Eagle Owls take nightlife hooting, hunting cues from moon

Full moons have profound effect on Europe’s largest owl

Rabbits exhibit “lunar phobia” tactics as moonlight intensity increases.

Eagle owls in southern Spain were studied 8 years.

By Rex Graham
BirdsNews.com

Nocturnal predators and full moons are deadly combinations for many mammals. No wonder rabbits, deer mice and even bats exhibit “lunar phobia,” becoming less active and more vigilant as moonlight intensity increases. They tend to spend moonlit nights in thickets and other safer areas even if there is less food to eat there.

Full moons also have a profound effect on Europe’s largest owl, the Eagle Owl (Bubo bubo). Nesting males hoot more frequently and flash a white badge on their throats. But ornithologists couldn’t observe what else moon-crazed eagle owls were up to.

Sound recording of copulating eagle owls

New answers, more questions

Now, radio tagging and other sensor technologies are providing answers – and raising tantalizing new questions about the roll moonlight plays in the evolution of both predator and prey.

Conservation biologist Dr. Vincenzo Penteriani fitted 71 Eagle Owls with 30-gram backpacks containing radio tags and body-posture sensors. Penteriani worked with other conservation biologists Anna Kuparinen at the University of Helsinki, Maria del Mar Delgado and Rui Lourenço, both at Estación Biológica de Doñana and the University of Helsinki, and Letizia Campioni at Estación Biológica de Doñana. The team followed the owls’ nightly movements and activities from 2003 to 2010 in the Sierra Morena of southern Spain.

The researchers wondered how Eagle Owls had enough time for so much hooting and territorial posturing on intensely moonlit nights and also hunting harder-to-find rabbits and rats.

This direct attack by an Eagle Owl on a video camera reveals what to many rabbits is a lethal encounter.

“I was very intrigued by the idea that the less reflective tails of unhealthy individual rabbits might lead Eagle Owls to target them during moonlit nights,” Penteriani said. “We hypothesized that the eagle owl’s preference for substandard individuals could be due to the easier detection of slightly duller rabbit tails.”

Complex trade-off for eagle owls

Describing results of eight years of observing owls from sunset to dawn, Penteriani reported in Animal Behaviour that the response of a predator like the Eagle Owl to moon phases may represent a complex trade-off. Breeding adults may need to carefully balance the difficulty of overcoming anti-predator strategies of its prey, mostly rabbits (Oryctolagus cuniculus) and rats (Rattus rattus and R. norvegicus), with owls’ need to feed themselves and their young as well as advertise their home territory.

“Modern technology such as radio-tagging represents one of our best tools to discover nightlife,” Penteriani said. “Each night we focused on one individual.

A mercury posture sensor allowed his team to discriminate different behaviours by changes in the radio signal of the transmitters. “After more than 700 nights of continuous radio-tracking of more than 100 owls, we were easily able to discriminate between hunting, vocal displays, feeding young, roosting and other behaviours,” he said.

These are the highlights of his long-term study:

–Non-breeding, dispersed owls weren’t affected by the lunar cycle.

–Breeding Eagle Owls traveled longer distances at greater speed and also increased the number of flights per night, increasing in phase with the intensity of lunar light.

–Owls move much less at the time of a new moon.

–Non-breeding dispersed owls were not affected by the lunar cycle.

–Younger owls flew farther and faster than older owls.

–Female owls moved less than males, presumably so they could remain closer to the nest while males could range father to hoot and make territorial displays.

–Owls moved more frequently when hunting in denser habitats such as forests.

–Owls living in home ranges with few rabbits relied much more on rats and other smaller prey, which required them to devote more time and energy to hunting.

–Dark nights are more silent nights — owls use lunar light to increase the effectiveness of their territorial communication.

Owls and rabbit tails

“We also measured the reflectance of the tails of individual rabbits and found that those with duller tails were more likely to be taken by Eagle Owls,” Penteriani said. “These preyed rabbits were the substandard individuals of the whole population. In fact, the higher survival of rabbits in prime condition is consistent with previous studies on the relationship between viability and secondary sexual characters.”

Since Eagle Owls have evolved with rabbits, the suggestion that their predatory behavior is guided by visual cues is in agreement with the hypothesis that bright coloration may signal an unprofitable hunt.

“More fit individual rabbits may be signaling their greater ability to avoid predators compared to less fit, duller ones,” Penteriani said. “We may be dealing with a mechanism that has evolved in predators, which also plays role in rabbit sexual selection.”

Short URL: http://birdsnews.com/?p=315

Posted by Rex Graham on Jul 16 2011. Filed under Birds of Prey, Europe, Owls, Research, World. You can follow any responses to this entry through the RSS 2.0. You can leave a response or trackback to this entry