Systems in ActionTopic 1
The curious case of the burrowing owls
Step 1: Making observations
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 – 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’s the dung. Around the entrance to an owl’s burrow is a scattering of mammal manure, anything from shreds of cowpat to diced horse droppings and dollops of bison dung.
Baiting and waiting: an owl stands at the entrance to its burrow, surrounded by the dung that it has positioned to entice beetles.
Source: Levey, D, Duncan, RS, Levins, CF 2004, ‘Use of dung as a tool by burrowing owls’, Nature, 431, p. 39. Photographer: R.G.Wolff
Doug Levey and his students wanted to know what the dung was doing there.
Activity 2
What observation stimulated the investigation of the burrowing owls?
Step 2: Find background information
Doug Levey and his students made an interesting observation about the burrowing owls. What other information about burrowing owls could the students explore? They searched for articles written about burrowing owls and other birds with similar behaviours. They also found out as much information as they could about the biology of burrowing owls.
Burrowing owls, Athene cunicularia, live in open prairie and semi-desert regions from Canada to Chile. Reports of dung collecting go back many years. “In the past it was probably bison dung. Today it’s mainly cattle,” Levey says. Whatever its origin, the owls are very attached to their dung and will replace any that is removed. “This suggests it is very purposeful behaviour and not just a freak kind of thing,” says Levey.
Step 3: Formulate some possible explanations.
What ideas do you have? The students in this article came up with the following.
The dung might mask the scent of the owls’ eggs or nestlings (baby birds) so that they are protected from predators such as snakes and raccoons.
When owls eat they regurgitate the remains of their prey in pellets. The students analysed the pellets to identify what the owls ate. They found beaks and bones from small birds and rodents and the hard outer parts of beetles, some of which were dung beetles. Perhaps there was a connection between the dung beetles and the dung?
Activity 3
Think of a link between the owls and the dung beetles. What could have been the second possible explanation the students devised?
Step 4: Design and conduct the experiments.
The first experiment the students conducted investigated whether the dung was placed in front of the burrows to protect the young from predators.
Experiment 1
The students created 50 artificial burrows, placed five quail eggs in each and scattered dung around half of them. (Owl eggs are hard to locate, but quails’ eggs are available from quail breeders.)
Note: The placement of dung around the artificial burrows is called the experimental treatment. The burrows without dung are called the control treatment.
Activity 4
Why did the students leave half of the artificial burrows free of dung?
Experiment 1 results
The students found that dung or no dung, the predators found all but one of the clutches of eggs. They concluded that the presence of dung around the burrows did not protect the eggs from predators.
Their second experiment tested the idea that there was a link between the dung and the beetle remains.
Experiment 2
Fresh cow dung was placed in front of some of the burrows.
Experiment 2 results
The students found that there were more dung beetle remains in the regurgitated pellets of those owls that had dung placed in front of their burrows.
This indicated to the students that the dung beetles were attracted to the burrows that had dung. These dung beetles provided more food for the owls.
But there was a problem with this experiment. The students realised that this experiment was not a ‘fair test’. Why? The owls would not be able to collect wet, gooey, fresh cow dung in their talons!
Experiment 3
The students removed all traces of dung and beetle remains from the entrances of ten burrows, then distributed a typical quantity of dung around five of them and left the other five without dung.
The owls without dung around their burrows replaced the dung after four days. So the students collected the regurgitated food pellets and prey remains from each burrow after four days and identified the food that each owl was eating.
The students began a second trial in which they switched the groups. Those burrows that had dung in the first trial had the dung removed, and the burrows without dung in the first trial had the dung added.
Experiment 3 results
In both trials of the experiment, the owls with dung outside their burrows ate ten times as many dung beetles as those without dung.
Activity 5
What conclusion can you draw from these results?
Feedback
Activity 2
The observation that stimulated the investigation was that dung was present at the mouth of the burrowing owls’ burrows.
Activity 3
The students’ second explanation could have been:
The owls collected the dung to encourage dung beetles to come close to their burrows so that they could eat them.
Activity 4
Half of the burrows were left free of dung for comparison. If the predators leave the eggs in the burrows with dung but take the eggs from the burrows without dung, then it can be concluded that the dung has an effect.
Activity 5
The owls attracted dung beetles to their burrows using dung as ‘bait’.
Senior Biology1