Effects of Moving Housing:
A Five Year Look at the Lock 4 Colony
Ken Kostka
Purple Martin Preservation
Pittsburgh, PA

In 2005, a colony of Purple Martins was established on the bank of the Allegheny River at the US Army Corps of Engineers Lock 4 facility in Natrona, PA. One subadult pair, the result of a Forced Dispersal project, bred successfully. A complete account of this breeding attempt can be found at
http://purple-martin.org/MartinsBreedInNatrona.htm.  The martin house was located in what I will refer to as the south lawn, denoted by the red dot in the photograph at left. (click to enlarge)  The location of the north lawn is denoted by the yellow dot. Both lawns are about 50 feet from the water and about 200 feet apart. It should be noted that although this location is the bank of the Allegheny River, this part of the river is actually more like one long winding lake because of a series of fixed-crest dams that create wide, deep, pools used by commercial barges and pleasure boats.

    Both martin houses are wooden T-14’s placed about 20 feet apart, except for 2007, when the houses were in separate lawn areas. House #1 consists of compartments #1-14, and is always closer to the road, whether in the south or north lawn. House #2 consists of compartments #15-28, and is always closer to the river, whether in the south or north lawn. Note that in 2007, house #1 was moved to the north lawn, and in 2008, house #2 was moved to the north lawn. Housing was moved because of its proximity to an outdoor lunch bench and sanitary concerns based on the growth of the colony.

Breakdown of house location and occupancy by year:

2005                1 pair

house #1          south lawn        1 pair
house #2          south lawn        0 pair (house added July 20th) 

2006                3 pairs (+3 nesting failures)

house #1          south lawn        1 pair (+2 pair failed)
house #2          south lawn        2 pair (+1 pair failed)

2007               8 pairs (22% occupancy)

house #1          north lawn        2 pair
house #2          south lawn        6 pair

2008              9 pairs (25 % occupancy)

house #1          north lawn        7 pair
house #2          north lawn        2 pair

2009                20 pairs (56 % occupancy)

house #1          north lawn        13 pair (+2 pair failed)
house #2          north lawn        7 pair


2005    1 pair                           base year
2006    3 pairs                         200% increase
2007    8 pairs                         166% increase
2008    9 pairs                         12% increase
2009    20 pairs                       120% increase

Analysis/Conclusions. Moving a house as little as 200 ft. between seasons can have a considerable negative impact on the number of breeding pairs using that house. When house #2 was moved from the south lawn to the north lawn for the 2008 season, the number of pairs dropped from 6 to 2. Based on the 2 to 7 to 13 pair growth rate of house #1 when it was located in the north lawn from 2007 to 2009, during which the number of pairs nesting in it increased by 5-6 pairs per year, I would have projected a growth rate for house #2 of 6 to 12 to 18 pair in 2009, if it had not been moved. And since house #1 had 13 pairs in 2009,  the number of breeding pairs might currently be at 30-36, close to full occupancy, had no housing been relocated.

Several other factors may have affected colony growth. First, the north lawn is somewhat less open than the south lawn, with several large trees in the immediate area. Also, in 2008, the first year that both T-14's were located in the north yard, a major valve refit was done on the lock chamber, and a slew of corps workers descended on the facility, parking their cars in the north lawn, directly under the martin housing! I arrived to find the cars there a day later in mid-June, and quickly asked for permission to rope off a 15 ft. perimeter around the housing, which was allowed. The work went on for several weeks, with workers coming and going 24/7. Although the established pairs bred successfully, there may have been a few unestablished pairs hanging around that were discouraged from nesting by all the commotion. 

    2009 a major growth year for the colony, as it grew from 9 to 20 breeding pairs, making it the first year that the number of breeding Purple Martins outgrew the number of breeding Tree Swallows, which has held steady at about 12 for the past 4 years. Gone are the days when I had to battle Tree Swallows trying to encroach on the martin housing.  I do not plan to add any additional housing – martin or swallow. The Tree Swallow housing consists of 14 large plastic gourds with starling-resistant crescent entrance holes hung about 25 ft apart along a wire rope fence that runs the length of the lock wall. The gourds are four feet above the walkway and 30 ft above the water. I have occasionally seen martins investigating these “Tree Swallow” gourds.