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  • Ravens and foxes and beetles, oh my! Savvy scavengers confuse efforts to survey bird mortality

    Thursday, May 21, 2020
    News Releases

    Most people prefer to dine at a table with a view, and the same may be true for the Common Ravens on the Corner Brook campus of Memorial University of Newfoundland, Grenfell Campus.

    Megan Clarke, an undergraduate student in Grenfell's Environmental Science Program, investigated the activity of scavenger animals on campus and discovered that the local ravens had already learned to look beneath windows, where bird collisions often occur, to find dead birds.

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    Ms. Clarke's research, which was published in the journal PLOS ONE today, investigated the removal of small bird carcasses from locations around Grenfell Campus, with the hope of determining how local scavengers respond to the presence of an unexpected food source: birds killed when they collide with windows.

     "Collisions with windows are the number two cause of songbird mortality in North America," said Ms. Clarke. "To better understand when and why bird-window collisions occur, it is important to accurately document bird mortality. But because local scavengers may remove dead birds before researchers can find them, gathering accurate counts of mortality due to these collisions can be really difficult."

     Ms. Clarke and her co-authors, Dr. Ian Warkentin and Dr. Erin Fraser, both faculty members at Grenfell Campus, investigated which scavengers were present on Grenfell Campus and observed their scavenging behaviour.

     "If scavengers learn that certain windows or buildings are likely to have dead birds on the ground next to them, then those scavengers may remove dead birds more quickly from those locations than from others. This can make it difficult to identify locations that are especially deadly for birds," she said.

     Ms. Clarke obtained dead chicken hatchlings from a local hatchery and placed them around campus, both under windows and against blank walls with no windows, as well as in a local forest. She monitored carcasses with cameras that were triggered to take photos when the carcasses were disturbed.

    The results were striking and a little surprising. Only one species of scavenger was active on Grenfell Campus: Common Ravens.


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     "The ravens were absolutely aware of the windows as potential food sources. Twenty-six hatchlings were scavenged on campus and 25 of them were taken from beneath windows," said Ms. Clarke. "Only one hatchling was removed from against a blank wall. We expected the scavengers to learn about the presence of new food sources during our study and to respond over time. In fact, it seems like they already knew that windows were an important place to look for food."

    Common Ravens belong to the same family of birds as crows, jackdaws and blue jays. Members of this family are known for being particularly intelligent and for devising ingenious approaches to challenges and opportunities in their everyday lives. It seems that the Common Ravens on Grenfell Campus are no exception.

     But the raven activities were just half the story:

     "The biggest mystery in our project was in the forest," said Clarke. "The hatchlings were disappearing, but no photos were triggered to help us understand what was happening to them. One day when I was in the forest, I looked down and saw one of the hatchling's feet sticking up out of the ground; it had been buried."

    The culprits were burying beetles, a kind of insect that buries dead animals for later consumption. 

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    The beetles did not trigger the cameras because they were so small and the burials occurred slowly overnight, but they were important scavengers in the forest environment, removing 14 of the 18 scavenged carcasses. The remaining four were removed by red foxes and a Common Raven.

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    The results of Clarke's research have implications for surveys of bird mortality at buildings and other structures.

    "The Grenfell Campus ravens showed us that scavenging rate can vary significantly at a very small scale – the difference between a carcass being under a window or not can be huge – making it really difficult to account for scavenger activity across whole sites or groups of sites, said Ms. Clarke."

    Similarly, while burying beetles are well known to bury animal carcasses, they have received very little study in the context of mortality surveys. Burying beetles may significantly alter perceptions of bird mortality at structures that are located in forest environments, such as wind turbines and hydro or transmission towers.

    For more information: efraser@grenfell.mun.ca


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