and bats. Continent-wide, 20 more vertebrate species can be added to this list. No other North American woodpecker supports such a large diversity of successor tenants.

Pileated woodpecker cavities are prime real estate because they are considerably larger than those made by other woodpeckers. Some species, including American martens, boreal owls and Barrow’s and common goldeneyes, don’t fit into smaller cavities. Others, such as saw-whet owls, prefer more spacious lodgings when available. In winter, northern flying squirrels find pileated cavities the perfect place to curl up with as many as nine other individuals to beat the cold.

Scratching cavity trees might seem a frivolous pastime, but for biologists like Bonar it has a serious purpose. Having visually inspected 1,144 pileated woodpecker holes and scratched or knocked on trees containing an additional 4,610 cavities over a five-year period, he has solid grounds for declaring pileated woodpeckers “a keystone species, with an essential functional role in maintaining the cavity-using wildlife community in boreal forests.” Even the squirrels he disturbed might agree it was worth losing a little shut-eye to confirm the ecological importance of Canada’s largest woodpecker.