Nest Predation and Nest-Site Selection of a Western Population of the Hermit Thrush ’

The Condor90~5 1-57 0 The CooperOrnithological Society 1988

NEST PREDATION AND NEST-SITE SELECTION OF A WESTERN POPULATION OF THE HERMIT THRUSH ’

THOMAS E. MARTIN AND JAMES J. ROPER Department ofZoology, Arizona State University,Tempe, AZ 85287

Abstract. Audubon’sHermit Thrushes(Cutharus guttatus auduboni) in centralArizona havea low nestingsuccess (7 to 20%)due almost exclusively to nestpredation. We examine the siteschosen for nestingand comparethem to nonusesites randomly selected within the vegetationtypes associated with nests.Hermit Thrush nest sitesdiffer from nonusesites primarily in that nest siteshave more small (l- to 3-m tall) white firs (Abiesconcolor) in the patch(5-m radiuscircle) surrounding the nest.Hermit Thrushesnest almost exclusively in smallwhite firs and theydo not foragein or near them.Hermit Thrushesmay selectnest sitesthat have a largenumber of otherpotential nest sites(i.e., small white firs) near the nest becausepredation risk is therebyreduced. Indeed, nestswith a high probability of predationwere surrounded by a lowerdensity of smallwhite firs than moresuccessful nests. However,low predationnests also were more concealed than high predation nests. Nest- site selection appears to be a function of characteristicsin the immediate vicinity of the nest (concealment,overhead cover, nest orientation), but also on a larger scalesurrounding the nest. Considerationofnest-site selectionon this largerscale may castlight on the question of whether nest sites limit territory and habitat selectionby birds. Key words: Daily mortality; nestconcealment; nest orientation; nest predation; nest-site selection;nest-patch selection; nesting success.

INTRODUCTION Nest-site selection is closely tied to fitness be- Best 1985). However, the nest patch may be causeof the effectson offspring production (e.g., equally important to selectionof sitesfor nesting. see Martin, in press a). Consequently, nest-site Nonrandom selection of nest patches has only choice should be molded by nest-site character- been examined a few times (i.e., MacKenzie and istics that influence the number and quality of Sealy 198 1, Clark et al. 1983, Petersen and Best young that can be successfullyfledged. Habitat 1985) and none of these studies attempted to characteristicsthat influence probability of nest relate vegetation characteristicsof the nest patch predation may be particularly important because to nesting success.Yet, studiesin aquatic systems nest predation often is the primary source of provide a basis for expecting foliage density to nesting mortality for a wide range of bird species influence predation probability at a scaleas large (Ricklefs 1969). or larger than the nest patch; increasesin vege- Nests may be affectedby habitat at two spatial tation density in foraging patches of aquatic scales:(1) the nest site (characteristicswithin the predators often reduce predation risk by con- immediate vicinity of the nest) and (2) the nest cealing prey or inhibiting predator search effi- patch (characteristics of the habitat patch sur- ciency (e.g., Crowder and Cooper 1982, Ander- rounding the nest). Previous work has focused son 1984, Cook and Streams 1984, Leber 1985). on the nest site, examining effects of overhead Indeed, Bowman and Harris (1980) found rac- cover on energy costs (e.g., Calder 1973, Wals- coon (Procyon lotor) foraging efficiency de- berg and King 1978, Walsberg 1981), nest ori- creased,search time increased,and fewer clutch- entation relative to solar exposure (e.g., Giesen es of bird eggswere found in enclosures where et al. 1980, Schafer 1980, Cannings and Threlfall understory foliage density was artificially in- 198 1, Zerba and Morton 1983), and effects of creased.Thus, foliage density in the nest patch nestconcealment on probability ofpredation (e.g., may impede random and intentional nest dis- Keppie and Herzog 1978, Nolan 1978, Best and covery by inhibiting transmission of chemical, auditory, or visual cues. An alternative hypoth- esis that may operate simultaneously or inde- ’ ’ Received 13 February 1987. Final acceptance8 pendently is that predation probability may de- July 1987. creasewith increasesin density of the particular 52 THOMAS E. MARTIN AND JAMES J. ROPER

foliage types that are used as nest sites; such in- ence of parents, eggs,nestlings) of each nest were creasesmay reflect the number of potential nest recorded every 3 to 5 days. Nests that fledged at sitesthat predators must examine which reduces least one young were considered successful.Ob- their chancesof finding the actual nest (Martin, servations of fledging, fledglingsnear the nest, or unpubl.). These alternatives can be addressedby parents feeding new fledglingsin the general area examining effectsof the nest patch on probability of the nest were taken as evidence of a successful of nest predation and by specifically examining nest. Depredation was assumedwhen the nest or predation probability as a function of numbers eggsor nestlings (when too young to fledge) dis- of potential nest sites surrounding nests. appeared. Although most nestswere found prior In this paper, we present data on nesting suc- to onset of incubation, some nests were not and, cessand nest-site and patch choice of Audubon’s so, nest successand mortality were calculated Hermit Thrush (Catharusguttatus auduboni) and using the Mayfield method (Mayfield 196 1, 1975) then examine nesting success and predation as modified by Johnson (1979) and Hensler and probability relative to the numbers of potential Nichols (198 1). Half the number of days between nest sites and other habitat characteristics as- subsequent visits over which a nest was depre- sociated with actual nests. As we will show, Au- dated was added to the number of previous days dubon’s Hermit Thrush is particularly appro- the nest survived to obtain the total number of priate for this analysis becausenest-tree selection days a nest survived. Tests of differencesin nest- in central Arizona is highly specific,which allows ing successwere conducted using the z-test de- reasonableestimates of numbers of potential nest scribed in Hensler and Nichols (198 1). sites. Four nestsin 1985 and nine nestsin 1986 that were found by observing parents were never vis- STUDY AREA AND METHODS ited more closely than 10 m to check effects of Study sitesare drainagesdominated by big tooth human visitation on probability of predation be- maple (Acer grandidentatum) in the understory causesuch effects can sometimes obscurethe im- and located on the Mogollon Rim in Central portanceof nest concealment (Westmoreland and Arizona at 2,300 m elevation. These drainages Best 1985). These nests were checked from 10 vary in area and numbers of coexisting bird m or more using binoculars and observations of specieswith a total of 29 speciesrecorded (Mar- parental activity at the nest to determine whether tin, in press b). These sites have a mixed over- or not the nest was active. When no activity was story with ponderosa pine (Pinus ponderosa), found, the nest was approached to verify pre- white fir (Abiesconcolor), douglas-fir (Pseudotsu- dation. ga menziesii), white pine (Pinus strobiformis), Nest-site characteristics were measured after quaking aspen (Populus tremuloides),and Gam- termination of nesting. Plant speciesused as the be1 oak (Quercus gambelii). Saplings of canopy nesting substrate, height of the nest above the trees, plus maple and New Mexican locust (Ro- ground and height of the nest tree were measured binia neomexicana)are the dominant understory by meter stick, or by ocular estimation in the woody species(see Martin [in pressb] for further three casesof large trees. Orientation of the nest description). These drainages contrast with sur- relative to the main stem was recorded in 45” rounding forest which is primarily characterized octants. Nest concealment was indexed by esti- by open ponderosa pine with Gambel oak in the mating percent foliage cover in a 25-cm circle subcanopy and little understory vegetation. centered on the nest from a distance of 1 m from Red squirrels(Tamiasciurus hudsonicus), gray- above and from the side in each of the four car- neck chipmunks (Eutamias cinereicollis), long- dinal directions. Minimum (MINSC) and aver- tailed weasels (Mustela frenata), House Wrens age (AVESC) side cover were used for analyses. (Troglodytes aedon), and Steller’s Jays (Cyano- Habitat characteristicswithin a 5-m radius cir- citta stelleri) are present as possible nest preda- cle around each nest were measured at all nests tors on Hermit Thrushes (Martin, unpubl. data). in 1985 and 1986 and for a few nests in 1984. From mid-May to early July in 1984 through Included within this sampling were a few nests 1986, 15 maple drainageswere searchedfor Her- that were found in the first 2 weeks of the breed- mit Thrush nests. Nests were located by observing seasonbut which never contained eggs.These ing parents with nesting material or by simply nests were probably depredated before being searchingthe vegetation. Date and status (pres- found by human observers.However, thesenests NEST-SITE SELECTION 53 were assigned a status of “unknown” and not provided more adequate sample sizes for anal- included in analysesof habitat characteristicsrel- yses. ative to nesting success.Habitat variables mea- Analysis of variance was used to test univar- suredwithin the circlesincluded numbers ofwhite iate differences in habitat variables between firs between 1 and 3 m tall (WFSM) because groups. Variables that discriminated between Hermit Thrushes almost always nest in such lo- groups were identified by stepwisediscriminant cations (see later). Numbers of all other conifer function analysis. Covariance matrices were speciesbetween 1 and 3 m were also counted tested for homogeneity using Box’s M criterion (CONSM). Numbers of treestaller than 3 m were (SPSS X 1986). For each pair of groups tested, counted for all conifer species(CONBIG). Num- the matrices showed significant heteroscedastic- bers of maple stems were counted for all stems ity (P < 0.05). Discriminant function analysis less than 5 cm dbh (MASM), between 5 and 15 was then based on the pooled within-group co- cm (MAMED), and greater than 15 cm (MA- variance matrix and using the Malhalonobis dis- BIG). Numbers of locust stemswere counted for tance (Minimum D2) between group centroids as all stemsless than 5 cm (LOSM) and greater than the criterion for maximizing separationof groups. 5 cm (LOMED). Locusts were always smaller This method is most appropriate when covari- than 15 cm dbh, so the large class was not nec- ante matrices are not homogeneous(Hand 198 1, essary. All other deciduous woody stems were Williams 1983). Finally, original variables se- counted for all stems less than 5 cm (DECSM) lected by DFA were correlated with the discrim- and greater than 5 cm (DECBIG). A separate inant function to examine their importance. intermediate group was not included becausethis RESULTS group included very few stems. All habitat variables were also measured in GENERAL NESTING SUCCESS nonuse plots in 1986. Nonuse plots were ran- Daily mortality rates did not differ between nests domly located within the general vegetation type that were and were not visited by humans (z = associatedwith nest sites of Hermit Thrush and 0.42, P > 0.64; Table la). Probability of nesting other ground and understory nesting species. successvaried between 7 and 20% (Table lb). Nonuse sitesfor Hermit Thrushes (HTNU here- Only one of the nest lossesover the 3 years could after) and other species(OSNU hereafter) were be attributed to some causeother than predation, located by pacing 50 m from the nest sites in a so mortality reflectspredation rates. Daily mor- direction parallel to the drainage. In this way, tality rateswere surprisinglysimilar among years; the vertical position on the side slopeswas main- daily mortality for 1985 did not differ (z = 0.08, tained. Vegetation structure changes with in- P > 0.92) from 1984, and 1986 did not differ creasing distance up the side slopes and, thus, (z = 0.08, P > 0.92) from 1985 (Table lb). Con- HTNU sites represented randomly sampled sequently, all 3 years were pooled for subsequent vegetation within the same vegetation type se- analyses. lected by Hermit Thrushes. OSNU sites provided more complete coverage of the full range of NEST-SITE CHARACTERISTICS habitat sitesavailable becauseother speciesnest- Ninety-three percent of the nest trees (n = 57) ed in other vegetation zones. were white firs between 1 and 3 m tall. The rest Habitat variables were compared between were maples (5%) and white firs taller than 3 m Hermit Thrush nest sites and the two types of (2%). Mean nest-tree height was 172.0 k 16.28 nonuse sites. In addition, habitat variables and (x k SE) and nest height was 107.8 -t 9.16 cm. nest-site characteristicswere compared between Nests were most common in the southwestquad- two groups of Hermit Thrush nests defined as rant of the nest tree (x2 = 17.00, P < 0.025) (Fig. high predation rate (HPHT) and low predation 1). rate (LPHT) groups. These groups were defined as neststhat were depredatedduring the eggstage NEST-PATCH SELECTION (HPHT nests) vs. those that were depredated More small white firs occurredon Hermit Thrush during the nestling stageor were successful(LPHT nest patchesthan on HTNU patches(Table 2a). nests). Nests were grouped in this way because Numbers of small white firs also were most im- nesting successwas extremely low (only three portant in differentiating between Hermit Thrush nests successfully fledged) and such grouping and OSNU patches,but number of locust stems 54 THOMAS E. MARTIN AND JAMES J. ROPER

