THE EFFECT OF DISTURBANCE ON MOURNING DOVE NESTING SUCCESS DAVID WESTMORELAND AND LOUIS B. BEST Departmentof Animal Ecology,Iowa State University,Ames, Iowa 50011 USA ABSTRACT.--Disturbedand undisturbed Mourning Dove (Zenaidamacroura) nests were comparedfor differencesin daily survivalprobabilities and discrepanciesin the relationships between nest-sitefeatures and nesting outcome.At 3-day intervals, attending adults were flushed from disturbed nests(n = 51), whereas undisturbed nests(n = 50) were checkedfrom a distanceso that adultsdid not flush.Disturbed nests had significantly(P < 0.1) lower daily survival probabilities;this trend was evident during both the incubationand nestling stages, but was significantonly during the former. For disturbednests, two nest-sitefeatures (nest- bowl depth and nest support)were related to success(P < 0.1). In contrast,success of un- disturbednests was relatedto four variables(substrate height abovenest level, relative light intensity above the nest, nest concealment,and nest width). These results indicate that standardnest-checking procedures influence nesting success and confoundinterpretation of relationshipsbetween nest-site features and nesting outcome. Received10 December1984, accepted30 April 1985. MANY studies of the relationship between find nests, the value of concealment under nat- nesting successand nest-sitefeatures have re- ural conditions may be obscured.In the sum- suited in conflicting findings. For example, mer of 1983, we tested this hypothesiswith some authors (Meanley and Webb 1963, Hol- Mourning Dove nests.Any negative effect that comband Twist 1968) have reportedthat nest- nest checkshave on nestingsuccess should be ing successfor Red-winged Blackbirds (Age- particularlyevident for this speciesbecause the laius phoeniceus)is positively related to nest adults have noisy wingbeats [sometimes ac- height, whereas others have found a negative companiedby a distractiondisplay (Nice 1923)] relationship (Goddard and Board 1967, Ortego when flushed from the nest, and the eggsare and Hamilton 1978) or none at all (Francis 1971, noncryptic (white). The former could direct a Lenington 1980). Similar contradictoryresults predator's attention to the nest vicinity, and have been reported for Field Sparrows(Spizella the latter probably makes the nest more con- pusilla;Best 1978, Evans 1978) and Mourning spicuous.Under natural (undisturbed) condi- Doves (Zenaida macroura;Nice 1923, Schroeder tions, Mourning Doves attend the nest contin- 1970). Although researchershave studied sev- uously (Harris et al. 1963). eral bird speciesin diverse settings,most have Bart (1977) examined nest records for found no relationship between nest conceal- Mourning Doves and found evidence that ment and nestingsuccess (Roseberry and Klim- nesting successis reduced by researcherdis- stra 1970, Caccamise 1977, Best 1978, Gottfried turbance,but his results also can be explained and Thompson 1978, Best and Stauffer 1980, by reporting bias(Bart and Robson1982). Nich- Lenington 1980;but seeNolan 1978,Wray and ols et al. (1984) reported that daily vs. weekly Whitmore 1979, Murphy 1983). Becausepre- visits to Mourning Dove nests did not differ- dation often is the major causeof nest failure entially affectnesting success, but they did not in avian communities (Nolan 1963, Best and study undisturbed nests. Our objectiveswere Stauffer 1980), one would expect concealment to comparenesting successbetween disturbed to have a direct causal effect on nesting out- and undisturbed nests and to determine if dis- come. turbance obscuresrelationships between nest- The reason nest-site features often seem un- site featuresand nesting outcome. related to nesting successmay be that research- STUDY AREA AND METHODS er disturbance confounds the normal relation- ship between nest microhabitat and nesting The study was conductedfrom May through Au- outcome.If nest checksprovide additional cues gust 1983at Big Creek StatePark, Polk County, Iowa. or enhance existing cues that predatorsuse to A 110-ha sectionof the park containsseveral kilo- 774 The Auk 102:774-780. October1985 October1985] Disturbanceand Nesting Success 775 meters of windbreaks; each windbreak consists of a lesseggs or nestlingswere found on the groundbe- multiflora rose (Rosa multiflora) hedge flanked on one neath the nest. When the nest structure was dam- or both sidesby double rows of jack pine (Pinusbank- aged,we consideredmammals responsible. If the nest siana)or white pine (P. strobus).Most trees are less was intact, the failure was attributed to arian pred- than 6 m tall, and land use between windbreaks is ators. Snakesand chipmunks(Tamias striatus) were rotated between corn, alfalfa, oats, or nonuse. The rare in the study area, and we discountedthe