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SHORT COMMUNICATIONS The Condor 86:483-485 was seen in any of the unparasitizednests, including two 0 The Cooper Ornithological Society 1984 that were checked on the day prior to hatching. No damage was seen on the two infertile host eggsin DAMAGED WESTERN FLYCATCHER parasitized nests (Table 1). One of these eggsremained in EGGS IN NESTS CONTAINING the nest after the cowbird chick had fledged, still without sign of damage. BROWN-HEADED COWBIRD The severity of damage was correlated with the age of CHICKS the cowbird chick (Table 1). Small nicks were present on an egg in a nest with a cowbird as young as two days old, but extensivedamage was not seenuntil the chick was five PATRICIA M. DOLAN days old. In no case did flycatcher young remain in the nest AND with the cowbird chick for as long as two days after host PHILIP L. WRIGHT hatching. Most of the host eggsdisappeared within a day or two after they showed signs of hatching. We assumed them to have producedyoung that subsequentlydied and The young of many brood parasitesbehave in ways that were removed by the parents. harm the host young with which they share a nest. By ejecting or injuring their nestmates, they reduce compe- DISCUSSION tition for parental care. To our knowledge,no specificanti- We can offer severalpossible explanations for the damage host behavior has been reported for chicks of the Brown- observed. headed Cowbird (Molothrus ater). Instead, the young par- (1) Visitorsto the nest.Animals visiting the nest would asite’s advantage over nestmates is usually attributed to have an opportunity to damage eggs.Possible visitors in- its hatchingearlier than the hosts,to the preferential treat- cludeoredators and adult female cowbirds(Mavfield 1961). ment it receivesbecause of its larger size, and possibly to Either of these, however, could have pierced-or removed damage done to the host young by accidental trampling the eggs,rather than just nicking them. Neither would be (Friedmann 1963). expectedto leave the eggsin the nest and return to inflict While studying the nesting of the Western Flycatcher more damage at a later time. (Empidonax dtjicilis) near Missoula, Montana, we noticed (2) Accidentaltrampling by the cowbirdchick. Sharply damagedflycatcher eggs in nestscontaining cowbird chicks. localized damage like the early nicks and small, steeply We describehere the incidence and extent of this damage inclined dents could not have been caused by crushing to the flycatcher eggs, and consider several hypotheses under the cowbird’s weight. The absence of damage to concerningits cause. infertile eggssuggests that damage was related to activity of an emergingchick within the egg,rather than to random OBSERVATIONS accidental crushing. However, the shells of the infertile Of 12 incubated Western Flycatcher nests with known eggsmay have been strongerthan the other shellsbecause contents, 5 were parasitized.One parasitizednest was dif- no minerals had been extracted by a developing chick. ficult to reach and was checked only irregularly, and a (3) Damage to the shell by the emerging chick, later secondwas destroyedduring incubation. Of the sevenun- dentedinward bv tramulina bv the cowbirdchick. The oc- parasitizednests, four were relatively inaccessible,and one currence of isolated damage-on opposite sides of some was destroyed before hatching. eggsand the variation in the location of damage from one Hatching of the host eggswas delayed in the two most egg to another suggestthat the damage to the shell was crowded parasitizednests, occurring two to four days after not related to normal hatching activity. the end of the normal incubation period of 14 to 15 days (4) Damage by a hostparent peckingat hatching eggs. (Davis et al. 1963). Incubation heat to host eggsmay have The two nests with the heaviest damage were abnormally been reducedboth by “shading” by the largercowbird egg, late in hatching.Intervention by a host parent in response and by the reductionin incubation time following hatching to abnormal hatching is a possibility. In the less crowded of the cowbird chick on day 11 or 12. In one case,a host third nest, however, hatching occurred in the normal in- eggrolled inside a pieceof the shell from which the cowbird terval and damage was slight. No damage was seen in had emerged, adhered to it, and was subsequentlypartly unparasitizednests. insulatedfrom incubation heat. As a result of the delay in (5) Pecking by the cowbirdchick directedat cuespro- host hatching, host eggswere still present in the nest up ducedby emergingchicks. Possible stimuli include vibra- to six days after the cowbird chick had hatched. tions and soundsresulting from prehatchingvocalizations Damage included “nicks,” superficial chips on other- or movements. This hypothesisis consistentwith all the wise intact shell, and “dents,” depressionsof the shell evidence presented. surfacecovered by a mosaic of shell fragments adhering Pecking and biting of nestmates occurs in at least two to the shell membrane. The membrane was exposedwhere other brood parasites. Young of the Lesser Honeyguide isolated fragments had fallen off, but in no case was the (Indicator minor) bear hooks on the end of the premaxilla membrane ruptured. The smallest dents were less than 2 and mandible which are driven into the host young in mm in diameter, with steeply sloping sides. The most savage“grasping” bites (Friedmann 1955). Attacks by a severely damaged eggshad several dents. In some cases, honeyguideon its second day after hatching left a newly the dents were distinct, even located on opposite sides of hatched host chick covered with specksof blood. Young the egg.In other cases,the dents were contiguousor over- honeyguidesalso attacked eggs,but were unable to punc- lapping, extending over much of the upper surfaceof the ture them. Young of the Striped Cuckoo (Tapera naevia) egg. are equipped with bill hooks similar to, but much smaller We found at least one damaged flycatcher egg in each than, those of the Lesser Honeyguide (Morton and Fara- of the three accessibleparasitized nests that survived to baugh 1979). Twisting bites by a cuckoo only three days hatching. Damage was seen only during the one or two old causedsubcutaneous hemorrhages and open wounds, days before host hatching (Table 1). No similar damage and the host chick died within 24 h of hatching. [4831 484 SHORT COMMUNICATIONS TABLE 1. Chronology of damage to host eggsin three Western Flycatcher nests, each containing a cowbird chick. Descriptions are given from the first detection of damage to the last day when viable eggswere present in the nest. Age of Num- cow- Days ber bird since of host Damage to host eggs chick incubation eggs in Date (days? began nest Egg 1 Egg2 5~~3 W4 Nest 49 25 July 16 Shell dented and chipped; One dent, Two isolated No chipping membrane exposedalong chips of dents or dentingb U-shaped area; on oppo- shell missing site side of egg, one isolated dent Nest 55J” 21 July 16 Three dents (1 to 2 mm) and A few tiny a few nicks nicks 29 July 18 Several coalesceddents and Tiny nicks, two isolated dents shell cracked, hatching 30 Julyd 19 Downhill from fallen nest Gone Nest W 29 July 13-14 One dent (1 mm) No chipping or denting 30 July 14-15 Cracked across,hatching ’ Minimum estunate of age; chick may have been up t” one day older. OAn undeveloped egg with a translucent shell, “ver half full of fluid whxh moved freely when the egg was Wed. This egg faded t” hatch and remained in the nest at the time the cowbird chick fledged. ENest also contained a second cowbird egg, which failed t” hatch. *Nest fell about 1 m from niche in sandbank to base of roadcut. The host parents continued t” feed the cowbird next to the fallen nest for at least two days. eA dead flycatcher egg that had been punctured and contained only a remnant of dry material. Directed pecking at suchan early age would be unusual itive disadvantagethat they would seldom remain long in for an altricial passerine chick. However, young of the the nest. For example, in 69 parasitizednests of the Eastern Giant Cowbird (Scaphidura oryzivoru) begin pecking at Phoebe (Suyornisphoebe), host eggsusually hatched when moving objectsshortly after their eyesopen, and can preen the cowbird was four days old, yet no damage to eggsor nestmatesand accuratelysnap at small moving objectsby young was seen (Klaas 1975). Effective pecks would just the age of five days (Smith 1968). have begun,and any dented eggswould soonhave hatched. If young cowbirds do behave aggressivelytoward host Young phoebesusually disappearedwithin 24 to 48 h of eggs(and, presumably, toward host young), why has evi- hatching in a nest with a cowbird nestling (Klaas, pers. dence of this tendency not been detectedduring the many comm.). studiesconducted on other host species?Pecks delivered If the host young were covered with down or feathers in a typical host nest are well hidden from direct obser- when pecking began, and were closer to the cowbird in vation, and detection depends on noticing damaged eggs size and development, they would be more likely to remain or young. in the nest longer, but signs of injury would be partially Our first notice of damaged eggs did not result from concealed. For example, eggs of the Kirtland’s Warbler especially careful or detailed examination. On the con- (Dendroica kirtlandiz] hatch two days after cowbirds in trary, even a casual glance into the nest would have re- parasitized nests, and may live several days, even long vealed the large U-shaped area of crushedshell on one of enough to fledge (Mayfield 1960). Effective pecks would the eggs,and the sharp dents on another. However, we not begin until the host was in its third day, and damage did not see such severe denting until the cowbird was at short of severelacerations would soon be obscuredby the least four days old, after 16 days of incubation.

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