Starlings and oval-holed nest boxes
By Kevin Berner
NABS Research Chairnmn
Many researchers have experienced greater success in attracting
bluebirds to Peterson nest boxes than standard nest boxes. My field
tests determined that it is the oval hole of a Peterson box and not its
wedge-shaped design that most attracts Eastern Bluebirds. W. H. Davis,
in a 1997 paper, also identified the oval hole as being preferred over
other entrances, such as slots and round holes.
There are, however, several advantages of standard nest boxes over
Peterson boxes. First they are much simpler to build for someone with
limited construction skills because they don't require cutting wood at
unusual angles. They are also much lighter and do not require as rugged
a mounting post as does the Peterson box.
While uncommon, on occasion European Starlings will use Peterson
boxes. D. M. Lehmann wrote in 1997 of controlled tests which determined
that starlings can readily escape a simulated nest box with a 1 3/8-inch
by 2-inch oval hole. I suspect that although starlings can negotiate
these oval entrances, they are reluctant to do so because of the cramped
interior space of a Peterson box. If oval holes were used on the larger
standard nest boxes, it would be expected that starlings would be more
likely to attempt to nest in these boxes. In fact, starlings have nested
in several oval-holed standard shaped boxes along the Route 20 Research
Trail in New York State.
My goal was to determine if there is a size of oval hole preferred by
bluebirds over round holes that also minimizes the chance that starlings
can pass through it.
METHODS
In the spring of 1998, I captured six starlings and placed each of
them in a box with both 1 3/8-inch and 1 1/4-inch-wide oval escape
holes. Both holes were two inches tall. Starlings were allowed to escape
through the wider hole before attempting passage through the narrower
hole.
I also converted all of my research nest boxes to pairs of standard
nest boxes with oval holes. Each pair included one nest box with a 1
3/8-inch-wide hole and the other with a 1 1/4-inch-wide oval hole to
determine if bluebirds would dis-of that size.
There is a greater motivation to escape a box after being handled than
to enter a cavity to use as a nest site.
Bluebirds did show a strong preference for the wider 1 3/8-inch hole,
using those boxes for 36 of 40 nesting attempts (a nesting attempt is
defined as a building a nest and laying at least one egg). Swallows
used more of the narrow-holed boxes. Starlings did build a nest and
lay two eggs in one of the nest boxes with the wider hole. House
Sparrows and House Wrens were a minor presence in these tests (see
table).
Results of field test of standard and
narrow oval holed nest boxes.
|
Species |
Number of nesting attempts |
|
1 3/8 in. X 2 1/4 in. |
1 1/4 in X 2 1/4 in. |
| Eastern Bluebird |
36 |
4 |
| Tree Swallow |
24 |
35 |
| House Wren |
0 |
3 |
| House Sparrow |
2 |
0 |
| European Starling |
1 |
0 |
DISCUSSION
These tests show that starlings can use standard nest boxes with the
Peterson-size oval hole. I have had many of these boxes in the field
over several years with only one starling nest attempt to date, although
I have watched starlings easily enter and examine these boxes on several
occasions. A slightly smaller oval hole may reduce the chances of
starling use.
As expected, bluebirds showed a strong preference for the larger oval
holes. Bluebirds have consistently shown a preference for oval holes
over the 1 1/2-inch round holes that have a smaller overall size. A few
bluebirds did successfully nest in the narrower oval as well.
A better test might have been to compare the narrow oval to round
holes to see if bluebirds still preferred the oval hole shape. My
ultimate goal is to determine if there is an oval hole size that is
preferred over a round hole while eliminating use by starlings. A 1
1/2-inch or 1 9/16-inch round hole is effective at excluding starlings
from nest boxes.
At this point, I would still recommend oval-holed standard nest boxes
as long as the boxes are monitored regularly to ensure that starlings
are not allowed to use the boxes. if regular monitoring is not possible,
then a round hole would be preferable.
ACKNOWLEDGMENTS
All the boxes used in this study were built by Herman Bressler and
paid for by the New York Bluebird Society. The Bluebird Recovery Program
in Minnesota covered my transportation expenses.
Literature consulted for this article:
