Providing evidence to improve practice

Individual study: A comparison of the breeding ecology of four cavity-nesting birds nesting in boxes and tree cavities at the San Joaquin Experimental Range, California, USA

Published source details

Purcell K., Verner J. & Oring L. (1997) A comparison of the breeding ecology of birds nesting in boxes and tree cavities. The Auk, 114, 646-656


Few studies have compared reproductive parameters of bird species using nest boxes versus tree cavities. This study compared laying date, nesting success, clutch size, and productivity of four bird species (western bluebird Sialia mexicana, plain titmouse Parus inornatus, house wren Troglodytes aedon, ash-throated flycatcher Myiarchus cinerascens) nesting in boxes compared with natural tree cavities.


Study area: The study area consisted of oak-pine woodlands at the San Joaquin Experimental Range in Madera County, California, USA.

Nest boxes: From 1989 to 1991, 44 boxes, were monitored in a 30 ha plot in a cattle-grazed area of the study area. The boxes were made of redwood (average basal area 137 cm²); half of the boxes had entrance hole diameters of 3.2 cm and half of 3.8 cm. Boxes were placed at distances of between 67 to 120 m apart, hung at a 2 m height on trees. Eleven boxes were oriented in each of the four cardinal directions (N,S,E,W).

From 1992 to 1994, 92 nest boxes (minimum distance between boxes 90 m) were monitored: 36 boxes were placed in half of an area ungrazed by cattle (since 1934) and another 56 on half of the previously used grazed woodland site. Equal numbers of boxes were randomly assigned to orientation (N,S,E,W) and equal numbers of the two entrance diameters were used.

Nest box monitoring: From 1989 to 1991, nest boxes were checked every four to seven days and from 1992 to 1994, approximately every four days. Contents of the boxes were removed in the autumn of each year after nesting had ended.

Natural nests and monitoring: Nests in cavities were located in the grazed and ungrazed plots. From 1989 to 1991, nests were monitored every four to seven days (using, as required, a ladder, an inspection mirror and a light source). From 1992 to 1994, most nests were checked by using a flexible fiberscope. Nests were also monitored, in 1993 and 1994, in other grazed areas of the study area to increase sample sizes for some species. Cavity depth, basal area, volume and diameter were measured.

Nests that could not be examined were observed for activity. Nesting success, predation and clutch size were measured in nest boxes and natural cavities.


Western bluebirds gained the most advantage from nesting in boxes. They initiated egg laying earlier, had higher nesting success (more than twice that of bluebirds nesting in cavities), lower predation rates 0.0087 in boxes vs. 0.0227 in cavities), and fledged marginally more young in boxes (0.17) than in cavities (-0.38) but did not have larger clutches or hatch more eggs (values presented are deviations from yearly averages).

Plain titmice nesting in boxes, had marginally lower predation rates (0.0068 in boxes vs. 0.0109 in cavities), hatched more eggs (0.39 vs. -0.43), and fledged more young (0.36 vs. -0.18). They did not have higher overall nesting success, nor initiate clutches significantly earlier in boxes.

House wrens in boxes, laid larger clutches (0.37 vs. -0.45), hatched more eggs (0.27 vs. -0.34), and fledged more young (0.24 vs. -0.83) and had marginally higher nesting success (success of wrens nesting in boxes it was 1.3 times higher than in cavities) and lower predation rates (0.0050 in boxes vs. 0.0126 in cavities).

Ash-throated flycatchers experienced no apparent benefits from nesting in boxes versus cavities.

No significant relationships were found between clutch size and basal area or volume of cavities for any of these species. Given the different responses of these four species to nesting in boxes, the effects of nest box addition on community structure should be considered.

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