impor- windbreaks are spacedat intervals of about 60 m. tanceof small nocturnal mammals(mice, shrews,etc.) Easternred cedar (Juniperusvirginiana) is a common becauseat least one adult dove was always present invader in perennially uncultivated areas. at the nest overnight. We located nestsby walking between pine rows To comparedifferences in nestingsuccess, we cal- and flushingnesting parents. All rowswere searched culatedthe daily nest survival probability (Mayfield weekly, and uncultivatedfield edgeswere searched 1975) and applied the statisticaltest developedby irregularly. When nests were discovered,we deter- Henslet and Nichols (1981). For these we used one- mined the age of eggsor nestlingsby using the de- tailed testsat a rejectionlevel of P < 0.1. This relaxed scriptionsin Hansonand Kossack(1963). We consid- rejectionlevel increasesthe power of the test (Hen- ered a nestingcycle initiated on the day the first egg sler and Nichols 1981) and has been the convention was laid. Nestswere randomly assignedto either dis- of previousresearchers (Coon et al. 1981,Nichols et turbed or undisturbed treatments. For the disturbed al. 1984). We used t-tests to compare nest-site vari- treatment,we approachednests directly, flushedthe ables between successful and unsuccessful nests attendingadult, and examinedthe nest contents.We within eachtreatment; a P < 0.05 rejectionlevel was checkedundisturbed nests from a distance(usually used for these analyses. >10 m) so that the attending adult did not flush. Mourning Dovescontinuously attend the nest until RESULTS the young are near fledRing age (Nice 1922),so pres- ence of an adult was taken as evidence that the nest Of the 150 nests found, 32 failed and 13 were wasstill active.In the latter half of the nestling stage, abandoned between initial discovery and the we often could see the nestlings directly. first revisit. We eliminated 4 additional nests Nestsin both treatmentswere checkedevery 3 days; from the undisturbed category becausethey when a nest failed, we assumed that it had survived could not be checkedwithout flushing the par- for 1.5 daysafter the previousvisit. Becauseall nests ent. The remaining 101 nests were used for were disturbed when discoveredinitially, we elimi- analyses.Median clutch age at discovery for natedfrom analysesall data for the interval between nest discoveryand the subsequentvisit. Nests that these nests was 5 days. failed during this period were not usedin analyses. Forty-three percent of all (101) nestssuccess- After a nest failed or the young fledged, we re- fully fledged at least one young; the daily s•r cordedthe speciesand height of the nest substrate, vival probability for all nests combined was nest height above the ground, nest width, depth of 0.946,yielding a nesting-successestimate of 21% the nest bowl, horizontal distance from the nest to for a 28-day nesting cycle. We attributed 79% the periphery of the substratecanopy, and, as an in- of the nest failures to avian predation. Doves dex of nest support, the total (sum) diameter of nested primarily in jack pine (61% of nests), branchessupporting the nest. Nest-bowl depth was multiflora rose (13%), and eastern red cedar measuredby placing a straightedgeacross the nest (8%). The remaining nestswere placed in elm and recordingthe perpendiculardistance from it to the center of the nest. The substrate height above (Ulmusspp.), mulberry (Morusspp.), and honey nest level was calculated from nest and substrate locust (Gleditsiatriacanthos) trees and in various height measurements.Visual concealment at nest shrubs.Daily nestsurvival probabilities did not level was rated aspoor, fair, good,or excellent(yield- differ (P > 0.1) among the three primary sub- ing valuesof 1-4, respectively).These estimates were strates(treatments pooled). made about 5 m from the nest in each of the four Effectsof disturbanceon nestingsuccess.--Thir- cardinal directions; the mean was used as an overall ty-sevenpercent of disturbednests and 50%of index of concealment around the nest. We estimated undisturbed nests were successful.Daily nest concealmentabove the nest by recordinglight inten- survival probabilitieswere significantlylower sity immediately above the nest and expressingit as for disturbed nests (P < 0.1); this trend was a percentageof ambient light intensity. We considereda nesting attempt successfulif the presentduring both the egg and nestlingstages, young survived to 10 days of age (Coon et al. 1981, but was significantonly during the former (Ta- Nichols et al. 1984). Nestlings usually remain in the ble 1). By Mayfield estimates,disturbed nests nest until age 13-14 days but will fledge at 10 days were about 50% as successful as undisturbed if disturbed.Failures were attributed to predatorsun- nests.