Tile Auk 114(4):646-656,1997

A COMPARISON OF THE BREEDING OF NESTING IN BOXES AND TREE CAVITIES

. KATHRYNL. PURCELL,',~JARED VERNER,~ AND LEWIS ORING'

I Progrnnt in Ecology, Emlutio~t,nnd Corrservntioa Biology, University oJNcunrln, 7000 Vnlley Rond. Reno, Nmdn 89512, USA; atid 2 Pacific Southwest Rescnrch Stntio% USDA Forest Seruicr, 2081 Eost Sierrn Auer~rre,Fresm, CnliJornin 93710, USA

ABSTI~ICT.-Wecompared laying date, nesting success, clutch size, and productivity of four that nest in boxes and tree cavities to examine whether data from nest boxes are comparable with data from tree cavities. Western Bluebirds (Sinlin nrexicnnn) gained the mast advantage from nesting in boxes. They initiated egg laying earlier, had higher nesting success, lower predation rates, and fledged marginally more young in boxes than in cavities but did not have laracr clutches or hatch more eggs. Plain Titmice (Pnnis iriorrmtrrs) nesting

earlier in boxes. ~oiseWrens (~roglo&tes~eiion) nesting in boxes laid larger clutches, hatched more eggs, and fledged mare young and had marginally higher nesting success and lower predation rates. Ash-throated Flycatchers (Myinrclirrs cincmscens) experienced no ap- parent benefits from ncsting in bows versus cavities. No significant relationships were found between clutch size and bottom area or volume of cavities for any of these species. These results suggest that researchers should use caution when extrapolating results from nest- box studies of reproductive success, predation rates, and productivity of cavity-nesting birds. Given the different responses of these four species to nesting in boxes, the effects of the addition of nest boxes on community structure also should be considered. Rrcrived 19 September 1996, nccepted 21 April 1997.

NESTBOXES MAY BE USED AS A CONSERVATION cause investigators generally remove old nests TOOL to augment populations of secondary cav- from boxes between breeding seasons (Nilsson ity-nesting birds. Much of what we know about 1986; M~ller1989, 1992; Rendell and Verbeek cavity nesters is based on nest-box studies, but 1996a, b, c). In addition, high densities of nest this information may not be representative of boxes may result inartificially high densities of individuals breeding in natural cavities (van breeding birds (Nilsson 1984a), which may in- Balen et al. 1982; Nilsson 1986; Mder 1989, fluence aggressive interactions and competi- 1992; Robertson and Rendell 1990). Studies that tion for nest sites (Gowaty and Wagner 1988, compared birds nesting in tree cavities versus Robertson and Rendell 1990), predation rates nest boxes have shown lower predation rates (Dunn 1977), and mating behavior (Alatalo and (Nilsson 1975,1984a, b; Robertson and Rendell Lundberg 1984, Gowaty and Bridges 1991). 1990), larger clutch sizes (Nilsson 1975,1984b; More information on individuals breeding in Robertson and Rendell 1990), and more young cavities is needed to understand factors that fledged (Nilsson 1986, East and Perrins 1988, regulate populations of secondary cavity nest- Alatalo et al. 1990, Kuitunen and Aleknonis ers and influence community structure. 1992) from nest boxes than from tree cavities. If tree cavities are smaller than nest boxes, Ectoparasite loads also may differ between cav- differences in clutch size could be influenced by ities and nest boxes because of the greater the area of cavity bottoms (Ludescher 1973, amount of nesting material in boxes (Pinkow- Nilsson 1984a). A positive relationship be- ski 1977, Rendell and Verbeek 1996c) or be- tween clutch size and nest-box size has been demonstrated for Great Tits (Parus major; Lohrl 1973, 1980; Karlsson and ~iisson197j; vanBa- Present address: Pacific Southwest Research Sta- tion, 2081 East Sierra A~~~~cF~~~~~,ca1ifarnialen 1984), Pied Flycatchers (Ficeduln hypoleucn; q17'1n.-. .- , IISA- -. . . Karlsson and Nilsson 1977, Gustafsson and E-mail:ls=k.purcell/oul=s27lO2aQmhs. Nil~son1985),CollaredFlycatchers(Ficedulnal- fsws.attmail.com bicollis; Gustafsson and Nilsson 1985), and Tree 646

October 19971 Nest Boxes velsus Cavities

Swallows (Tach~~cinetabicolor; Rendell and Rob- METHODS ertson 199'3). his relationship also was found From 1989 to 1991, we monitored 44 boxes on a for Willow Tits (Pnrus inontanus) and MarshTits 30-ha, gridded plot in the grazed portion of the (Parus pnlustris) nesting in cavities (Ludescher study area. Distances between boxes ranged from 67 1973), f6r Tree Swallows nesting in cavities to 120 m. Boxes were placed on the suitable tree near- (Rendell and Robertson 1989), and for the est to a designated grid marker, at a height of ap- open-nesting Barn Swallow (Hirundo rustica; proximately 2 m. Boxes were placed on wooden hfdler 1982). Mixed results have been found posts (n = 6) when no tree was within 25 m of a des- for European Starlings (Sturmrs vulgaris), al- ignated marker. Equal numbers of boxes were ori- though differences in clutch size may have re- ented in each of the four cardinal directions. Half of sulted from differences in age composition of the boxes had entrance diameters of 3.2 cm and half had diameters of 3.8 cm. These entrance diameters females nesting in small and large boxes were chosen to assure use by most potential species (Karlsson and Nilsson 1977, Karlsson 1978, but to exclude European Starlings. Boxes were con- Clobert and Berthet 1983). Box size had no ef- structed of redwood and had an average bottom area fect on clutch size of Eastern Bluebirds (Sinlin of 137 + SD of 7.1 cm2 (range 105 to 147 cm2).Boxes sialis; Pitts 1988). Great Tits, Pied Flycatchers, were checked every four to sevendays. A small flash- and Collared Flycatchers fledged more young light and mirror were sometimes used to determine from larger boxes (Lohrl1973, Gustafsson and nest contents. Nilsson 1985). Thus, species appear to respond We monitored 92 nest boxes from 1992 to 1994. differently to box size, with smaller species re- Thirty-six were located on half of the ungrazed area; another 56 were located on half of the previously sponding the strongest and the largest species used grazed site, which was extended to accommo- responding not at all to increases in box area date them. All boxes were moved. Nest boxes were (Karlsson and Nilsson 1977). again placed on the suitable tree nearest to a desig- Few studies have compared reproductive pa- nated grid marker, except in three cases in which rameters of bird species using nest boxes ver- wooden posts were used. The minimum distance be- sus tree cavities. In this paper, we present data tween boxes was 90 m. We based this spacing, which on nesting success, clutch size, productivity, we believe minimized the likelihood of multiple box- and laying date for populations of four species es in territories, on several years of spot-mapping of birds that use cavities and nest boxes. Stud- data and a thorough knowledge of the distribution and density of these species in these sites. Therefore, ies of coexisting individuals eliminate biases of we did not expect box density to affect bird densities. year, habitat, and geography. We also explored Equal numbers of boxes were randomly assigned to relationships between clutch size and bottom orientations in the four cardinal directions and equal area of cavities. numbers of the two entrance diameters were used. Nest boxes were checked approximately every four davs. Entrance holes of the boxes occasionally were en- This study was done at the San Joaquin Experi- larged by woodpeckers. We replaced doors on boxes mental Range in Madera County, California. The with enlarged entrances before each breeding sea- study area consists of relatively homogeneous foot- son, but not within a season. Boxes with enlargeden- hill oak-pine woodlands ranging in elevation from trances often were used by Ash-throated Flycatchers 215 to 520 m. Winters are cool and wet. and summers (Mvinrclrus. " cincmsccns). No predator-exclusion de- are hot and dry. Annual precipitation averages 48.6 vices were used, and nestlings were not banded or cm. with most falline- as rain between November and weighed. Contents of nest boxes were removed in the March. Monthly mean air temperatures range from fall of each year. about 6°C in January to about 27°C in July. The study Nests in cavities were located and monitored on area has been lightly to moderately grazed by cattle both the grazed and ungrazed plots. Nests also were since at least 1900. A "natural area" of about 29 ha monitored in other grazed areas of the study area, es- has been ungrazed since 1934. Dominant overstory pecially in 1993 and 1994, to increase sample sizes for trees include a sparse cover of foothill pine (Pirmssa- some species. Most cavities in this habitat were in live bininno), blue oak (Qtcerctcs dosglnsii), and interior trees, primarily oaks, and lasted for many years. Some live oak (Q. wislizenii). Understory species include were natural cavities formed by limbs that had died buck brush (Cennothus ceaentss), chaparral white- and fallen out, and where heart rot resulted in a hol- thorn (C. Icecoderazis), redberry (Rhnmnas crocen), and low stem. Others were excavated by. primary. cavity Mariposa manzanita (Arctostaphylos uiscidn mnripo- nesters, including Acorn Woodpeckers (Melnnerpesfor; sa). nricirorus), Nuttall's Woodpeckers (Picoidcs rzrrttnllii), 648 PURCELL,VERNER, AND ORING [Auk, Val. 114 and Northern Flickers (Colrrptes numtus). From 1989 to piro-Wilk statistic and for homogeneity of variances 1991, nests accessible by extension ladder were using Levends test and the folded form of the F sta- checked using an automotive inspection mirror and a tistic (SAS Institute Inc. 1988: 943). Variables were small light bulb. Other nests were observed for activ- transformed when appropriate. ity but not checked directly. Nests werechecked every Because we hypothesized that birds nesting in four to seven days. From 1992 to 1994, we checked boxes would have higher reproductive success and most nests by climbing trees and examining the nest productivity than birds nesting in cavities, power contents with a flexible fiberscope that allowed us to was calculated using one-tailed tests with a = 0.05 see clearly into the cavity and to count eggs and nest- (Abramowitz and Stegun 1964). We calculated power lings accurately (Purcell1997). Nests that could not be based on what we considered to be bialagically examined directly because of an unstable nesting sub- meaningful differences. Power calculations based on strate were observed for activity. observed differences are intrinsic to the data and, for Cavity depth was measured from the bottom of the a nonsignificant result, power can never be greater entrance to the nest cup. The bottom area of cavities than 0.50 (1. Baldwin pcrs. comm.). was estimated by placing a leaf of known size in the Regressions of clutch size on nest-bottom area and nest with a "pick-up" tool. The diameter was then nest volume have no intuitive, biologically meaning- estimated using the fiberscope. When cavities were ful slope or effect size against which to test power. not circular, two diameters were estimated and area Values from the literature suggest a fairly constant was computed as an ellipse. Volume was calculated slope of about 0.030 eggslcm' for Great Tits, al- as depth X area. though.. higher., values were found for Marsh and Wil- We analyzed data unly from ncsts in wllich at least loss tits, and lawcr values for I'ied Flycarchc~rs(Karl+ vrwegg was lad. Fmt-cxg dales were derrrrnmed by son end Nllasm 197;. Loltrl l'J8ll. van llalen 19841. backdating, assuming one egg laid per day. ~heiThis slope wasused to determineeffect size for pow- clutch size was unknown, the species' average clutch er calculatioms for area relations. Two effect sizes for size was used for backdating. "Number hatched" volume relationships were used: 0.0014 eggslcm3 and "number fledged" are based on nests that (calculated from van Balen's 119841 data on Great hatched at least one egg and fledged at least one Tits) and 0.0003688 eggs/cm3 (W. Rendell pers. young, respectively. Some of our data undoubtedly comm. for Tree Swallows). came from nests of the same individuals in multiple years. Because we did not find all of the nests each year, and annual survival typically is less than 50% in these species (Price 1936; Kendeigh and Baldwin 1937; Drilling and Thompson 1988; Martin 1988, Four bird species-Ash-throated Flycatcher, 1993,1995; Martin and Li 1992), we believe the influ- Plain Titmouse, House Wren (Troglodytes aedon), ence of nonindependence was minimal., and Western Bluebird (Sialia mexicana)-used Statistical nna1ysis.-We estimated nesting success nest boxes in sufficient numbers to compare and daily mortality rates of nests based on Mayfield's with natural nest sites. White-breasted Nut- methods (1961, 1975). Only the first nesting attempts hatches (Sitta carolinensis) and Bewick's Wrens were included, based on initiation dates and sequen- (Thryomanes bewickii) each made only two nest- tial occupancy of cavities and boxes. Values were cal- ing attempts in boxes. culated separately for all nest failures and for nests Box occupancy rates increased steadily from known to have been lost to predation. Variances were 2-0,r, u in 1989 to 68?0 in 1994. This was due main- calculated following Hensler and Nichols (1981). Dif- ferences among daily mortality rates within species ly to the fact that Plain Titmice initially did not were tested using a chi-square statistic following use boxes in large numbers, perhaps due to site Sauer and Williams (1989). In analyses of nesting suc- fidelity (see East and Perrins 1988). In 1989, tit- cess and nest predation rates, nests were pooled mice nested in only two boxes, representing across years. Although we expect that year effects may only 18% of the boxes used. By 1991, they ac- exist, small within-year sample sizes and the low counted for 71% of the boxes occupied. When power of these tests precluded testing for them. Year boxes were relocated in 1992, use of boxes by effects and the corresponding interaction terms were titmice fell to 41% of all boxes used but in- included in analyses of laying date. We found signif- creased to 89% by 1994. In contrast, use of box- icant differences across years for clutch size, number es by Western Bluebirds remained fairly steady, of eggs hatched, and number of young fledged for Plain Titmouse (Parus inornntus), although none of the ranging from 18 to 27% of boxes used. Use of interaction terms was significant for any species. For nest boxes by House Wrens was variable, rang- these measures of productivity, we standardized the ing froin none in 1990 and 1991 to 12 (13%) in data by testing deviations from the yearly means. 1992 and 1993. The abundance of House Wrens Variables were tested for normality using a Sha- also varied during this period (Verner et al. October 19971 Nest Boxes versus Cnvities 649

TABLE1. Mortality rates, predation rates, and nesting success (SD in parentheses) for Ash-throated Fly- catcher, Plain Titmouse, House Wren, and Western Bluebird. Predation rates are based on failures from predation only. P-values compare differences between nest boxes and cavities.

Daily Mayfield Daily Mayfield Nest - mortality nesting predation predation substrate it rate P success rate P rate Ash-throated Flycatcher Boxes 33 0.0134 (0.0040) 0.545 0.62 0.0098 (0.0034) 0.259 0.29 Cavities 52 0.0170 (0.0044) 0.55 0.0159 (0.0042) 0.43 Plain Titmouse Boxes 111 0.0117 (0.0017) 0.784 0.62 0.0068 (0.0013) 0.065 0.24 Cavities 171 0.0124 (0.00'19) 0.60 0.0109 (0.0018) 0.36 House Wren Boxes 36 0.0050 (0.0020) 0.059 0.83 0.0050 (0.0020) 0.059 0.17 Cavities 47 0.0126 (0.0035) 0.63 0.0126 (0.0035) 0.37 Western Bluebird Boxes 47 0.0133 (0.0030) 0.033 0.59 0.0087 (0.0024) 0.035 0.29 Cavities 39 0.0297 (0.0071) 0.29 0.0227 (0.0062) 0.61

1997), and nest-box use reflected these trends. speaes (Table 2). The average date of the first Use of boxes by Ash-throated Flycatchers egg was earlier in boxes than in cavities for all dropped from 27% of boxes used in 1989 to four species, although significantly so only for 15% in 1994. In seven cases, Ash-throated Fly- Western Bluebirds (Table 2). Bluebirds were catchers used boxes in which other species had variable in their laying dates, but the difference nested and failed earlier in the season; six of was significant because it was large (18 days). these instances occurred during the last two Ash-throated Flycatchers also had a wide range years of the study. of laying dates but a small difference between Nestirtg success.-Daily mortality rates were boxes and cavities (4.6 days earlier in boxes). lower in boxes than in cavities for all four spe- Nesting of titmice and House Wrens was very cies, although significantly so only for Western synchronized; each species had a small differ- Bluebirds and nearly so for Aouse Wrens (Table ence in laying dates (ca. 2 and 4 days earlier in 1). Nesting success followed the same pattern. boxes, respectively). The four species respond- Success of bluebirds nesting in boxes was more ed differently to nesting in boxes, as indicated than twice that of bluebirds nesting in cavities; by a significant species X nest type interaction success of wrens nesting in boxes was 1.3 times (P = 0.0002) in a two-way ANOVA. that of wrens nesting in cavities. Daily preda- Productivity.-Based on data standardized tion rates also were lower in boxes than in cav- across years, Plain Titmice and House Wrens ities for all species, significantly so for Western laid larger clutches, hatched more eggs, and Bluebirds, and marginally so for Plain Titmice fledged more young in boxes than in cavities and House Wrens (Table 1). Power to detect ab- (Table 3). We detected no differences in clutch solute differences of 0.10 in nesting success and size, number of eggs hatched, or number of predation rate, extrapolated back to the corre- young fledged between boxes and cavities for sponding differences in daily mortality and Ash-throated Flycatchers and Western Blue- predation rates, was low for all four species birds (Table 3). For both species, the power to (0.20 to 0.48). Power to detect an absolute dif- detect a difference of one egg or nestling ex- ference of 0.25 in nesting success and predation ceeded 0.99 in all cases. For Ash-throated Fly- rate was high for Plain Titmice (0.94 and 0.98, catchers, power to detect a difference of half an respectively) but low for the other three species egg or nestling was 0.98 for clutch size, 0.93 for (0.55 to 0.73), indicating that biologically mean- number of eggs hatched, and 0.88 for number ingful differences may have been undetected. of young fledged. The differences for the latter Lqing date.-Laying dates differed across two, however, were in the opposite direction years between boxes and cavities for all four from that hypothesized. For Western Bluebirds, 650 PURCELL,VERNER, AND ORING [Auk, Vol. 114

TABLE2. Results of two-way ANOVA testing effects of nest type (boxes vs. cavities) and year on layingdate for boxes and cavities. P-values compare nest type, year, and nest type X year interaction terms between nesl boxes and cavities.

Nest tvoe Lavine datea II Effects F P Ash-throated Flycatcher Boxes 132.35 (10.35) 31 Nest type 0.11 Cavities 136.96 (14.66) 46 Year 4.31 Nest type X Year 0.25 Plain Titmouse Baxcs 80.68 (8.16) 108 Nest type 0.08 Cavities 82.62 (9.48) 139 Year 11.30 Nest type X Year 0.94 House Wren Boxes 109.03 (5.33) 35 Nest type 2.09 Cavities 113.36 (7.78) 42 Year 4.57 Nesl type X Year 3.71 Western Bluebird Bones 88.48 (16.41) 40 Nesl Type 5.39 Cavities 106.90 (16.93) 21 Year 6.35 Nest twe X Year 1.46

power to detect a difference of half an egg or the greater number of young fledged from box- nestling was 0.94 for clutch size, 0.77 for num- es. Because Western Bluebirds nesting in boxes ber hatched, and 0.67 for number fledged. Al- versus cavities did not differ in clutch size or though the number of young fledged was only number of young hatched, it is not surprising marginally significant for bluebirds (Table 3), thatnest type wasnot related to thesemeasures the mean number fledged was higher in boxes of productivity, although laying date was (Ta- in every year, and power calculations indicated ble 4). Nest type, and not laying date, was re- a 33% chance of a Type I1 error. sponsible for the greater number of young blue- The species X nest type interaction term was birds fledged from boxes (Table 4). significant in a two-way ANOVA that included Relationships between clutch size and nest area.- all four species for number of eggs hatched (P The bottomareas of cavitiesusedby Ash-throat- = 0.04), and marginally so for number of young ed Flycatchers averaged 65 2 36.7 cm' (range 20 fledged (P = 0.096). Ash-throated Flycatchers to 201 cm2); those of Plain Titmice averaged 62 nesting in boxes tended to hatch fewer eggs and 2 40.7 cm2 (range 13 to 284 cm'); House Wrens fledge fewer young, whereas the other three averaged 48 i 32.4 cm2 (range 13 to 177 cm2); species showed the opposite pattern. and Western Bluebirds averaged 73 2 35.0 cm2 If birds nesting in boxes begin nesting ear- (range 24 to 177 cm2). Regressions using data lier, and there is a negative correlation between from cavities showed no significant relation- laying date and productivity, the higher pro- ductivity inboxes could be due to earlierlaying ships between bottom area of the cavity and dates in boxes. We examined this possibility us- clutch size (Table 5), number of eggs hatched, or ing ANCOVA, including laying date as a covar- number of young fledged. Power to detect a iate. Because none of the interaction terms in slope of 0.030 was high (>0.99) for clutch sizes any of the models was significant, we used the for all species (Table 5). We also examined vol- simple models without interactions. Both nest ume of nest cavities, which can influence clutch type and laying date contributed to the higher size (van Balen 1984, W. Rendell pers. comm.). productivity of Plain Titmice nesting in boxes All regressions of clutch size (Table 5), number (Table 4). For House Wrens, the larger clutches of eggs hatched, and number of young fledged and greater number of young hatched in boxes on volume of the nest cavity were nonsignifi- were directly related to nest type (Table 4), and cant. Power to detect a slope of 0.0014 was >0.97 both nest type and laying date were linked to for all tests (Table 5). Power to detect a slope of October 1997) Nest Bores ver-strs Cnuitirs 651

0.0003688 ranged from 0.24 to 0.085 for clutch size (Table 5).

Studying mian life lzistories: Boxes versus cmi- ties.-Our results revealed marked differences in several aspects of breeding ecology between birds nesting in boxes and those nesting in cav- ities. Not all species benefitted from nesting in boxes. Of the four species considered here, the resident Western Bluebird gained the most from breeding in nest boxes. Bluebirds readily occupied boxes and generally used them when- ever they were available in a territory. Bluebird densities were low compared with the other three species, and their nesting was not syn- chronous. Bluebirds fledged more young from boxes even though clutch sizes did not differ between boxes and cavities. Plain Titmice and House Wrens gained some benefits from nesting in boxes, e.g. larger clutches, more eggs hatched, and more young fledged, but not all differences were statistical- ly significant. Although tests had low power, we suspect that the higher nesting success and lower predation rate for House Wren nests in boxes versus cavities (see Table 1) would have been significant with larger sample sizes. Be- cause House Wrens are migrants and arrive af- ter the resident species have established terri- tories, cavity availability may be more limiting for them than for residents. If competition for nest sites is severe, migrants should be at a dis- advantage because the best cavities would be occupied by earlier nesters (von Haartman 1957, 1968). In Finland, von Haartman (1968) found that no cavity nesters had very late breeding seasons, which he attributed to com- petition for nest sites. Other studies have shown that House Wrens are aggressive and destroy nests of conspecifics and other species, perhaps in an attempt to usurp nest sites (Ken- deigh 1941, Belles-Isles and Picman 1986, Finch 1990, Kennedy and White 1996). In contrast, Ash-throated Flycatchers appar- ently gained no benefit from nesting in boxes compared with cavities. Ash-throated Fly- catchers are late-arriving migrants that nested considerably later than other cavity nesters in the study area (Table 2), putting them at an even greater disadvantage in competition for nest sites. Cavities chosen by Ash-throatedFly- 652 PURCELL,VERNER, AND ORINC [Auk, Vol. 114

TABLE 4. Results of ANCOVA and Type I11 sums of squares for standardized reproductive parameters (de- viations from yearly means) that included nest type (nest boxes w. cavities) and laying date (Julian date of first egg) for Plain Titmouse, House Wren, and Western Bluebird. Plain Titmouse House Wren Western Bluebird

Clutch size Nest type 5.05 0.026 6.07 0.017 0.09 0.761 Laying date 4.23 0.042 2.62 0.111 3.88 0.054 n 160 59 56 Number hatched Nest type 17.39 0.000 3.35 0.073 1.07 0.308 Laying date 14.20 0.000 0.38 0.541 11.19 0.002 n 143 52 39 Number fledged Nest type 8.66 0.000 8.93 0.005 4.27 0.047 Laying date 19.16 0.004 5.11 0.030 0.24 0.625 I) 140 37 15 catchers often were in very decayed wood with Aleknonis 1992), this is not always the case. For large openings that appeared to be unsuitable example, Robertson and Rendell (1990) found for other species and were highly susceptible to no difference in nesting success of Tree Swal- predation. Successful nests of Ash-throated lows nesting in boxes versus cavities, which Flycatchers had smaller entrance holes than they attributed to predation by raccoons (Pro- depredated nests (Purcell1995). The large size cyon lotor) and black rat snakes (Elaphe obsoleta) of this species and their aggressive nest defense in boxes in the second year of the study. Nils- may help to reduce predation (Murphy 1983, son (1984a) found lower predation rates in box- Martin 1992). es for Great Tits and Pied Flycatchers but not Although most studies have found higher re- for Blue Tits (Parus cnertrleus) or Marsh Tits. Al- productive success and/or lower nest preda- though we do not know why species differed tion in boxes versus cavities (e.g. Nilsson 1975, in their response to nesting in boxes, the dif- 1986; East and Perrrins 1988; Kuitunen and ferences have biological relevance and need further study. TABLE5. Regressions of nest box bottom area and Differences in nesting success and nest pre- volume on clutch size for Ash-throated Flycatcher, dation rates between birds nesting in boxes ver- Plain Tilmouse, House Wren, and Western Blue- sus cavities may be large, as we found forWest- bird. P-values test the significance of the regres- ern Bluebirds and House Wrens. Predation sion of area or volume on clutch size. rates of Great Tits and Pied Flycatchers nesting

Slope R2 P 11 Powera Powerb in boxes were less than one-third those of birds nesting in cavities (Nilsson 1984b), and nesting Ash-throated Flycatcher success of European Tree-Creepers (Certhin Area 0.007 0.078 0.232 20 1.000 Volume 0.000 0.090 0.199 20 1.000 0.847 fnn~iliar~s)in boxes was nearly double that of Plain Titmouse birds nesting in cavities (Kuitunen and Alek- nonis 1992). Similarly, clutch size tends to be Area 0.002 0.004 0.709 41 1.000 Volume 0.000 0.002 0.815 40 1.000 0.731 larger among birds nesting in boxes. Great Tits House Wren laid smaller clutch sizes in boxes, although Area 0.002 0.003 0.815 22 1.000 sample size was small (Nilsson 1975). For Pied Volume 0.000 0.010 0.665 21 1.000 0.534 Flycatchers, a nonsignificant difference in Western Bluebird clutch size between boxes and cavities was at- Area 0.011 0.167 0.148 13 0.994 tributed to later laying dates in boxes (Alatalo Volume 0.000 0.001 0.930 14 0.970 0.238 et al. 1990). First clutches of European Tree- 'Power bared on a slooe of 0.030 for area md0.0014 for volume. Creepers did not differ in size between boxes Power bared on u rlopc of0.0003688 for volume and cavities (Kuitunen and Aleknonis 1992). October 19971 Nest Boxes versus C~uitics 653

We have assumed that birds using cavities and species' coexistence may need to be recon- and boxes differ only in the type of nest site. sidered. For example, cavity nesters generally This assumption may be violated if there is a are believed to have higher nesting success than preference for nesting in boxes, if the supply of open nesters (Lack 1954, Nice 1957, von Haart- boxes is limited, or if the birds nesting in boxes man 1957, Ricklefs 1969; but see Nilsson 1986, are older, more experienced, and/or in better Martin and Li 1992). Because most of what we condition than those nesting in cavities (John- know about the reproductive ecology of second- son and Kermott 1994). A review of age-related ary cavity nesters is based on studies using nest variation in reproductive performance of birds boxes, and because nesting success may be en- showed that first-time breeders generally bred hanced by nesting in boxes, this belief may be later, had smaller clutches, and produced fewer erroneous. In addition to the differences we fledglings than did older birds (S~ther1990). have shown, the use of nest boxes may influence Hatching success showed the least difference, the frequency of extrapair copulations and egg with equal numbers of studies finding higher dumping (Gowaty and Wagner 1988, Gowaty hatching success for younger and older females and Bridges 1991; but see Barber et al. 1996), among . The most pronounced dif- competition for nest sites (Gowaty and Wagner ferences were later laying dates for young fe- 1988, Robertson and Rendell 1990, Merila and males. Differences in reproductive perfor- Wiggins 1995), and dispersal. mance between birds nesting in boxes versus We do not mean to invalidate studies based cavities due to age and experience of breeders on nest boxes. On the contrary, nest-box studies would be most important when the preference have contributed greatly to our knowledge of for boxes is strong and nest sites are limiting. the breeding ecology of secondary cavity nest- In an area where nest sites were very limited, ers. Nest boxes are useful tools that facilitate Robertson and Rendell (1990) found a ratio 4.8: the study of aspects of breeding biology that 1 of older to yearling Tree Swallows nesting in otherwise would be difficult to study, and there boxes. In an area without boxes, the ratio of is no a priori reason to believe that behavioral older to first-year birds nesting in cavities was responses to nesting in boxes are maladaptive nearly equal. Because our birds were unband- (Koenig et al. 1992). Instead, we advise caution ed, we do not know whether older birds nested in designing and interpreting studies using proportionately more often in boxes. nest boxes, and we encourage additional stud- Our objective was to determine whether re- ies of birds nesting in natural situations and productive parameters differqd between birds comparisons of birds nesting in boxes versus nesting in tree cavities and those nesting in tree cavities. boxes. Our study design did not allow us to ad- Relationships between clutch size and nest dress the mechanisms that caused these differ- area.-Based on other studies, the range of vari- ences. Data comparing successful and depre- ability we observed in the area of cavity bot- dated nests in cavities may provide some in- toms was sufficient for us to detect an effect on sight. Successful nests of Plain Titmice and clutch size if one existed. We found no such re- House Wrens had smaller bottom areas, al- lationship. In an analysis of several species, van though the bottom areas of nest boxes are larg- Balen (1984) reported a steady increase in er than those of most tree cavities (Purcell clutch size with bottom area up to about 150 1995). Successful nests of House Wrens and cm'. Studies using boxes with bottom areas of Western Bluebirds were lower than depredated 314 cm2 showed only a weak relationship be- nests (Purcell 1995), as were the nest boxes tween clutch size and nest area (Lohrl1973, van used here and in other studies. Obviously, Balen 1984). Because Great and Blue tits have many other factors could contribute to the ob- large, variable clutch sizes, a correlation be- served differences, and studies designed to test tween clutch size and cavity size is more likely specific hypotheses are needed. to be detected, although this relationship has Our results indicate that reproductive param- been found in species with smaller clutchsizes. eters estimated from nest-box studies may not Studies of clutch/area relations in cavities be representative of birds nesting in tree cavi- might be more informative, because adaptive ties. Assumptions concerning reproductive ecol- responses to cavity size should have evolved in ogy, life-history patterns, community structure, natural situations. Alatalo et al. (1988) also 654 PURCELL,VERNER, AND ORING [Auk, Vol. I14 found no relationship between clutch size and Perrins 1988). A previous study that involved bottom area of cavities in Pied Flycatchers. blocking cavity entrances in our study area Cavities in their study were larger than nest failed to show that cavities were limiting (Wa- boxes. Alatalo et al. suggested that boxes typ- ters et al. 1990). We suspect, however, that lim- ically used in nest-box experiments are outside itation of nest cavities increased over the course the range of sizes of natural sites. In our study of our study owing to an increase in the num- area and that of Nilsson (1984a), bottom areas ber of European Starlings. Point-count surveys of cavities averaged smaller than those of box- from 1989 to 1994 indicate that starling num- es, although variability was large. Perhaps fac- bers have more than doubled in our study area tors other than bottom area (e.g. laying date, (unpubl. data), cavities formerly occupied by age, population density, food availability, pre- other species have been taken over by starlings, dation risk) have a more important influence on and aggressive interactions have been ob- clutch size. served between starlings and other cavity-nest- Nest boxes nnd conservation.-Nest boxes have ing species (K. Purcell pets. obs.). been used to augment nesting sites for a wide Although the addition of next boxes to a hab- variety of bird species in many parts of the itat may precipitate changes in the relative world. Although well intentioned and often abundances of bird species (both cavity nesters highly successful, most of these efforts have and open nesters), we believe that nest-box pro- proceeded without consideration of the effects grams generally are beneficial. We also believe of nest boxes on the rest of the bird community. that nest boxes may provide an opportunity, For example, when nest boxes increase the pro- scarcely used in the past, to investigate ques- ductivity of bluebirds, do numbers of bluebirds tions of interspecific competition in bird com- increase to the detriment of other species? If we munities. were concerned only with bluebirds, then put- ting up nest boxes would be an appropriate ~ - -. . Conservation measure. Our management con- cerns are rarely so uncomplicated, however, We were aided in the field by numerous assistants: Parker Backstrom, Sue Balch, Sandy Bloomfield, and general'y they maintaining and Kathleen Brubaker, Barbara Crouse, DougCubanski, protecting entire communities or ecosystems. JeffDavis, Doug Drynan, Matt Georgeff, Brian Gib- Hogstad and et al. re- son, Steve Hawkins, Kitt Heckscher, Tames Tones. ported increased abundances of cavity nesters Sheila Kee, Kim I

Flycatchers nesting in natural cavities. Pages HOGSTAD, 0. 1975. Quantitative relations between 323-330 in Population biology of birds hole-nesting and open-nesting species within a (J. Blondel, A. Gasler, J.-D. Lebreton, and R. passerine breeding community. Norwegian McCleery, Eds.). Springer-Verlag, Heidelberg. Journal of Zoology 23261-267. ALATALQR. V., AND A. LUNDBERG.1984. Polyterri- JOHNSON,L. S., AND L. H. KERMOTT.1994. Nesting torial polygyny in the Pied Flycatcher Ficeduln success of cavity-nestingbirds using natural tree hypoleucn-evidence for the deception hypothe- cavities. Journal of Field Ornithology 6536-51. sis. Annales Zoologici Fennici 21:217-228. KARLSSON,J. 1978. Clutchsize in relation to nest-box BARBER,C. A,, R. J. ROBERTSON,AND P. T. BOAG.1996. area in the Starling Strrrrurs ualgnris. Anser (Sup- The high frequency of extra-pair paternity in plement) 3121-123. Tree Swallows is not an artifact of nestboxes. Be- KARLSSON,J., AND S. G. NILSSON.1977. The influ- havioral Ecology and Sociobiology 38:425-430. ence of nest-box area on clutch sizein some hole- BELLES-ISLES,J. C., AND J. PICMAN.1986. House nesting passerines. lbis 119:207-211. Wren nest-destroying behavior. Condor 88:190- KENDEIGN,S. C. 1941. Territorial and mating behav- 193. ior of the House Wren. University of Illinois BOCKC. E., A. CRUZ,JR., M. C. GRANT,C. S. AID,AND Press, Urbana. T. R. STRONG.1992. Field experimental evidence KENDEIGH,S. C., AND S. P. BALDWIN.1937. Factors for diffuse competition among southwestern ri- affecting yearly abundance of passerine birds. parian birds. American Naturalist 140:815-828. Ecological Monographs 791-124. CLOBERT,J., AND P. BER'IHET. 1983. Les jeunes ha- KENNEDY,E. D., AND D. W. WHITE. 1996. Interfer- bitent petit ou impact de la reduction du volume ence competition from House Wrens as a factor inttricur du nichoir sur le comportement d'une in the decline of BewicKs Wrens. Conservation population nicheuse d'6taurneax sansonnets Biology 10:281-284. (Stamis vulgnris L.). Annales de la Societe Ray- KOENIG,W. D., P. A. GOWATY,AND J. L. DICKINSON. ale Zoologique Belgique 113:183-192. 1992. Boxes, barns, and bridges: Confounding DRILLING,N. E., AND C. F. THOMPSON.1988. Natal factors or exceptional opportunities in ecologi- and breeding dispersal in House Wrens (Troglo- cal studies? Oikos 63:305-308. dytes nedon). Auk 105480491. KUITUNEN,M., AND A. ALEKNONIS.1992. Nest pre- DUNN,E. 1977. Predation by weasels (Musteln nivnl- dation and breeding success in Common Tree- is) on breeding tits (Pores spp.) in relation to the creepers nesting in boxes and natural cavities. density of tits and rodents. Journal of Ornis Fennica 69:7-12. Ecology 46:633-652. LACK, D. 1954. The natural regulation of animal EAST,M. L., AND C. M. PERRINS.1988. The effect of numbers. Oxford University Press, London. nestboxes an breeding populations of birds in L~HRL,H. 1973. Einfluss der Brutraumfliche auf die broadleaved temperate wo,odlands. lbis 130: Gelegegrosse der Kohlmeise (Pnrrrs nmjor). Jour- 393401. nal fiir Ornithologie 114:339-347. FINCH, D. M. 1990. Effects of predation and com- LOHRL,H. 1980. Weitere Versuche zur Frage "Bru- petitor interference an nesting success of House traum und Gelegegrosse" bei der Kohlmeise, Wrens and Tree Swallows. Condor 92674-607. Pnrns rrmjor. Journal fiir Ornithologie 121:403- GOWATY,P. A,, AND W. C. BRIDGES.1991. Nestbox 405. availability affects extra-pair fertilizations and LUDESCHER,F. B. 1973. Sumpfmeise (Pnrss p pnlas- conspecific nest parasitism in Eastern Bluebirds, tris L.) und Weidenmeise (I! rrrontnnas snlicnrirrs) Sinlin sinlin. Animal Behaviour 41:661-675. als synipatrische Zwillingsarten. Journal fiir Or- GOWATY,P. A,, AND S. J. WAGNER.1988. Breeding nithologie 114:3-56. season aggression of female and male Eastern MARTIN, T. E. 1988. Nest placement: Implications for Bluebirds (Sinlin sinlis) to models of potential selected life-history traits, with special reference conspecific and interspecific egg dumpers. to clutch size. Amercian Naturalist 132:900-910. Ethology 78:238-250. MARTIN,T. E. 1992. Interaction of nest predation GUSI'AF~SON,L., AND S. G. NILSSON.1985. Clutch and food limitation in reproductive strategies. size and breeding success of Pied and Collared Current Ornithology 9563-197. flycatchers Ficedtrln spp. in nest-boxes of differ- MARTIN,T. E. 1993. Nest predation among vcgeta- ent sizes. lbis 127380-385. tion layers and habitat types: Revising the dog-

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