Pairing houses: How do you do the math?

By Linda Janilla

Pairing bluebird housing to prevent takeover by Tree Swallows seems to be a controversial topic.

Whenever I read pairing study results, I have a nagging question regarding the evaluation of the collected data.  When a person pairs houses on a trail, in effect they are decreasing their nesting sites.  Therefore, should not data analysis be done by counting the available nesting sites, not nest boxes?  In essence, they are offering half of their trail to Tree Swallows.  One of the housing pair is meant to be a Tree Swallow box, not a bluebird box.

For example: Alan has a 100-nestbox trail.  He has paired his boxes.  Each pair is 100 or more yards from the next pair.  He now has 50 sites to offer nesting bluebirds.

Brenda has a 100-box trail.  She does not pair.  Each nestbox is 100 or more yards from the next box.  She now has 100 sites to offer bluebirds.

If each gets 47 bluebird nests, should not the percent of occupancy be figured on nestbox sites?  Adam has 50 sites and gets 47 nests: His nest site occupancy is 94 percent.  Brenda has 100 sites and gets 47 nests: Her nest site occupancy is 47 percent.

Similarly, if one is judging fledging rates, should not the numbers be also calculated by nesting sites not box count?

Common sense tells me that when I place a pair of nest boxes, I am NOT expecting two bluebird pairs, but ONE nesting pair.  Therefore, I have provided one site for bluebirds.  Of course, if someone is comparing trails and then counts box numbers instead of site numbers, an unpaired trail could appear to have a higher bluebird occupancy rate.

My personal experience: 10 years on a golf course bluebird trail of 48 houses.  The first years I had unpaired boxes.  I offered 48 sites.  One year, one quarter, or 12, of those were occupied by bluebirds (25 percent bluebird occupancy).  Tree Swallows were taking most of the boxes.

Then I paired the houses, in essence offering 24 sites.  Two-thirds, or 16, houses were occupied by bluebirds.  If you figure percentage on my site numbers, I had a 66.66 percent bluebird occupancy.

Many box pairs consisted of a Tree Swallow/bluebird combination.  Never did I have a swallow/swallow combination.  My boxes were paired 15 feet apart.  Each pair was over 100 yards from the next pair.  In subsequent years, the numbers were similar.

The end result for me was that pairing increased my bluebird occupancy numbers by decreasing swallow occupancy.  The golf course had many ponds, and Tree Swallows were there in great numbers.  The trail is on the extreme eastern edge of Minnesota.

In my backyard, I also pair boxes, and I have observed that by doing do I have ended the battles between Tree Swallows and bluebirds over a single nestbox: I normally have one bluebird pair and one swallow pair in the backyard.

(Ms.  Janilla lives at 13325 4th St. N., Stillwater, MN55082.)