Owl
Guards: Preferences and Breeding Success at an Established Colony
Ken
Kostka
Purple Martin Preservation Alliance
http://www.purple-martin.org
Pittsburgh, PA
9/21/05
Introduction: Placing owl guards on martin housing has become a
commonly practiced management technique. These guards help to prevent owls and
other large raptors from getting close to the entrance hole of Purple Martin
nesting cavities and extracting the martins. Some landlords are reluctant to
install owl guards in the belief that the barriers might discourage martins by
impeding their entrance and exit of the entrance hole area. This study was
intended to determine whether martins are more likely to choose compartments
with or without owl guards in T-14 martin houses.
Methods: I have used owl guards for the past two seasons at the
Saxon Golf Course colony near Saxonburg, PA. These guards consist of ˝” square
strips of wood placed 3 inches apart and attached vertically to the porches of
each section of the T-14’s. (see photo) The colony had 100% occupancy (28
pairs) in 2004 when all compartments had owl guards. I wanted to see how
frequently martins would choose to nest in compartments without guards if given
the choice, and to see which compartments would be chosen first - guarded or
unguarded. Therefore, I removed guards from half (14 compartments) of the
two T-14’s (28 compartments total) prior to their arrival for the 2005 season.
There were an equal number of guarded and unguarded compartments at all heights
and facing all compass directions. Both houses were wooden T-14’s, each
containing 14 identical compartments. All entrance holes were the Conley
Excluder.
Two factors could have biased the results: First, there is
a high degree of site fidelity among Purple Martins and many returning martins
had either nested in or had fledged from the owl-guarded compartments in 2004.
Since they had become accustomed to them, they may have come to ignore or prefer
them, even though they may have initially “disliked” them and considered them an
obstacle. Second, there is also a significant amount of cavity fidelity among
Purple Martins (Kostka, unpub. data), and returning breeders may have chosen the
same cavity regardless of whether it still had guards. For these reasons, I do
not intend to draw any hard conclusions with regard to true preference for or
against owl guards in cavity-hunting martins. True preference results could only
be established by testing the preference of first year breeders, and would have
to take into account whether the test subjects fledged from guarded and
unguarded cavities, since this could affect their choice as breeding adults. My
main objective was to determine if the martins, overall, considered the guards
to be a significant obstacle or inconvenience. If they did, then it seems
logical to conclude that more martins would tend to choose the unguarded
cavities first.
I used clutch initiation date to
determine which compartments were chosen first and presumably preferred. Only
20 of 28 compartments were used in this experiment because I blocked the last
eight compartments claimed as part of a
Forced Dispersal Project that was undertaken in early June. There is no CID
(clutch initiation date) for these compartments since no eggs were laid before
they were closed, but all had nearly completed nests.
Data: [terms for data table: WH-X =
compartment #; 0X/XX/05 = clutch initiation date; OG= Owl Guards present; NO =
No Owl guards present; XF = # of young fledged)
WH-02 05/13/05 OG 3F
WH-08 05/13/05 OG 5F
WH-18 05/14/05 OG 5F
WH-14 05/15/05 NO 6F
WH-20 05/16/05 OG 4F
WH-12 05/17/05 NO 3F
WH-11 05/25/05 NO 4F
WH-07 05/25/05 NO 1F
WH-28 05/26/05 OG 6F
WH-19 05/29/05 OG 4F
WH-01 05/31/05 OG 2F
WH-25 05/31/05 OG 4F
WH-27 06/01/05 OG 4F
WH-24 06/03/05 NO 4F
WH-10 06/04/05 OG 4F
WH-13 06/05/05 NO 0F
WH-06 06/05/05 NO 2F
WH-03 06/06/05 OG 0F
WH-21 06/06/05 OG 4F
WH-22 06/07/05 NO 2F
Results: Four of the first five
compartments (80%) and nine of the first 13 compartments (69%) claimed had owl
guards. 12 of the 14 guarded compartments (86%) and 8 of the 14 unguarded
compartments (57%) were among the first 20 chosen. A total of 45 nestlings
fledged from the guarded compartments, and 22 nestlings fledged from unguarded
compartments. An average of 3.75 nestlings fledged from guarded compartments and
2.75 young fledged from unguarded compartments.
Discussion: Since most of the compartments
claimed, both initially and overall, had owl guards, it is unlikely that the
martins at this colony considered the guards a highly significant obstruction.
Furthermore, the pairs nesting in guarded compartments fledged an average of one
extra nestling. I did remove starling nests (and trapped the starlings) from
WH-21 and WH-22 on 04/18/08; these were unguarded compartments and certainly
prevented martins from claiming them. But this occurred early in the season,
and I doubt it significantly affected the results. I lost 13 nestlings (found
dead) that were 20+ days old, and found a large owl feather under a tree about
150 ft. from the housing, making me suspect I was experiencing owl predation
(owls capturing one or both parents and causing the young to starve); therefore,
I eventually replaced all the owl guards in mid-July, with many nestlings still
unfledged and many fledglings still returning to the box at night. This also
may have affected the compared reproductive success results.