Helping Bluebirds to Nest
Document Actions
It seems like a simple concept. Build and erect nest boxes in the proper habitat and they will come – the bluebirds. As with many situations in life, however, complications can mount quickly. The Great Swamp National wildlife Refuge in northern New Jersey is a very challenging place for bluebirds to raise a family. The Swamp has a variety of nest box competitors and predators.
Background
A Bluebird nest box trail consisting
of 47 nest boxes has been monitored on a weekly basis during the last two
breeding seasons. These nest boxes have been built and installed specifically
for bluebirds who are dependent to a large degree on these manufactured homes.
Unfortunately, the other denizens of the Swamp don’t know or don’t care that the
boxes were built for bluebirds. In the last two years five different cavity-nesting birds have nested in the boxes as well as two different mammals. In
order of nesting frequency the birds are Tree Swallows, Eastern Bluebirds, House
Wrens, Tufted Titmice and House Sparrows. Bluebirds can fledge two broods in the
Swamp. One pair of bluebirds laid four sets of eggs to fledge two broods. Their
second and third nestings suffered predation. A nest box can be used by several
species in the same year. Tree Swallows and bluebirds are the predominate
occupants of the boxes. House Wrens not only compete for empty boxes but also
destroy the eggs of nesting birds by pecking them. Then, the wrens may nest in
these abandoned boxes. House Sparrows are an invasive, non-native species
introduced from Europe. Their eggs are destroyed when found in the nest
boxes.
House mice are an all too common year-round resident of
the nest boxes. They construct their own nests and typically raise litters of 4
young in the boxes. The mice along with their nests are removed whenever they
are found. The mice tend to recolonize the same box repeatedly and can prevent
birds from nesting. The mice can also enter boxes in use by birds and disrupt
their breeding. In 2001, 47 mice nests usually containing one or more mice were
removed from 14 different nest boxes. This means that the mice nested in about
30% of the available nest boxes. The mice also reestablished nests in the same
box on average over 3 times in the April through July bird breeding period.
Relocating the mice a distance from the box slows their recolonization. Use of a
stovepipe predator guard (Figure 1) has been extremely successful in preventing
mice from occupying the nest boxes. For the 2002 nesting season, 17 posts were
fitted with stovepipe predator guards. No mice or mice nests have been found in
these boxes.
During the 2002 season mice continued to nest in
boxes with conical predator guards or no predator guards at about the
same frequency as in 2001. The plan is to install the stovepipe
predator guards on all posts.
One flying squirrel took up residence in a nest box after displacing a pair of bluebirds. The squirrel subsequently had a litter of two. The box was close enough to a tree so that the squirrel could glide to and from the box. This nocturnal rodent is common but rarely seen.
There are also cohabitators of the boxes (wasps) and parasites (blowflies). Paper wasps build umbrella-shaped, colony nests that are usually located on the inside roofs of the boxes. They occasionally kill Tree Swallows whose dead bodies are found in the box. When present, paper wasp nests and wasps are removed with care, as they are skilled at stinging the humans monitoring the boxes. Wasps have not been found in boxes used by bluebirds. Evidently, bluebirds do not use nest boxes that have paper wasp nests. Bluebirds may also prey on any wasps trying to establish a household in a bluebird-occupied box. Mud dauber wasp nests are also found occasionally in the nest boxes. These solitary wasps are docile and not likely to sting birds or monitors.
Blowfly larva are parasites and feed on the blood of nestlings. This usually is not fatal. After a clutch fledges, the old nest is removed from the box in the hope that the impact of blowflies on succeeding clutches will be reduced.
There are also visitors to the nest boxes looking for a free meal. Predator guards (conical or stove pipe) are installed on most posts to prevent or at least reduce snake and raccoon predation. The snakes leave no signs of entry. The nest is still in good condition but empty after their visit. Raccoons have ripped open boxes to get their meal. Both prefer eggs to nestlings – just like humans. Predation of nests almost always involves eggs. Once the eggs have hatched there is a high probability (about 90%) that the young will fledge.
While it is not possible to positively identify the predator in all cases, total egg predation can be accurately measured. Bluebird nest predation rate was 21% in 2001 and 43% in 2002. Tree Swallow nest predation was 10% in 2001 and 30% in 2002. Unlike bluebirds, Tree Swallows are very reluctant to leave their nests when they are brooding eggs or young hatchlings. Sometimes Tree Swallows need to be physically moved by hand in order to count the eggs in a nest. Tree Swallows also dive bomb nest monitors as they approach their boxes. The combination of their aggressive behavior towards potential threats and their reluctance to leave their nests may be the reasons for the lower egg predation rate for Tree Swallows.