N rates. The second most important discriminant was minimum side cover, with greater minimum side cover at nests with lower predation rates (Table 3a). Nests with lower predation rates were associated with more small maple stems and lower nesting heights,but the low correlations of these variables with the discriminant function show that their importance is weak (Table 3a). High predation nests were associatedwith fewer small firs than low predation nests,but high predation nestsstill had more small firs than nonuse sites (Table 3b).

DISCUSSION Nest concealment, as indexed by minimum side cover, was significantly greater at low predation neststhan at high predation nests.Similar results S have been obtained by other woodland bird stud- FIGURE 1. Nest orientationof Hermit Thrushesin ies (e.g., Keppie and Herzog 1978, Nolan 1978, treesused for nesting(n = 5 1). Lengthof barsreflect Murphy 1983, Westmoreland and Best 1985; but number of neststhat were found with an orientation in eachoctant: NNE (4), ENE (l), ESE (4), SSE (5), see Best and Stauffer 1980). On our sites, this SSW (16), WSW (8), WNW (9), NNW (4). result not only reflects positioning of the nest within a selectedfir, but also reflectsqualities of greater than 5 cm dbh also was important (Table the fir selected.The density and fullnessof boughs 2b). In both analyses, the number of correctly of small white firs are influenced by grazing her- classifiedcases indicated that birds were selecting bivores to varying extents and birds appeared to specific features in their nesting patches. selectfirs with high cover density (pers. observ.) General cover densities of firs was not measured NESTS WITH LOW VS. HIGH RATES OF but may be important to refining discrimina- NEST PREDATION tions; firs with a particularly high cover density Four variables discriminated between nestswith may not require as many surrounding firs to be high and low predation rates (Table 3a). Again, a suitable nest site. The fact that discrimination numbers of small white firs were most important between low and high predation nests was so in discriminating between the groups, with more highly accurate when both numbers of small firs firs associated with nests with lower predation and minimum side cover were incorporated into

TABLE 1. Numbers of successfuland unsuccessfulnests and the nesting successbased on numbers of nests, plus numbers of days those nests were observed to survive and daily mortalities and predicted nest success under the Mayfield method for (a) nests that were visited and neststhat were not visited by humans during the nesting attempt and (b) for all nests in each year.

No.successful/ % Nests PredIcted nest Yea1 unsuccessful successful Days observed Daily mortality S”CceSS (96) (a) Visited vs. unvisited nests Visited 3/13 18.8 150.5 0.086 (O.OOOSp 8.8 Unvisited l/12 7.7 118.5 0.101 (0.0008) 5.6 (b) Nest successamong years 1984 3/13 18.8 139 0.094 (0.0006) 7.0 1985 2/8 20.0 88 0.09 1 (0.0009) 7.6 1986 2/17 10.5 181 0.094 (0.0005) 7.0 All 7/38 15.6 408 0.093 (0.0002) 7.2

aVariance as calculated under the methods of Hensler and Nichols (1981). NEST-SITE SELECTION 55

TABLE 2. Mean (SD) of habitat characteristicsthat are important in discriminating between Hermit Thrush (HT) nest sites vs. other (OSNU) and Hermit Thrush (HTNU) nonuse sites, plus the univariate ANOVA of differencesin the means and results of stepwise,discriminant function analysis.

