Habitat Differences are examined in study of Eastern Bluebird

By Victoria J. Byre and Mary Hennen

Although a number of detailed studies of the the Eastern Bluebird (Sialia sialis) have been conducted, there still are aspects of its life history, habitat preference, productivity, and population dispersal for which more information is needed.  In Illinois, for example, there are no quantitative data on life expectancy, survival rate, or mortality factors other than nest mortality.  Also very little quantitative data exists on nesting success, percent of a population returning to a site, or population dispersal between nearby nesting sites.

In this paper we examine Eastern Bluebird populations in three separate and distinctly different habitats in DuPage County, Illinois.  Data on nest success, productivity, nest site selection, and dispersal between sites are compared.  Several management techniques also are suggested.

The study area

DuPage County, which lies approximately 27 miles (45 km) west of Chicago, is a heavily populated and rapidly developing section of northeastern Illinois.

The three study areas - the Morton Arboretum in Lisle, 1,500 acres (600 ha), McKee Marsh in Winfield, 600 acres (240 ha), and Fermilab in Batavia, 6,900 acres (2800 ha) - lie along approximately the same latitudinal line, and each is separated from the next by 3 to 6 miles (6-10 km) of suburbia.

The Morton Arboretum bluebird trail, which consisted of 73 boxes by year three of the study, is located in open, savanna-type woodlands.  Large oaks are the dominant trees, and the ground cover is composed of Eurasian grasses that are mowed one to three times per year.

At McKee Marsh, 5.6 miles (9 km) northwest of the arboretum the 41box trail is in open, meadow-type grassland that borders an 80-acre (32 ha), man-made marsh.  In contrast to the arboretum site, none of the boxes at McKee Marsh are under a wooded canopy; most border floodplain woodlands or wooded fence rows, and some are very much in the open, situated in meadows on exposed hilltops.

Fermilab, a high-energy physics research laboratory, the grounds of which are designate a National Environmental Research Park, is approximately 3.5 miles (6 km) southwest of McKee Marsh.  Most of the 18 bluebird boxes that exist on the site border a 10-year-old reconstructed prairie.  Indian grass and big bluestem are the dominant grasses.  Part of the prairie usually is burned each year in early spring or late fall.  Scattered oak trees and an old apple orchard provide canopy cover for a few of the boxes.  Five of the 18 boxes are located along the edge of an old pasture.

All three study sites had bluebird trails established four to eight years prior to this study.  The boxes at all sites had been monitored only irregularly, however, and only at the arboretum site did a few pairs of bluebirds consistently fledge young.

Study methods

Only Peterson-style nest boxes, separated by a distance of at least 55 yards (50 m) and usually greater than 100 yards (90 m) (except for a few placed within 10 to 20 feet (3-6 m) of another box to reduce tree swallow competition) were used at all three study sites.  Thirty-one boxes were added to the arboretum site at the beginning of the second year of the study, and three more were added at the start of the third year.  The number of boxes at the other two sites was kept virtually constant during all three years of the study.  Most boxes were mounted on either smooth metal poles or steel fence posts (T-posts).  A few boxes at the Fermilab site were attached to trees.  By the end of the second year of the study approximately 90 percent of the 130 nest boxes were protected with at least one type of predator guard, usually either a heavy coating of grease on the pole, a wire cat and coon guard, or both.

Throughout the nesting period (April to mid-August) from 1989 through 1991, all boxes were monitored a minimum of one to two times per week.  Species use was determined by presence of adults or characteristics of nests or eggs.  A nest was defined as a box containing at least one egg, and a successful nest had at least one fledged young.

House Wrens were discouraged from nesting in the bluebird boxes by placing the boxes away from shrubby, brushy areas and thickets.  Tree Swallow competition was reduced by placing an additional nest box within 10 to 20 feet (3-6 m) of each of the nest boxes that was favored by the swallows.

House Sparrows, in the few instances in which they attempted to nest, were discouraged by removing the nesting material immediately.  Wasps and their nests were removed from in or under the nest boxes, and, where wasps were a persistent problem, petroleum jelly was applied to the inside roof of the box discouraged them from attaching their nests. Bluebird nest and egg characteristics, including laying and hatching dates, clutch size and color, length and width of eggs, number of nestlings and fledglings, and presence of ectoparasites such as blowfly larvae were recorded.

Bluebird nestlings were banded when 8-12 days old with a standard aluminum U.S. Fish and Wildlife Service size 1B band and a unique combination of one to three plastic, butt-end type, color leg bands.  Adult bluebirds, which were banded in a similar manner, were captured in mist nest or usually in the nest box either by placing a hand over the entrance hole or by using a cardboard "trap door" that had been taped to the inside top of the entrance hole and propped up with a stiff grass stem' The trap door, which was tripped when an adult bird entered the box, flapped down over the entrance hole, trapping the bird inside

To minimize possible desertion of the territory or nest, adults were not captured until the latter stages of the nesting cycle.  Most captures took place near the end of incubation or when very young (one day to five days old) nestlings were present.  Banding was not attempted in rainy weather or when the temperature was below normal, and nests were never disturbed for more than 10 minutes at any one time.  Adults were banded whenever possible, especially at the Morton Arboretum site, but priority was placed on banding all nestlings at each study site. individual bluebirds, identifiable by their color bands, were observed with binoculars and spotting scopes to document behavior and dispersal to new sites.

Nest-box characteristics and the habitat surrounding each nest box were assessed to try to determine significant nest site selection factors and relevance to nesting success.  Fifteen variables were measured at each nest box during 1990 and some again during 1991.  Measurements were taken by a single observer within a five-day period during the nesting season.  Entrance hole length and width, and height from the ground to the bottom rim of the hole were measured with a tape measure. The direction each box faced was determined with a hand-held compass.  Distance to nearest perch and nearest closed canopy cover were measured with a surveyor's measuring wheel, as were distances to the nearest water, road, and building. Distances over 110 yards (100 m) were estimated by pacing.  Height of the grass within a 10-foot (3 m) radius of each box was estimated by standing a tape measure parallel to three to five representative stems, measuring the heights in their natural positions, then averaging the results.  The density of the grass within this same area was estimated as sparse (up to 50 percent bare ground), high (thick carpet with little or no bare ground visible), or medium (between the two extremes).  Dominant grasses were estimated by visual inspection of grasses within the 10-foot (3 m) radius.  A larger area, 110-yard (100 m) radius, around each box was rated as to general habitat type: savanna, consisting mostly of large trees such as oaks; widely scattered deciduous trees or orchards; tree-lined fence row along grassland or pasture, or grassland within 55 yards (50 m) of woodlands; open grassland.

The results

Bluebirds nested at all three study sites during each year of the study.  The Morton Arboretum site consistently had the highest percentage of box use by Eastern Bluebirds and the highest productivity as measured by number of young fledged per successful nest.  The Fermilab site was consistently the least productive.

Of 208 bluebird nests attempted in the study area during 1989-1991, 149 (72 percent) were successful in that at least one young fledged.  Eighty-five percent of the successful boxes were mounted on round metal poles and were equipped with at least grease as a predator deterrent.  Annual variation in success ranged from 66.7 percent (1990) to 78.3 percent (1989).  An average of 3.9 eggs was laid per nest, and an average of 3.5 young were fledged per successful nest.  For all seasons and study areas combined, approximately 65 percent of the eggs laid resulted in fledged young.

When comparing the three study areas, differences in nest success and productivity become apparent. Over the three-year nesting period, bluebirds at the Morton Arboretum site had an average success rate of 76.6 percent (nests which fledged at least one young), and an average of 68.2 percent of the eggs laid resulted in fledged young.  The bluebird nests at the McKee Marsh site, on the other hand, had average success rate of 63.5 percent and only 56.6 percent of the eggs laid resulted in fledged The Fermilab site had too few bluebird nests to make valid comparisons.

During the first year of the study, House Wrens were the major known cause of bluebird nest failure, resulting in a 6.7 percent loss of bluebird eggs.  This rate dropped to 3.0 percent during the second year and rose to 4.7 percent during the third year.  House Wren competition was most prominent at the arboretum site which has a greater abundance of the brushy, shrubby habitat that the wrens prefer.

During the second year of the study, predation by raccoons became the major known cause of nest failure.  Raccoons caused no loss of eggs at any of the sites in 1989, but were responsible for a 10.7 percent loss of bluebird eggs during 1990. At McKee Marsh during that year, 251988 percent of the bluebird eggs were lost to raccoons.  With the installation or application of anti-predator devices, the overall failure due to raccoons dropped to 4 percent in 1991.

Tree Swallow competition was a cause of bluebird nest failure only at McKee Marsh site (11.5 percent egg loss in 1990), where boxes were situated in much more open habitat than at the other two sites. The placement of a second nest box within 10 to 20 feet (3-6 m) of a box at which bluebird and Tree Swallow competition was occurring almost always resulted in successful nestings by both species, one in the original box and one in the new one.  In one instance, however, Tree Swallows nested in both boxes at a distance of only 8 feet (2.5 m) apart, a rare occurrence for such a territorial species.

Human interference with nests or young was not a major factor until the third year of the study, but did cause 7.4 percent of the eggs to fail that year, mainly from vandalism to occupied nest boxes. Unknown causes of failure include such factors as infertile or abnormal eggs, faulty incubation, unexplained abandonment of eggs or young, unexplained disappearance of one or both adults, and fluctuations in food supplies and weather conditions.  Most of these factors are extremely difficult to verify, hence the large percentage of unknown causes of failure each year.

Banding

Forty-five adult Eastern Bluebirds  (36 females and nine males) and 579 nestlings were banded during the course of the study.  Forty-seven of these (three adult females and 44 nestlings) were banded in before the formal initiation of the study (with USFWS bands only) at the Morton Arboretum site.

Thirty-eight individuals or 8.6 percent of the 444 bluebirds banded from 1988 through 1990 returned to the study sites for one or more years (birds banded in 1991, of course, could not be included in this data).  Thirty (19.4 percent) of the bluebirds (23 nestlings and seven adults) banded in 1989 returned in 1990, 1991 or both.  Seven of the nestlings that returned as adults to nest on the study sites in 1991 had been banded in 1989 and were not in evidence at any of the sites during 1990.  All seven of the year-old, banded male nestlings that returned to nest at the Morton Arboretum in 1990 were paired with unbanded females.  And, although all but seven (17 out of 24) of the adult females and all nestlings at the Morton Arboretum were banded in 1990, 18 of the 29 females nesting there in 1991 had no bands.

Five bluebirds had a consecutive three-year history, and five bluebirds had a three-year history but with the middle year missing.  Seventeen bluebirds returned for two consecutive years during the study.  A few individuals returned to the same box from which they were fledged or in which they had raised young the previous year, but most birds moved to other boxes within the same study area from one year, or even from one brood, to the next.

While most of the returns were seen at the same site at which they had been banded, there was evidence of interaction between the three study areas.  A female bluebird banded as a nestling at Fermilab in 1989 nested at the arboretum in 1990, and a male banded as a nestling at the arboretum in 1989 nested at McKee Marsh in 1990.  Also, two males banded as nestlings from the same nest at McKee Marsh in 1989 returned in 1991 after a year spent elsewhere, one to Fermilab and one to the arboretum.

Management recommendations

During our study, the number of Eastern Bluebird nesting pairs increased from 11 in 1988, before regular monitoring began, to a peak of 49 in 1990, The number dropped to 42 in 1991, despite a slight increase in the number of boxes available.  Also, despite an almost constant number of boxes available from 1989 through 1991, the number of nests peaked at 87 in 1990, up from 46 the year before, but dropped slightly to 75 in 1991.  These preliminary results help confirm the assumption that availability of suitable nesting cavities is but one of the many interdependent and complex factors involved in Eastern Bluebird population dynamics.

Seventy-nine percent of the bluebird nesting activity took place at the Morton Arboretum where the habitat consists of open grassy areas with scattered trees and shrubs.

The variable that figured predominately in the majority of use-versus-no-use cases was grass height.  In every use-versus-no-use situation, mean grass height was shorter for the use category; the grass height averaged 24 inches (62 cm) at the arboretum, 27.5 inches (70 cm) at McKee Marsh and Fermilab.  The Preference for shorter grass was strengthened when the hole height also was considered.  The mean hole height 67 inches (171 cm) for the arboretum was higher than that found at the other two sites (49 inches [124 cm] for McKee Marsh and 60 inches [152 cm] for Fermilab).

Our results concur with past studies of bluebird habitat, confirming the Eastern Bluebird's preference for open areas with scattered trees and sparse or relatively short herbaceous vegetation.  The importance of short grass or sparse vegetation to effective foraging by bluebirds may be the key factor in this habitat preference, more important, for example, than entrance hole height or the grass height to entrance height ratio.

Competition from other cavity nesting species is a factor that can greatly influence bluebird nesting success and one that can never be completely eliminated.  Although House Wren competition for nest boxes was reduced by placing the boxes away from the shrubby areas preferred by the wrens, wrens were still a major cause of bluebird nest failure during all three years of the study.  Because House Wrens are smaller than bluebirds, there is little that can be done to make a bluebird nest box wren-proof and still acceptable to bluebirds.  Several hole designs and wren guards have been tried on bluebird boxes, and a wren guard designed by H. S. Pollick was tried on several of our boxes.  Results of our tests and those of others show that wrens still easily were able to gain access to nest boxes to build nests or to destroy bluebird eggs.  Hence, the only at least partially effective way to reduce competition from House Wrens (the birds, their nests and eggs are protected under the Migratory Bird Treaty Act) is by trial and error: relocate the nest boxes used by wrens, usually to more open areas, until they find them unacceptable.

Boxes placed in open areas should be monitored closely for Tree Swallow competition. if a second box is expediently placed within 10 to 20 feet (3-6 m) of the contested box, the chances of having both species nest successfully will be increased.

The fact that raccoon-caused nest failures rose from 0 percent in 1989 to 10.7 percent in 1990 may represent learned behavior by the raccoons.  By using several anti-raccoon management techniques - mounting houses on smooth metal poles, greasing the poles with white lithium grease, and attaching wire guards to the fronts of the boxes - overall egg loss due to raccoons was reduced to 4 percent the following year.  It is safe to assume that eventually a raccoon will find and try to investigate any nest boxes in its territory.  Precautions taken to prevent them from successfully gaining access initially will save much time and effort later, as raccoons are very persistent once they have found a food source.  Nest boxes always should be mounted at least 51 inches (I .3 m) above the ground on smooth round poles on which is applied a liberal coating of grease at least 20 inches (0.5 meter) from top to bottom (from one foot above ground to within one foot of the box; carnuba car wax may be a less messy alternative).  In high-risk areas, an anti-predator device also should be attached below or onto the front of the nest box.  The Noel cat and coon guard is highly recommended.  The "bird guardian," a tube-like commercially available predator guard that fits over the entrance hole, has not proved satisfactory.

Human-caused nest failures best can be prevented by strategic placement of the nest boxes, especially away from frequently used paths or roads, and through informative signage and public education.

While a number of Eastern Bluebird returned to their previous year's nesting areas the following year, a surprisingly large number of unbanded adult birds appeared each spring, despite the fact that all nestlings and most adult females the majority of which nested successfully - were banded at each study site during previous years.  This finding is somewhat contrary to a study done by B.C. Pinkowski in which the number of Eastern Bluebirds nesting in an area gradually increased after the initial provision of nest boxes, apparently because young birds that were reared in the area were returning there to breed, and adults that were successful in the area were returning there for subsequent nestlings.  While our bluebird population also increased quite dramatically, many of the birds were new to each site each year and took advantage of the abundance of nest boxes provided.  This high turnover rate was surprising and certainly would not have been suspected had the banding study not been done.

Preliminary banding returns also indicate that a large number of nestlings from our study areas spent their first adult summer somewhere other than their hatching site, then returned to their hatching side during their second adult year to nest.  Whether these two-year-olds nested somewhere else during their first summer is unknown.  Our banding returns also show that local bluebird populations do intermix.  Areas of higher productivity, such as the Morton Arboretum site, may provide a source of birds for areas of lower productivity, such as the McKee marsh and Fermilab sites.  Likewise, areas of higher-quality habitat may draw birds to them from less suitable areas.

Many factors are involved in the ultimate size and stability of a local Eastern Bluebird population.  The availability of suitable nesting cavities is certainly a major factor, but food supply, predators and competitors, habitat characteristics, and local and regional weather conditions also are important factors that influence size and success of local populations.  It seems likely that if good quality habitat within the Eastern Bluebirds' nesting range contains properly monitored nest boxes bluebirds eventually will colonize the area, provided the regional population base is sufficiently high.  These local areas, especially those which provide predator-proof nesting boxes, can aid in increasing the Eastern Bluebirds' nesting success.

ACKNOWLEDGMENTS

We wish to thank the administrations and staffs of the three study areas for allowing us to conduct bluebird studies on their sites and for aiding us with equipment and labor!  Thanks especially to Dick Wason (deceased) and Chris Whelan of the Morton Arboretum, Dan Ludwig of the DuPage County Forest Preserve, Rudy Domer of Fermilab and Ed Miller of Governoes state University.  Jeff Hardt put in uncountable hours monitoring bluebird boxes especially at the Morton Arboretum and McKee Marsh sites, and Joan Harmet put in many hours of data entry.  Ron Olsen and his DuPage County Boy Scout Troop constructed a large number of nest boxes for us.  This study was funded in pan by the Chicago Academy of Sciences and by a grant from the North American Bluebird Society.

LITERATURE CITED

Information from the following publications was used in preparation of this article:

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