Familiarity Breeds Contempt with Nesting Birds
(From July 2018)
I was still separating junk mail from utility bills near my mailbox when the red-breasted dive-bomber buzzed my ears, veered left and landed overhead to hurl insults and curse me from the safety of our arborvitae trees.
You bet I was offended. I’ve never heard such profanity, disrespect and poor grammar from Wisconsin’s state bird in nearly 26 years of retrieving our mail at this address. And here we thought robins were such friendly, cheerful chirpers. Pfft! Think again. This one had all the grace and patience of bouncers at bar-time.
And those insults rained down in stereo seconds later when the robin’s spouse winged over to help, heaping on scorn and ridicule as if it were the president himself. Sheesh. Given such contempt, you’d have thought I was a Cooper’s hawk or the neighbor’s free-ranging cat.
Finally, a thought occurred. I returned to the mailbox and scanned our crabapple tree’s branches. Sure enough. I spotted a robin’s nest 8 feet up, with grassy curls hanging below like jellyfish tentacles.
I returned five minutes later with an 8-foot stepladder and my camera. The robins weren’t fooled. “You’re no scientist!” they shrieked. “You’re not even a photographer! You’re the press! We had you pegged days ago! Scram! Get away from our young’uns! Loser!”
I didn’t argue. I snapped some photos from atop the ladder, rolled some video while ducking their strafing runs, and then descended the ladder and fled down the driveway.
Once indoors, I contacted Stanley Temple, the famed UW-Madison ornithologist, for his insights into nesting robins. Temple suggested a better idea: “Let’s discuss how birds defend their nests,” he said. “It’s something I’ve researched and written about.”
Temple was being modest. His work with nesting birds goes far beyond mere peer-reviewed papers in scientific journals. If bird researchers could earn meritorious medals, Temple would own a Purple Heart. While banding rough-legged hawk nestlings one day in Alaska in 1968, Temple stood up just as an adult hawk buzzed his head to express its irritation. The resulting head-on collision knocked bird and biologist out cold onto the cliff ledge.
“I’ve been hit by great horned owls, too,” Temple said. “It felt like someone fast-pitched a softball into my head. That’s why a lot of biologists wear a hardhat when checking the nests of great horned owls and other large birds of prey.”
Most bird-on-human conflicts don’t end in knock-out collisions, of course. In most cases, the birds just grow increasingly bold as they recognize human intruders and lose their fears. Former Green Bay Packer receiver Jeff Janis, for example, posted an Instagram video a few days ago showing a bird dive-bombing him as he ran along a trail.
Temple said robins, bluejays, house wrens and red-winged blackbirds are notorious for harassing people who venture near their nests. The more people the birds encounter and “chase off,” the more the birds’ aggression gets reinforced, further emboldening them.
Roughly 40 years ago, however, researchers explained those increasingly aggressive flybys with the “parental investment” theory. That is, they believed adult birds defended their offspring in proportion to how much time and effort they had invested in rearing them. As the breeding season progressed from egg-laying to incubation to hatching and then feeding nestlings, the parents become increasingly bold and protective.
Temple said this theory nicely fit the increasingly aggressive patterns ornithologists documented in their studies. Researchers sometimes visited the birds daily to tally the increased frequency of dives, pecks and calls, and the increasingly closer contacts. The pattern stayed consistent for a wide variety of birds as their nesting cycles progressed.
In 1986, however, Temple suggested to one of his students, Richard Knight, that repeated visits to the same nesting birds might affect their behavior. Sure enough, Knight and Temple discovered that birds quickly learn to recognize regular visitors who present little danger. Likewise, when Knight and Temple placed taxidermied mounts of crows near red-winged blackbird nests, the blackbirds learned to attack the mounts more aggressively than live crows, probably because they learned not to fear the stuffed birds.
Knight and Temple also documented that birds attacked people with less vigor if a person stared at the attacker. The birds perceived stares to be more threatening than an averted gaze.
The researchers also discovered that urban-dwelling crows confronted and attacked humans threatening their nests, while rural-nesting crows quietly retreated. Why? Rural crows had learned they might get shot if they made a fuss with people nearby, whereas urban crows never suffered any harm from people.
Knight and Temple tested their findings further by visiting individual robins and blackbirds only once at different nesting stages. Birds they repeatedly visited steadily increased their nest-defense intensity, while once-visited birds did not.
Temple said the birds learned researchers were no threat to them, but could still threaten their nest. Therefore, their aggression increased steadily as they lost their initial fear. Further, because the researcher always left after a short visit, the bird felt its attack drove the person away. This positive reinforcement further emboldened their aggression. The same thing happens when birds attack runners and bicyclists passing near a nest.
Temple said Knight’s research didn’t totally invalidate the “parental investment” theory. It likely plays some role in the parents’ behavior, but scientists were wrong to think it explained everything about a bird’s nest-defending actions.
“The moral of the story is that when you’re studying smart animals like birds, you have to be careful how you, as an observer, are influencing their behavior,” Temple said.
Realize, too, that familiarity breeds contempt even among tiny birds like house wrens. Temple and his wife place wren houses around their garden in hopes the little birds will prey on caterpillars and other plant-eating bugs. The downside is that wrens often peck their heads while they garden.
“They’re not a bird you’d expect to be so bold, but they lose their fear and get that positive reinforcement from repeatedly attacking us without being harmed, and then seeing us retreat,” Temple said. “And so the next time we garden, they come after us more aggressively.”
Whoever thought gardening, or checking your mailbox, required such raw courage?