Discriminant function analyses Univariate analyses Wilk’s Minimum Correlation Variable HT (46)” OSNU (29) F P lambda D’ P coemc1ent

(a) Hermit Thrush vs. other speciesnonuse sites WFSMb 17.52 (15.76) 7.62 (7.38) 10.02 0.002 0.879 0.563 0.002 0.781** LOMED 6.00 (13.92) 18.86 (23.08) 9.09 0.004 0.816 0.923 0.001 -0.744** Correctly classified 80.4% 69.0% (b) Hermit Thrush vs. Hermit Thrush nonuse sites HT (46) HTNU (22)

WFSMb 17.52 (15.76) 7.59 (6.40) 8.05 0.006 0.893 0.541 0.006 1.000 Correctly classified 76.1% 81.8%

a Sample size. b Numbers of white firs between I and 3 m in height. =Numbers of New Mexican locust stems greater than 5 cm in dbh. **p < 0.01. the discriminating model suggests that both fac- afternoon sun at this cool, high elevation (Fig. tors are important in discriminating nest sites 1). Thus, suitability of firs as nest sites may be from nonuse sites. Indeed, the significant cor- further constrained by the availability of nest relation of minimum side cover with the dis- sites with dense cover and an opening for a nest criminant function documents its interacting im- with a southwest exposure. portance. Moreover, quality of a fir as a nest site may NEST-PATCH SELECTION also be influenced by the distribution of cover Hermit Thrushes select their nesting sites with within the fir. Hermit Thrushes apparently po- regard to surrounding habitat characteristics. sition their nests to take advantage of the warm Comparisons of Hermit Thrush nest patches with

TABLE 3. Results of univariate and discriminant function analysesof (a) high (HPHT) vs. low (LPHT) predation Hermit Thrush nest sites and (b) high predation sites (HPHT) vs. Hermit Thrush nonuse (HTNU) sites.

Discnminant function analyses Univariate analyses Wilk’s Minimum Correlation Variable HPHT (27) LPHT (12) F P lambda D’ P coefficient

(a) High predation Hermit Thrush nest sites vs. low predation nest sites WFSMb 10.50 (4.31) 30.92 (15.58) 22.17 0.000 0.619 2.70 0.000 0.708** MINSC” 52.88 (19.35) 68.75 (21.23) 5.20 0.029 0.500 4.38 0.000 0.343* MASM* 49.38 (33.33) 48.08 (38.18) 0.01 0.916 0.473 4.89 0.000 0.015 NHe 112.96 (65.64) 79.91 (32.53) 2.70 0.109 0.449 5.39 0.000 -0.247 Correctly classified 96.2% 83.3% (b) High predation Hermit Thrush nests vs. Hermit Thrush nonuse sites HPHT (27) HTNU (22)

WFSMb 10.78 (4.47) 7.59 (6.40) 4.19 0.046 0.918 0.346 0.046 1.ooo Correctly classified 63.0% 68.2%

a Sample size. b Numbers of white firs between I and 3 m in height. cMinimum side ccwer around nests. d Numbers of big tooth maple stems less than 5 cm in dbh. *Nest height in cm. *P < 0.05. ** P < 0.01. 56 THOMAS E. MARTIN AND JAMES J. ROPER

