Preference for & and Breeding Success in 6" x 6" Compartments with 2" Round Entrance Holes vs. 6" x 12" Compartments with Excluder Entrance Holes

Ken Kostka
Purple Martin Preservation Alliance

Pittsburgh, PA

    These nest check data sheets below are for a 16 compartment Trio Castle at Moraine State Park during the 2003 and 2004 Purple Martin Breeding season. This house has eight 6" x 6" compartments with round holes and eight 6" x 12" compartments with starling proof Excluder entrance holes. (16 compartments x 2 years = 32 compartments)


-13 of 16 Excluder 6 x 12 compartments had nests with 1 or more eggs laid (81%)
 -9 of 16  Round  6 x 6 compartments had nests with 1 or more eggs laid (56%)

Results-Breeding Success

-11 of 16 Excluder 6 x 12 compartments had nests with 1 or more young fledged (69%)
 -6 of 16 Round  6 x 6 compartments had nests with 1 or more  young fledged (38%)

63 eggs laid in Excluder 6 x 12 compartments
40 eggs laid in Round  6 x 6 compartments

43 fledged from Excluder 6 x 12 compartments
19 fledged from Round  6 x 6 compartments


    While I have not tested for statistical significance, the martins certainly seemed to prefer and breed more successfully in the 6" x 12" compartments with Excluder entrance holes in 2003 and 2004, with more than twice as many martins fledging from them and a 25% higher occupancy rate. I have offered Excluder, crescent, or round holes on this house since 2001. In 2001, this house had 100% occupancy with no round-holed compartments, 3 crescent 6 x 6, 1 excluder 6 x 6, and 8 Excluder 6 x 12. (I kept some 6 x 6 compartments closed.) In 2002, the house had 8 Excluder 6 x 12 and 4 Round 6 x 6, with all 4 Round 6 x6 fledging young and 5 Excluder 6 x 12 fledging young. 

    I strongly suspect that the colony began experiencing owl predation in 2004, and this may explain the reduction in preference/breeding success for the 6 x 6 compartments.  Even though the house was equipped with owl guards, the 6 x 6 compartments are still vulnerable to owl predation, in this author's opinion.

    I still believe it is a good idea to offer a few round-holed 6 x 6 compartments when attempting to attract martins. One never knows what type of entrance a female might prefer. Round-holed 6 x 12 compartments could also be offered if starlings can be rigorously controlled on a daily basis.


    Dan Airola was kind enough to analyze this data for statistical significance.

He states:

     "I did a simple statistical (chi-squared) analysis of the occupancy and nest success data, and found occupancy to not be significantly different, but nest success was marginally significant (probability value between 0.05 and 0.10, meaning this is the probability that you could get such a result by chance in a series of tests, when there was actually no difference).
He further states:

    I also looked at your data to assess productivity for each treatment.  Although I am a western martin person, and have never managed a single nesting pair in a nest box, this seems like  the most important comparison to make.   I have read that smaller compartments inhibit egg laying, reduce juvenile survival due to crowding, and result in higher predation (as you noted).  The summary showed an average productivity of 3.0 young per 6x12 compartment with excluders (48 young/16 nesting attempts) and 1.19 young per 6x6 compartment with round entrances (19 young/16 nesting attempts).  I didn't statistically test this because the difference is so great as to surely be significant. 

Click Here to View MS Excel File detailing Analysis

2003 Breeding season:

2004 Breeding Season: