The Condor 88:190-193 0 The Cooper Ornithological Society 1986 HOUSE WREN NEST-DESTROYING BEHAVIOR’ JEAN-CLAUDE BELLES-ISLES AND JAROSLAV PICMAN Department of Biology, Universityof Ottawa, Ottawa KIN 6N5, Canada Abstract. House Wren (Troglodytesaedon) nest-destroying behavior was studied by experi- mentally offering 38 wrens nestswith eggs(or nestlings)throughout the nesting season.Individuals of both sexespecked all six types of eggspresented, regardless of the nest type and location. House Wrens also attacked conspecificyoung. Older nestlings(nine days old) were less vulnerable than three-day-old young. Our resultssuggest that nest-destroyingbehavior is inherent in all adult House Wrens but is inhibited in mated males and breeding females. It is suggestedthat nest destruction may have evolved as an interference mechanism reducing intra- and interspecific competition. Key words: House Wren; Troglodytes aedon; infanticide;nest destruction; competition. INTRODUCTION specificnestlings? (3) Is this behavior exhibited Destruction of eggs by small passerinesis a throughout the breeding season?(4) Do indi- relatively rare phenomenon which has been vidual House Wrens destroy neststhroughout observed mainly in members of two families, their breeding cycle? (5) How widespread is the Troglodytidae and Mimidae. Species this behavior among individuals from a pop- known to destroy eggsinclude the Marsh Wren ulation? (6) Is this behavior a local phenom- (Cistothoruspalustris; Allen 19 14); House enon or is it characteristic of all House Wren Wren (Troglodytesaedon; Sherman 1925, populations?(7) What is the adaptive value of Kendeigh 194 1); Cactus Wren (Campylorhyn- this behavior? thusbrunneicapillus; Anderson and Anderson METHODS 1973); SedgeWren (Cistothorusplatensis; Pic- man and Picman 1980); Bewick’s Wren This study was conducted at Presqu’ile Pro- (Thryomanesbewickii; J. Picman, unpubl. vincial Park, Northumberland County, On- data); the Gray Catbird (Dumetellacarolinen- tario, Canada, during the spring and summer sis;Belles-Isles and Picman, unpubl. data), and of 1984. House Wrens were breeding there in four speciesof Galapagosmockingbirds (Bow- nest boxes and hence were easy to study man and Carter 197 1). In contrast to “typical” throughout their breeding cycle. Thirty-eight predators that charcteristically consume eggs adult wrens (21 males and 17 females) were and nestlings,most of these speciesonly peck captured in mist nets or in a trap with a Red- and remove eggs and nestlings from the at- winged Blackbird (Agelaiusphoeniceus) nest tacked nests (Sherman 1925, Picman 1977, and eggas bait (Picman 1980). Captured wrens Picman and Picman 1980, Belles-Isles and were individually color-banded. Picman, unpubl. data). We conducted four seriesof experiments to Eggdestruction by the House Wren was first examine the wren’s nest-destroying behavior. described by Hill (1869). Thereafter, in the To determine the type of eggsand nests they early twentieth century, several isolated cases would attack, we offered them eggsof Yellow ofthis behavior by House Wrens were reported Warblers (Dendroica petechia), American (e.g., Widmann 1905, Ridway 1905, Wright Robins (Turdus migratorius),Blue-breasted 1909, Creaser 1925, Gardner 1925, Weigle Quails (Coturnixchinensis), Common Quails 1927, Lee 1927). However, most reported ob- (Coturnixcoturnix), and conspecificsin Yel- servations were based on circumstantial evi- low Warbler, American Robin, and Common dence, and the available information is there- Grackle (Quiscalusquiscula) nests as well as fore limited and often controversial. in nest boxes suitable for House Wrens (11 x In 1984 we began a long-term study on fac- 14 x 20 cm). The choice of eggsand nestsused tors determining mating patterns in the House during these experiments was determined Wren. A part of this project concerns the role mainly by their availability. In addition, traps of nest-destroyingbehavior in the evolution of baited with Red-winged Blackbird eggs and mating patterns in this species.We examined nestswere also presented. Experimental nests this behavior by asking the following ques- were attachedto vegetation (0.5 to 1.5 m above tions:(1) What kinds of nestsand eggsdo House ground) within 3 m of House Wren nesting Wrens attack?(2) Do thesewrens also kill con- boxes. To examine whether House Wrens at- tack neststhroughout their territory, robin nests with Blue-breasted Quail eggswere also pre- I Received 8 March 1985. Final acceptance3 September sentedat 10 to 40 m from their nest box. House 1985. Wren responsesto conspecific nestlings were P901 191 \ WREN NEST-DESTROYING BEHAVIOR TABLE 1. Summary of House Wren responsesto various eggs,regardless of sex, time in a seasonand stageof nesting. No. of % of positive birds Response trials with CObI Size* (mm) tested + _ brokeneggs Nest used House Wren pinkish, speckled 16 x 13 0 100 nest box Yellow Warbler creamy, spotted 17 x 13 100 Yellow Warbler Red-winged Blackbird light blue, spotted 24x17 26 20 2 ** Red-winged Blackbird American Robin light blue 28 x 20 5 3 2 100 American Robin Blue-breastedQuail creamy 29 x 20 28 28 0 75 American Robin Common Quail creamy, spotted 32 x 24 2 2 0 0 Common Grackle * Most of the eggmeasurements were taken from Harrison (1978). ** Unbreakableeggs (covered by severallayers of transparentglue). studiedby offering unmated males 3- and 9-day quailswere also pecked. In all sevencases when old House Wren nestlings in nest boxes. To House Wrens approached the experimental examine nest-destroyingbehavior throughout nests placed 20 m or farther from their nest the breeding cycle, we offered Blue-breasted boxes, they pecked the eggs.In four of these Quail eggsto the resident birds in robin nests. casesnests were located outside the aggressors’ Experimental nestswere presentedat the same territories. Previous reports mention 23 other location from nest boxes during pre-mating specieswhose nests were attacked by House (males only), pre-laying, laying, incubating,and Wrens (i.e., Widmann 1905, Sherman 1925, nestling stages.Within a given breeding stage, Creaser 1925, Lee 1927, Kendeigh 194 1). Only some birds were tested up to three times. 13 of these species breed in cavities, which Offering a nest with one egg (or young) was suggeststhat House Wrens exhibit a general- considered a single trial. No more than one ized type of nest-destroyingbehavior. nest per day was presented to a given wren. A Offering conspecific nestlings in five trials positive response was defined as pecking an resulted in three positive responsesby adult egg or nestling, whereas a negative response wrens.In thesecases, one female and two male occurred when the bird appeared to ignore the House Wrens pecked the young, and in two experimental nest while nearby (within 1 m instancesthe 3-day-old nestling was also re- from the nest). Individual trials were contin- moved from the experimental nest box. The ued for 2 hours, but were stoppedwhen a wren 9-day old nestling survived the attack after it responded positively. All experiments were was returned to its original nest. In two ad- conducted between 0600 and 1000. ditional cases, males removed nest material but did not inflict any injuries or remove the RESULTS AND DISCUSSION 9-day old young from nest boxes. In all five NATURE OF NEST-DESTROYING cases,adult wrens removed nest material from BEHAVIOR experimental nest boxes. These results suggest In general, House Wrens responded to nests that House Wrens may also have an important with eggsby vigorously pecking the eggs.Fol- impact on each other’s reproductive success lowing pecking, they removed broken eggs, through their attackson nestlings,even though often carrying them away before dropping them the older nestlings seem less vulnerable. To (see also Hill 1869, Widmann 1905). In 15% our knowledge only one direct observation of of our observations (n = 13), they also re- House Wren infanticide has been previously moved nesting material (see also Swanson reported although the age of the attacked nest- 1925, Sherman 1925, Weigle 1925, Baldwin ling(s) was not given (Baldwin 1925). In con- 1925). Damage inflicted on the nests ranged trast, killing of heterospecificyoung by House from the removal of some nest lining to the Wrens was reported by Weigle (1925), Dales destruction of one side of the nest. In only 2% (1926), and Kendeigh (1941). of our observations(2 females) wrens used the removed material as lining for their own nests. INCIDENCE OF NEST-DESTROYING House Wrens attacked all six types of eggs, BEHAVIOR regardlessof their size and color or type of At the individual level. Contrary to the pre- nests in which they were offered (Table 1). vious belief that only males exhibit nest-de- Wrens were successfulin breaking all smaller stroying behavior (Sherman 1925), we ob- eggsup to the robin size, but their successde- served both males and females pecking eggs. creased for the larger eggs (Table 1; Fisher’s House Wrens attacked eggs throughout the exact probability test for smaller versus larger breeding season from mid-May to late July. eggscombined, P < 0.001). Eggsof allopatric Males however, did not attack eggsonce paired species such as Blue-breasted and Common (Table 2; Fisher exact probability test for com- 192 JEAN-CLAUDE BELLES-ISLES AND JAROSLAV PICMAN TABLE 2. Summary of House Wren responsesto eggs (Catherpes mexicanus), Rock Wren (Sal- throughouttheir breeding cycle. pinctesobsoletus), and Winter Wren (T. trog- lodytes)did not attack experimentally offered Males FeIIlal.3