Dead nestlings are sometimes found in nest boxes. The observed causes for dead nestlings include:
-
Blowflies or paper wasps.
-
Death of parent due to avian or other predation.
-
Insufficient food supply prevents the entire clutch from fledging – survival of the fittest starts at an early age.
-
Parents abandoning nest – when the time comes, Tree Swallows will migrate leaving not fully developed young to die in the nest. This occurs with second broods.
-
On rare occasions a hatchling will not fledge (leave nest box) with nest mates and will eventually starve to death.
One of the disappointments of the Great Swamp nest box trail is that bluebirds over a two-year period used only 21% of the nest boxes. Tree Swallows used 68% of the boxes. This greater than 3/1 ratio of Tree Swallows to bluebirds is unusually high for bluebird nest box trails in New Jersey. The New Jersey Nestbox Record Program (NRP) overseen by the NJ DEP (Division of Fish and Wildlife) summarized annual nest box breeding activity in bluebird nest box trails for a ten-year period. For all locations except the Great Swamp, bluebirds used about the same or more nest boxes than Tree Swallows. The combination of open fields, lots of water, abundant supply of flying insects and available nest boxes found in the Swamp appears to be the ideal habitat for Tree Swallows. A review of the nest data indicates that steps can and are already being taken to increase the ratio of nesting bluebirds to Tree Swallows.
The trail has two types of bird
boxes. There is the traditional vertical box (Figure 1) and a long horizontal
box (top of Figure 2). The long box has an internal weir. The birds are supposed
to but don’t always build their nest behind the weir. In theory the longer
internal path to the nest provides protection from predators.
The boxes are also arranged in three different configurations:
Paired – Boxes on single posts about 30 feet apart.
Shared – Two boxes on a single post (Figure 2).
Standalone – A single box on its own post at least 300 feet from the next nearest nest box.
Both types of boxes are used in the three configurations. The graph below (Figure 3) shows the occupancy rates for Bluebirds and Tree Swallows for a two-year period (2001-02).
Results
As shown in the graph below, Tree Swallows use all of
the configurations and boxes at about the same frequency that varies from 63 –
75%. The smallest percentage (63%) of boxes used by Tree Swallows is paired
boxes. Bluebirds nest in paired boxes with greater frequency (34%) than for the
other configurations. Bluebirds used only 8% of the shared boxes and 13% of the
long boxes. Paired boxes are preferred because both bluebirds and Tree Swallows
are territorial when it comes to nesting. Neither species will initially allow
another pair of same species to nest within about 30 feet. However, Tree
Swallows, unlike bluebirds, will allow another Tree Swallow pair to nest in the
other paired box once they have completed laying and begun brooding their
eggs.
The standalone boxes are predominately used by Tree Swallows who work in groups of 3-5 to harass a potential bluebird pair in the contest to use the box. The Tree Swallows usually win these contests especially when it involves a potential second nesting for the bluebirds. Bluebirds begin nesting earlier than Tree Swallows and have a better chance for securing a standalone box for their initial nesting.
The number of paired nest boxes was increased from 8 in 2001 to 22 in 2002. The most significant change observed was that the number of bluebird pairs increased their frequency of second nestings from 50% in 2001 to 75% in 2002.
To encourage increased bluebird
nesting, the plan is to configure all nest boxes as pairs located within 30 feet
of each other. Based on the data above this would increase the bluebird usage of
nest boxes from 21 to 34%. It is also planned to replace all long boxes with
conventional vertical nest boxes and to eliminate the shared box configuration.
This should further increase the percentage of bluebirds using the nest
boxes.
If you have a study or
project that you conducted on behalf of cavity-nesting birds, please share it
with us. All text must be submitted in electronic format. Images can be
submitted in either electronic or paper format.
Click here for submission
requirements and criteria.
NestWatch
159 Sapsucker Woods Ithaca, NY
14850
You can also e-mail your photo as an
attachment to:
nestwatch-support@cornell.edu
Thank you in advance for your contribution.
Please note, all submissions become the property of the Cornell Lab of
Ornithology.