J. Field Ornithol., 62(4):492-496

BRONZY TOLERATE INTRUSION ON FORAGING TERRITORIES BY FEMALE GOLDEN-WINGED SUNBIRDS THAT PERFORM "BEGGING" DISPLAY

DALE F. LOTT Departmentof Wildlifeand FisheriesBiology Universityof California Davis, California95676 USA

Abstract.--Female Golden-wingedSunbirds ( reichenowi) are subordinantto both sexesof Bronzy Sunbirds (N. kilimensis).Nevertheless, they were often able to avoid being evictedfrom foragingterritories of Bronzy Sunbirdsby using a wing-quivering, tail-spread display strongly evocativeof food-beggingdisplays. The female Golden-wingedSunbirds appeared to be exploiting communicationthat functionsin the reproductionof Bronzy Sunbirds.

NECTARINA KILIMENSIS TOLERA LA INVASI•)N DE SU TERRITORIO DE FORRAJEO POR PARTE DE HEMBRAS DE N. REICHENOWI QUE EXHIBEN CONDUCTA DE PEDIR ALIMENTO Sinopsis.--Lashembras de Nectarinareichenou•i se subordinan a ambossexos de N. kilimensis. No obstante,muchas veces evitan ser expulsadasdel territorio de forrajeode la filtima, haciendotemblar susalas y replegandoel rabo, evocandola conductade pedir alimento.N. reichenou•iparece estar explotandoaspectos de conductade comunicaci6nque son parte funcional de la reproducci6nde N. kilimensis. Some social behavior "manages" (Owings and Hennessy 1984) or "manipulates" (Krebs and Dawkins 1984) the behaviorof others.I have observedfemale Golden-winged Sunbirds (Nectarinia reichenowi) that are otherwisesubordinant to Bronzy Sunbirds(N. kilimensis)use displays to overcomethe defenseof a foragingterritory by Bronzy Sunbirds.These displaysmay exploitthe BronzySunbirds' responses either to foodbegging by juveniles or solicitationof courtshipfeeding by a mate.

SUBJECTS AND STUDY SITE Bronzy Sunbirdsand Golden-wingedSunbirds were sympatricat the studysite near the southshore of Lake Naivasha, , at an altitude of 1900 m. BronzySunbird males divided a densestand of (Leono- tis nepetifolia)growing on irrigated land into foraging territories. Golden- Winged sunbirdssometimes entered these territories. Both species'bills conformclosely to the shapeof the leonotisflower, and both speciesfeed from the flowersefficiently (Gill and Wolf 1978). BronzySunbirds socially dominate Golden-winged Sunbirds (Gill and Wolf 1979, pers. obs.). Male Bronzy Sunbirdsdefended their territories by flying directly at intruders,most of which fled immediately.When an intruder did not flee, the Bronzy male landed 10-15 cm from it, then moved closeruntil the intruder left. The occasionalaggressive ap- proachesby female Bronzy Sunbirds or Golden-winged Sunbirds took the same form. Bronzy Sunbirdsbreed throughoutthe year in east Africa. Most re-

492 Vol.62, No. 4 "Begging"Sunbirds Tolerated in ForagingTerritories [493 productionoccurs in late March throughApril (Brown and Britton 1980). There were no nestson any of the territoriesI studied,but two Bronzy Sunbirdmales consistently tolerated a particularbanded Bronzy Sunbird female while evictingall other female Bronzy Sunbirds. During March and April 1984, Golden-wingedSunbird females were observedin four of eight territories defendedby color-bandedBronzy Sunbird males. These eight territories were observedfor a total of 1341 min, in 1009 of which the maleswere present.Only one territory was observed at a time. Observations were made on one banded and at least one unbandedGolden-winged Sunbird female.

RESULTS Golden-winged Sunbird females interacted a total of 63 times with territorial Bronzy Sunbird males, 10 times with Bronzy Sunbird females residentin territories,and six timeswith Golden-wingedSunbird females temporarilyoccupying a territory in the absenceof the territorial Bronzy Sunbirdmale. On 21 occasionsintruding Golden-winged Sunbird females, which are readily distinguishedfrom Bronzy Sunbird femalesin all plum- ages by yellow-edgedwing and tail feathers, respondedto aggressive approachesby male or female Bronzy Sunbirdswith a display.Each such display was performedby a female Golden-wingedSunbird perchedon the vertical stalk of a leonotisplant. The display began with the female spreadingher feet apart on the stalk. She turned her body to a 90ø angle to the stalk and fanned her tail feathers.The extent of this fanning varied greatly, even in the same individual. She spread her wings to varying degrees,but they were nevermore than abouthalf extended.Frequently she fluttered or quiveredher wings slightly. Sometimesshe rotated her bodyso that her belly was turned partially up. There was no indication that she vocalized.I never observedfemale Bronzy Sunbirdsdisplaying similarly. Following this display,the aggressionwas terminatedon 20 of the 21 occasions,and the female Golden-winged Sunbird was able to resumeforaging. The aggressionwas never terminated without a display. At leasttwo different Golden-wingedSunbird females displayed a total of 18 times to Bronzy Sunbird malesand were toleratedafter each display (16 of those 18 displayswere by one banded Golden-winged Sunbird female to one Bronzy Sunbird male). Golden-winged Sunbird females displayedthree timesto Bronzy Sunbird femalesand were toleratedtwice. In contrast,they displayedfour times to other Golden-wingedSunbird females and were never tolerated. The Golden-wingedSunbird females appeared to benefit from the displays,especially when they were directedat Bronzy Sunbird males. The durationof the Golden-wingedSunbird female'stime in the territory was recordedafter each of 18 encounterswith either a Bronzy Sunbird male or female in which the Golden-wingedSunbird female displayed. The averageduration of the Golden-wingedSunbird female'stime in the territory after a display was 314 s in the 17 timed encounterswith a Bronzy Sunbird male and 300 s in the onetimed encounterwith a Bronzy 494] D.F. Lott J.Field Ornithol. Autumn 1991

Sunbird female. In the course of a longer-than-averageintrusion, the residentBronzy Sunbird aggressivelyapproached the intruder every few minutes,whereupon the intruderagain displayedand againwas tolerated. In the 33 encounterswith a Bronzy Sunbird male in which a Golden- winged Sunbird female did not display,the averageduration of her stay was 10 s, and in the sevensuch timed encounterswith Bronzy Sunbird femalesthe averageduration was 21 s. This data set involved pooling, and so was not suitablefor significancetests using inferential statistics. The data did suggest,however, that the display benefitedthe Golden- winged Sunbird females:they fed continually while in the territory.

DISCUSSION I proposethat the tail-fanning,wing spreadingdisplay induces a dom- inant Bronzy Sunbird to tolerate a subordinateGolden-winged Sunbird in the Bronzy sunbirds'territory. An alternativeinterpretation is that the Golden-wingedSunbird female dominatedthe Bronzy Sunbird male by this display and remainedin the territory as a dominant.I doubt this alternative interpretation. Male Golden-winged Sunbirds were present in low numbersat this sitebut never held a territory, and no male Golden- winged Sunbird was ever in a Bronzy Sunbird'sterritory for more than a few secondsbefore being evicted.Perhaps the Golden-wingedSunbird femaleswere not assertingdominance but signalling that they would defendthemselves against aggression, thus disuadingthe Bronzy Sunbirds via a generalizedaggressive signal. This interpretationseems unlikely for two reasons:1) when the aggressivebird continuedto press after the display, the Golden-wingedSunbird always left and never re- taliated;and 2) of all the sunbirdsof both sexesof severalspecies removed from territories,only Golden-wingedSunbird femalesgave this display. When in a Bronzy Sunbird's territory during its absence,intruding female Golden-wingedSunbirds occasionally repelled other intruders.A Golden-wingedSunbird female residentin a Bronzy Sunbird male'ster- ritory displacedVariable Sunbird (Nectariniavenusta) males twice and Malachite Sunbird(Nectariniafamosa) males three times by flyingdirectly at them and landing above or near them. She approacheda Bronzy Sunbird female in the sameway, but without effect.This suggestedshe couldnot dominatethe female Bronzy Sunbird. When a Golden-wingedSunbird female removedan intruder, the ter- ritory holder's resourceswere defendedby the tolerated intruder. This is similar to resourcedefense achieved by territorial Pied Wagtails (Motacilla alba) tolerating a secondconspecific (Davies and Houston 1981). The payoff was different in this case,however, because the intruder consumed rather than defendedresources most of the time it was present,did not removeintruders when the territory holder was present,and was only able to removecertain of the intruders.Consequently, it was not clear that the Bronzy Sunbird territory holder derived a net economicbenefit from tolerating the Golden-wingedSunbird female. It seemedclear that Golden-wingedSunbird femaleswere able to feed Vol.62, No. 4 "Begging"Sunbirds Tolerated in ForagingTerritories [495 in the territoriesof Bronzy Sunbird malesby exploitingthe responsesof both Bronzy Sunbird males and femalesto their displays.The Golden- wingedSunbird females gained improved foraging opportunities. I cannot think of any way in which this outcomebenefited Bronzy Sunbirds.The males'responses seem likely to have beenselected to servea reproductive function that inhibited aggressionto facilitate mate acceptance.Tail spreadingand quiveringare includedin the courtshiprepertoires of at leastsome species of sunbirds(Skead 1967). The females'responses seem likely to havebeen selected to facilitatefeeding their young.The pattern of fanningthe tail and droopingand quiveringthe wings was reported as a juvenilebegging display in severalspecies of sunbirds(Skead 1967), and the wing quiveringcomponent was observedin juvenile Bronzy Sun- (van Someren 1956). The Bronzy Sunbird males van Someren observednever fed either their incubatingmates or their young.Even if Bronzy Sunbirdmales never did so, inhibition of aggressionin response to an appropriatedisplay by either their young or their mate would be selected,as would females' responsesto their young. Communication within dyads(i.e., mate-to-mateor offspring-to-parent),too importantto ignore,is particularlyvulnerable to exploitationby a secondsender (Markl 1985) such as the Golden-wingedSunbird females. The Golden-wingedSunbirds' displays are in someways similar to WoWs (1975) report of "prostitution"in female hummingbirds.Wolf observedthat femalehummingbirds sometimes participated in courtship and copulationwith conspecificterritory-holding males several weeks before either reached their seasonally-limitedreproductive readiness. During the courtshipperiod, the femalefed in the male'sterritory. Female Golden-wingedSunbirds were also allowed to forage, but the Golden- winged/BronzySunbird interaction differed from the hummingbirdphe- nomenonin interestingways. The male hummingbirdsinitiated courtship displays,whereas it was the female Golden-wingedSunbirds that dis- played first. (As L. Wolf [pers. comm.] has pointed out, however,the femalehummingbird initiates interaction by repeatedlyreturning to the territory and this couldeven function as a display.)Moreover, the female hummingbirdwill repay courtshipand intrusiontolerance with poten- tially fertile copulationsin a few weeks, whereas the Golden-winged Sunbird will not. Finally, Wolf noted that the male hummingbird'sbe- havior may contributeto his reproductivefitness immediately by insem- inating an occasionalprecociously fertile female or developinga rela- tionshipwith an individual female beforethe breedingseason that could increasehis mating successwhen the breedingseason started. This female Golden-wingedSunbird's behavior appeared to be a clearercase of man- aging or manipulating behaviorto the disadvantageof another via socialsignals than did "prostitution"in hummingbirds. It is puzzlingthat the BronzySunbird males' responses to this display were not more widely exploited.Bronzy Sunbird females,which seem certain to be more similar to a mate or offspring than Golden-winged Sunbird females, should have been able to exploit this trait in males. 496] D.F. Lott J.Field Ornithol. Autumn 1991

Perhapsthe behaviorof the Bronzy Sunbird maleswas a simplemisfiring that was not exploited often enough in nature to produce significant selectionagainst it. But if that is true, how did selectionfavor the Golden- winged Sunbird females'display?

ACKNOWLEDGMENTS

Earlier versionsof this manuscript were reviewed by S. Minta, E. H. Burtt, Jr., D. Owings,N. Thompson,L. Wolf, K. Yasukawa,and threeanonymous reviewers. Preparation of the manuscriptwas supportedby federal funds administeredthrough the Agricultural Experiment Station as Hatch Project 3915.

LITERATURE CITED

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