Bat House Forum

[IowaNate]

   How do retailers and other bat house makers calculate how many bats a house can hold? I have a method for determining how many bats can fit into my houses comfortably... but it is limited in research.

  Given my old nursery house that was occupied by 250 adult bats, and measuring the size of the space the bats took up on each partition...I came up with an average size of my little browns. When in a 3/4" crevice, the average little brown was 1.25" wide and 2.25" long.

  Each crevice was 12" wide and 21 " tall...so 12/1.25= 9.6 (which I always round down) 9 bats wide...21/2.25= 9.3 (9 bats hanging "nose to tail").

    So 9 x 9 is 81 bats per crevice, and there are four crevices 81 x 4= 324 bats

  This house has held no more than 250 bats for about 7 years, which is less than what I thought for maximum occupancy. So 250/324= .77, just about 3/4 what I figured it would hold.

   Now I have seen many photos of bat houses packed so tight that the bats are protruding from the bottom of the house. I see so much information on houses said to hold 50, 100, or 300 bats...how are these figures being calculated?

    Just food for thought...I look forward to comments and speculations!

[Joe Spencer]

I believe the figures are often based on the designers or builders assumption and not based on some of the facts that you graciously showed above.  Bat houses that are seemingly overcrowded as we've seen in person or in photos are usually only that packed when roosting sites are scarce in a given area.  If an additional bat house were put up beside a seemingly overcrowded one, some bats would relocate to the additional bat house.  I saw this at a few of my sites where I had two bat houses and when I put up a third it would be occupied soon after with the numbers slightly decreasining in the initial two.  It also depends on what stage of growth the pups are at and whether the mother will relocate the pup or not prior to its independence.  These non-vacant houses are nursery colonies and not bachelor colony roosts which have far fewer bats in them.