OSNU patches showed that Hermit Thrush the fact that small firs do not provide a dense patcheshad fewer locust stems.This result seems thicket of foliage even when in a dense clump to reflect nest-patch selection on a moisture gra- because of the spacing between the firs. Maple dient rather than avoidance of locust; locust is thickets, which dominate the sites, provide a most abundant on the upper sidesof slopeswhere much denser thicket of foliage. Consequently, if conditions are drier. Hermit Thrushes tend to Hermit Thrushes were trying to select nest sites select nest trees at or near the bottom of the with dense foliage, they would do better by se- drainageswhere conditions are more moist (pers. lecting maple or selectingfirs surrounded by ma- observ.). Thus, moisture conditions seem to be ple. Yet, they rarely select such sites. The sig- important in nesting microhabitat choice, as also nificant effect of nest concealment suggeststhat found by Dilger (1956) in the northeasternUnited foliage density could be important, but the low States. correlation of maple stemswith the discriminant The density of small white firs was clearly the function suggeststhat general foliage density is most important factor discriminating Hermit not as important as numbers of potential nest Thrush nest patchesfrom nonuse patchesin both sites on predation probability. selected(HTNU) and other (OSNU) microhab- Territory (e.g., see Conner et al. 1986) and itats. Food limitation is an unlikely causeof these habitat (e.g., see Rosenzweig 198 1, 1985) selec- nest-patch choices; small white firs do not pro- tion have historically been considered primarily vide food resourcesbecause all observations of in terms of availability of food and foraging op- Hermit Thrush foraging showed they were not portunities, under the assumption that food is associatedwith small white firs (Martin, unpubl. most limiting to reproductive success.Food is data; also see Dilger 1956). Given that the Her- an important limit on reproductive success(see mit Thrush in central Arizona almost always se- Martin 1987), but the availability of nest sites lected small white firs for nesting, our results that minimize risk of nest predation may be just suggestthat Hermit Thrushes selectsites that are as important, given the amount of mortality at- associatedwith a large number of other potential tributable to predation (see Ricklefs 1969). Nest nest sitesin the patch surrounding the actual nest sites are conventionally thought to be abundant site. Such choices cannot reflect availability of (Ricklefs 1969, Lack 1971, but see MacKenzie sites for renesting attempts because Hermit and Sealy 198 1, Finch 1985), but ifnest-site needs Thrushes always move to different nest patches are considered at both the site and patch levels, for renesting (Martin, unpubl. data). then high quality nest sites may not be as abun- The most likely explanation is that suchchoices dant as conventionally assumed (Martin, un- are associatedwith lower probability of nest pre- publ.). Thus, future considerations of nest site, dation. The high rates of nest predation found territory, and habitat selection need to pay closer over the 3 yearsof this study suggestthat it should attention to the availability and suitability ofnest exert strong selection on nest site and patch sites based on habitat attributes surrounding a choice. Moreover, the fact that nests with low nest at multiple scales. predation rates had considerably more small firs surrounding the nest than high predation nests ACKNOWLEDGMENTS documents that numbers of small white firs in We thank L. Smith and R. Tollefson for field assistance the nest patch influence probability of nest pre- in 1986. J. Alcock, R. N. Conner, R. L. Rutowski, and dation. These results, then, support the hypoth- an anonymous reviewer provided helpful comments esis that the number of potential nest sites that on earlier drafts of this manuscript. This work was predators must examine influences the proba- supported by grants-in-aid from Arizona State Uni- bility of nest predation. Moreover, at least for versity and Whitehall Foundation, Inc. Hermit Thrushes on our sites, these results in- dicate that the effects of foliage on numbers of LITERATURE CITED potential nest sites may be more important to ANDERSON,0. 1984. Optimal foragingby largemouth predation probability than the effects on such bassin structuredenvironments. Ecology6535 l- factors as impeding travel of the predator, con- 861. BEST,L. B., AND D. F. STAUFFER.1980. Factors af- cealing activity of the parents from predators, or fecting nesting successin riparian bird commu- simply inhibiting transmission of auditory or nities. Condor 82:149-158. chemical cues.This latter conclusionderives from BOWMAN,G. B., AND L. D. HARRIS. 1980. Effect of NEST-SITE SELECTION 57

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