BURROWING owls are odd birds, even by owl standards. Not only are they small with peculiarly long legs, they nest in deep holes in the ground. During the day, all they seem to do is stand sentry-like outside their burrows and wait. And wait. And wait. No one is quite sure what they are waiting for. Nor does anyone have a good explanation for another bizarre behaviour. Burrowing owls collect stuff 鈥� seemingly any old stuff, from bits of foil and pieces of plastic to lengths of partly chewed centipede and squashed toads carefully scraped off the road. And then there鈥檚 the dung. Around the entrance to an owl鈥檚 burrow is a scattering of mammal manure, anything from shreds of cow pat to diced horse droppings and dollops of bison dung.
All this is very strange. But for Doug Levey鈥檚 students there was really only one question. Hanging about doing nothing was perfectly natural. Leaving stuff lying around the place was normal. But why the dung?
Levey is a zoologist at the University of Florida in Gainesville. He often takes his ornithology class to see wild birds, and burrowing owls are a favourite. During one field trip to watch them, his students asked the obvious question: what was the dung doing there? At first Levey assumed that cattle had simply defecated near the burrows. Then he realised the nearest cows and horses were several hundred metres away. The owls were collecting the dung themselves, picking it up in their talons and dumping it on their doorsteps.
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Burrowing owls, Athene cunicularia, live in open prairie and semi-desert regions from Canada to Chile. Reports of dung-collecting go back many years. 鈥淚n the past it was probably bison dung. Today it鈥檚 mainly cattle,鈥� Levey says. Whatever its origin, the owls are very attached to their dung and will replace any that is removed. 鈥淭his suggests it is very purposeful behaviour and not just a freak kind of thing,鈥� says Levey.
Masking the smell
So why do they do it? Some observers have suggested the dung might mask the scent of the owls鈥� eggs or nestlings, protecting them from predators such as snakes and raccoons. Such behaviour is not unknown. African waxbills deter would-be predators by perfuming their nests with the odour of bigger predators, courtesy of carnivore dung collected from the savannah. Perhaps the owls were using a similar but less scary ploy, using dung as camouflage rather than as a frightener.
But Levey and his students had another idea. In and around the burrows they found remains of the owls鈥� prey, including regurgitated pellets containing beaks and bones from small birds and rodents and the hard outer parts of beetles. Analysis of these leftovers showed that burrowing owls eat a lot of beetles, including several species of dung beetle. 鈥淲e put two and two together and suggested there might be a connection between the dung and the dung beetles,鈥� says Levey.
He and two students, Scot Duncan and Carrie Levins, decided to investigate. First they tested the idea that the dung was there to protect the young. They created 50 artificial burrows, placed five quail eggs in each and scattered dung around half of them. (Owls鈥� eggs are hard to come by but quails鈥� eggs are available from quail breeders.) Dung or no dung, predators found every clutch bar one.
Next they tested their hunch that there was a link between the dung and the beetle remains. 鈥淥ur first experiment was kind of silly in hindsight,鈥� admits Levey. 鈥淲e wanted to give the owls really good quality cow dung, so we got steaming-fresh, wet dung and plopped it down in front of the burrows to see if it made a difference.鈥� It did. Owls with warm, fresh dung got more beetles. But in the real world sloppy dung would slip straight though an owl鈥檚 talons. 鈥淲e started again, this time with dung that was dry enough for an owl to collect but moist enough to appeal to a dung beetle.鈥�
They removed all traces of dung and beetle remains from the entrances of 10 burrows, then distributed a typical quantity of dung around five of them and left the other five without. After four days 鈥� as long as an owl would go without replacing its stolen dung 鈥� they gathered all the pellets and prey remains and started a second trial, this time switching the groups around.
The results were unequivocal: owls with dung ate 10 times as many dung beetles as those without (Nature, vol 431, p 39). The owls were clearly 鈥渇ishing鈥� for dung beetles, using dung as bait. This explains why they spend so long standing stock still outside their burrows. 鈥淎sk a fisherman and he鈥檒l tell you, if you want to catch anything you have to be patient and wait. I think that鈥檚 what the burrowing owls are doing,鈥� says Levey.
Reports of tool use by animals invariably arouse controversy. Most are anecdotal and open to interpretation, with little hard evidence that the behaviour brings real benefits in the wild. For instance, there have been numerous reports of greenbacked herons dropping seeds, flowers, feathers, dead flies and even crackers into the water, then waiting motionless nearby. Occasionally they catch a fish, but there is no way of telling whether the 鈥渂ait鈥� made a difference.
Tool makers
Even the skills of the most famous tool-using birds, New Caledonian crows, have never been tested under controlled conditions in the wild. The crows spear plenty of grubs with their probes, but no one has checked how well they do without them. A captive crow called Betty astonished biologists by fashioning tools of exactly the right size and shape to obtain food from places she could not otherwise reach (快猫短视频, 17 August 2002, p 44). 鈥淭hat shows she鈥檚 wonderfully creative, but it can鈥檛 say how important the behaviour is in the wild,鈥� says Levey. 鈥淭he only way to find out if a tool makes a difference is to measure the animals鈥� success with the tool and without it. That鈥檚 very tricky to do in the wild.鈥�
The owl experiment shows beyond doubt that tool use 鈥� in this case a lure 鈥� can bring measurable benefits in an animal鈥檚 natural habitat. 鈥淎s far as I know no other study has quantified the benefits of tool use in such a precise way,鈥� says Gavin Hunt of the University of Auckland in New Zealand, who studies New Caledonian crows in the wild.
鈥淧erhaps they collected bits and pieces to attract a mate鈥�
Although a few dollops of dung on the doorstep brings in the beetles, the fact that the owls collect so much other stuff suggests the behaviour evolved for some other purpose, says Levey. Perhaps burrowing owls collected bits and pieces to attract a mate, much as male bower birds try to entice hens with offerings of flowers, stones and ring pulls from beer cans. But attempts to see what kind of objects attract an owl flopped. 鈥淲e laid out a choice selection, but they blew us out. They didn鈥檛 take up the offer,鈥� says Levey. Some material, including dung, might help to insulate the nest. And although dung didn鈥檛 mask the smell of eggs from predators, it might still protect chicks 鈥� an idea they chose not to test for ethical reasons. Hunt suggests that the very fact the owls are stationed outside the burrow all day would reduce the risk to chicks. Whatever it was encouraged their kleptomania, it brought a bonus 鈥� fast food that delivered itself to the owl鈥檚 door.
