Month: October 2014

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Information from “The Barn Owl” by Colin Shawyer

Prey Items and Diet Variation

Barn Owls swallow prey whole and are incapable of digesting much bone, which is why the contents of their pellets can prove very revealing. They tend to prey on small mammals such as voles, mice and shrews, with Field Voles being the most nutritionally important prey item in mainland Britain. Shrew are not as nutritious in terms of weight.

The amount of prey consumed must be especially high before breeding and throughout the nesting period, when prey items consumed for a pair of owls increases from around ten to forty per day. The breeding biology of the Barn Owl is largely adapted to the vole’s availability. The population of the vole peaks around Autumn, which is when inexperienced young owls are first hunting. By comparison,  the Wood Mouse population tends to peak early-winter, while shrew numbers peak mid-summer.

In the Spring shrews become more vocal and male voles become more active, while rats often move to farm buildings in late-Autumn. These changes are typically reflected by changes in the Barn Owl’s diet, as they make these prey items easier to find.

The number of mice taken over the summer may fall as they have more cover to hide in. Moles only tend to be taken between May and August, during which time they are closer to the surface. Starlings and House Sparrows feature more commonly in the Barn Owl’s diet during Winter.

Barn Owls do not appear to select voles, mice and shrews of any particular age or sex.

Pellets

Pellet length can provide an indication of the amount of live prey taken (Although Michael Toms has found a more reliable way to determine this -Look this up? Is it relevant to my investigation?) . Immature owls may digest bones more thoroughly than adults.

Pellets can provide the following information:

  • Age (amount of tooth wear or jawbone length)
  • Structure
  • Sex (From shape of pelvic bone)
  • Can also reveal previously unknown species in an are

 

This bears no relevance to my investigation as I will not personally be collecting the pellets I will be using, but I found the following warning amusing:

Barn Owls seem very active in their bombing, usually selecting an escape path overhead and…

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Planning Stage: What am I doing?

  • Do adult and young Barn owls have different nutritional requirements?
    • Presumably yes?
  • What is already known?
    • Find out!
  • Why is this interesting?
    • This could be used to gain a better understanding of Barn Owls’ dependence on other species from an ecological point of view and would therefore be relevant when considering habitat destruction and conservation.
    • Important consideration for reintroduction attempts
      • Reintroduction attempts have previously been opposed by leading conservationists on the following grounds:
        • Released individuals likely to starve and die
        • Released individuals may introduce disease or compete with existing wild population
        • Captive birds may not integrate well with wild birds
        • No resulting population increase
        • Genetic consequences
      • There was a DEFRA consultation paper on this in 2001 
      • Decline of Barn Owls in Britain in the century due to (according to Barn Owl Release in Lowland Southern England- A twenty-one year study by Meek, et. al):
        • Loss of field vole habitat
          • Agriculture intensification and town expansion
        •  Loss of nest sites
          • Barn conversions and Dutch Elm Disease;
        •  Severe winters
        • Disappearance of the corn rick (winter home of owls prey species) since the increased use of the combine harvester
        • Chemical poisoning by pesticides
        • Increased road casualties