A Starling--Deterrent Wood Duck Nest Box

A Starling--Deterrent Wood Duck Nest Box

FRANK B. McGILVREY, Patuxent Wildlife Research Center, Laurel, Maryland
FRANCIS M. UHLER, Patuxent Wildlife Research Center, Laurel, Maryland

Abstract: In many, parts of the United States, the starling (Sturnus vulgaris) has become a serious competitor for nest boxes erected for wood ducks (Aix sponsa). Research at the Patuxent Wildlife Research Center and at Eastern Neck National Wildlife Refuge near Rock Hall, Maryland, demonstrated that horizontal nest structures with semicircular entrance holes 11 inches in diameter were acceptable to nesting wood ducks but discouraged nesting by starlings. Starlings seemed to prefer boxes in open impoundments to those in wooded impoundments, whereas wood ducks seemed to show no preference.

Programs that provide nesting structures for wood ducks have become increasingly popular with both state and federal conservation agencies as well as with private organizations and individuals. Such programs are responsible for increased wood duck populations locally, and thereby enhance public enjoyment of this splendid waterfowl. However, as pointed out by Bellrose (1955), Webster and Uhler (1964:11), Grice and Rogers (1965), and Bellrose and McGilvrey (1966), predation on nests or competition for boxes can nullify the value of these programs. Over the years, a vertical box, 12 inches in diameter and 24 inches in length, with a 3- x 4-inch entrance near the top has evolved as the nest box preferred by wood ducks.

Fig. 1. Not available, was of a metal box with a 4- X 11-inch entrance. The guard was a 3- X 40-inch aluminum downspout. Opening the rear panel deterred wood ducks as well as starlings.

Unfortunately, the box that is ideal otherwise for the wood duck is also preferred by the starling. This exotic nuisance has become a serious competitor for nest boxes throughout much of the breeding range of the wood duck. Starlings first used the vertical, wooden nest boxes on the Patuxent Wildlife Research Center near Laurel, Maryland, about 1950. By 1954, they were nesting in most of the boxes. Vertical, metal nesting cylinders of the type described by Bellrose (1955:28-30) and Webster and Uhler (1964:5-8) , were erected in 1957. Starlings immediately accepted these boxes and soon monopolized them to the extent of almost excluding the wood ducks.

The objective of this study was to develop a nest box that would be used by wood ducks but not by starlings. Casual observation suggested that ducks would tolerate more light than starlings, and the research was designed to test this hypothesis.

METHODS AND MATERIALS
The basic experimental nest box was a horizontal cylinder 24 inches long and 12 inches in diameter; it was made of metal or of wire netting covered with roofing paper and had wooden ends (Fig. 1). To determine acceptability to ducks and starlings, entrances of the following sizes and shapes were tested:

1963-all semicircular, 4 x 11 inches;
1964-all wedge-shaped, 3 x 4 inches;
1965-half 4 x 11 inches and half 6 x 7 inches;
1966-1970-all semicircular, 4 X 11 inches.

In 1963 and 1964, 14 horizontal boxes had rear panels open slightly to allow light to enter.

About 40 vertical, metal boxes with 3x 4-inch elliptical entrance holes, as recommended by Bellrose (1955:22-23), were also available during 1963 and 1964. From 1965 to 1970, however, only the horizontal box was tested at Patuxent (Table 1). At Eastern Neck National Wildlife Refuge near Rock Hall, Maryland, in 1970, use of the metal horizontal boxes by starlings and wood ducks was compared with that of both metal and wooden, vertical boxes (Table 2).

The studies at Patuxent were conducted on 15 impoundments ranging from 2.5 to 53 acres. Six impoundments are dominated by marsh vegetation, five are mostly open water, three are flooded swamps with dead and fallen trees, and one is half marsh and half swamp. At Eastern Neck, boxes were erected on small ponds and tidal creeks.

Nest boxes were spaced 50 to 100 yards apart, as recommended by Bellrose (1955: 39). They were mounted on steel fence posts, 5 to 7 feet above normal water level. Aluminum sleeve predator guards, as described by Webster and Uhler (1964:2-3), or 3- x 38-inch aluminum downspouts (Fig. 1) were placed around each post. All boxes contained shavings as nest material. The number of available boxes ranged from 110 to 138 (Table 1). There were always a sufficient number of boxes in each impoundment to allow free choice by nesting hens,

RESULTS

Design of Box
In 1963, when starlings bad a choice between vertical boxes and horizontal boxes with large entrances, they used 40 percent of the vertical boxes but only 2.5 percent of the horizontal boxes (Table 1). In 1964, when entrance holes of horizontal boxes approximated those of vertical boxes, starlings used 55 percent of the vertical boxes and 36 percent of the horizontal boxes. Before the 1965 nesting season began, all vertical boxes were removed; during 1965-70, all horizontal boxes had larger openings. There was an abrupt decrease in use of boxes by starlings during the period 1965-70, even though there had been no apparent drastic change in the starling population between 1964 and 1970. The field test at Eastern Neck National Wildlife Refuge (Table 2) substantiated the findings at Patuxent: starlings readily used vertical boxes, whether the boxes were in ponds or tidal creeks, but they did not use the horizontal boxes with large openings.

Table 1: Nesting Attempts by Starlings and Wood Ducks in Boxes of Various types at Patuxent Wildlife Research Center
* These boxes were removed after the 1964 nest season.

Table 2: Use of nest boxes by ducks and starlings at Eastern Neck Wildlife Refuge, Maryland, 1970^a
^aSome boxes were used more than once and by both species.

Wood ducks nesting at Patuxent had no distinct preferences for any shape of nest box or for any size or shape of entrance hole. Moreover, even if they successfully hatched broods, most hens were not strongly attached to given nest boxes.

In the field test at Eastern Neck National Wildlife Refuge, wood ducks preferred vertical boxes with small openings to horizontal boxes with large openings.

Boxes with the rear panels left open one-third of an inch (in 1963 and 1964) were not used by starlings or wood ducks. Occasionally, however, at Patuxent, wood ducks have used nest boxes designed for mallards or black ducks; these boxes were open at both ends.

Location of Box
At Patuxent, starlings selected boxes in open impoundments more frequently than they did those in units full of dead and dying trees (Table 3). This was particularly striking in 1964, when starlings used over 50 percent of the boxes in the open but occupied only 1.1 percent of those in timbered areas. The abundance of suitable natural cavities in the timbered impoundments probably accounted for the lesser use of boxes in that habitat.

Table 3: Comparison of use of nest boxes by starlings in wooded and open impoundments at Patuxent Wildlife Research Center.

Although more wood ducks used boxes in the open than in wooded areas (Table 4), they did not exhibit the distinct preference shown by starlings. At Eastern Neck, both ducks and starlings used more boxes in ponds than in tidal creeks.

Table 4: Comparison of use of nest boxes by ducks in wooded and open impoundments at Patuxent Wildlife Research Center.

Use by Other Birds
Purple martins (Progne subis) and great crested flycatchers (Myiarchus crinitus) were the only other birds that nested regularly in the horizontal nest boxes. A single purple martin nest was found in 1963. More martins used the boxes each successive year; they used 30 boxes in 1970. Three or four pairs of great crested flycatchers nested annually. Both these species nested later than wood ducks and did not actively compete with them for nest sites.

DISCUSSION
These studies have shown that starlings will readily use vertical boxes with small entrances (3 x 4 inches). but they are discouraged from using horizontal boxes if the entrances are large (11 inches in diameter and semicircular).

We did not identify the factor that repels starlings from boxes with large openings. It may be the amount of light, the size of the opening, the position of the opening, or a combination of these. Bellrose (Bellrose and McGilvrey 1966) found that starlings were not discouraged when the light entering vertical boxes was increased by making additional holes (of 2-inch diameter) around the entrance. In a test we conducted on a pond near Patuxent from 1967 to 1970, a series of 3- x 4-inch holes around the entrance of five vertical boxes also failed to discourage starlings. These results suggest that the size of the entrance may be more important than the amount of light entering the box. Light entering both ends of a nest box, however, discourages both wood ducks and starlings from nesting in it.

Although wood ducks seem to prefer vertical wooden boxes, they will readily accept horizontal boxes. Wood ducks have proven to be much more adaptable than starlings to change in shape of box and size of entrance.

LITERATURE CITED

BELLROSE, F. C. 1955. Housing for wood ducks.
     Illinois Nat. Hist. Survey Circ. 45. 47pp.

____, AND F. B. McGILVREY. 1966 (1965). Characteristics and values of artificial nesting cavities.
     Pages 125-131. In J. B. Trefethen (Editor], Wood duck management and research: a symposium. Wildlife Management Institute, Washington, D. C. 212pp.

GRICE, D., AND J. P. ROGERS. 1965. The wood duck in Massachusetts.
     Massachusetts Div. of Fisheries and Game. Final P-R Rept., Project W-19-R. 96pp.

WEBSTER, C. G., AND F. M. UHLER. 1964. Improved nest structures for wood ducks.
     U. S. Fish and Wildl. Serv. Wildl. Leaflet 458. 20pp.

Acknowledgements
Received for publication March 8, 1971.
Reprinted from the Journal of Wildlife Management, Vol. 35, No. 4, October 1971, pp 793-797

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