The Auk 118(2):404-411, 2001 LEK-LIKE MATING SYSTEM OF THE MONOGAMOUS BLUE-BLACK GRASSQUIT JULIANA B. ALMEIDA1'3 AND REGINA H. MACEDOTM •Departamentode Ecologia,Universidade de Brasilia,70910-900 Brasilia, Brazil; and 2Departamentode Zoologia,Universidade de Brasilia, 70910-900 Brasflia, Brazil ABSTRACT.--Inthis study, we investigatedthe role of display and mating systemof the little known NeotropicalBlue-black Grassquit (Volatinia jacarina). Males form aggregations and executea highly conspicuousdisplay, resembling traditional leks. Number of displaying malesdeclined throughout the study period, thoughdisplaying intensity during the season showedno variation. Individual maleshad significantlydifferent displayingrates and also defendedterritories of very different sizes,ranging from 13.0 to 72.5 m2, but we found no associationbetween territory sizes and the averagedisplaying ratesof the residentmales. There alsois no associationbetween displaying rates of malesand size and vegetationstruc- ture of their territories. Four of seven nests were found within male territories and obser- vationsindicated that both sexesinvest equally in caring for nestlings.Results suggest that the Blue-blackGrassquit does not fit into the traditionallek mating system,contrary to what hasbeen proposed in the scarceliterature available. However, it is clearthat theseapparently monogamousbirds behavelike a lekking species.We speculateabout the possibilitythat aggregationof nesting territoriesin this speciesmay be due to sexualselection pressures, and suggestthat the Blue-blackGrassquit may be an ideal candidateto test Wagner's(1997) hidden-lek hypothesis.Received 2 August1999, accepted17 October2000. SEXUALSELECTION in lekking species has and Wolf 1979), females do not nest within been the object of much attention during the male territories,but may dependupon resourc- last decades.A lek can be broadly defined as es within them. It is possiblethat in somespe- any aggregationof malesvisited by femalesfor cies, male aggregationsfunction like typical mating purposes(see review in Ht•glund and leks: individuals form a monogamouspair Alatalo 1995). Lek mating systems--where bondwhile displayingin a lek closeto thenest- males are clustered but still maintain fairly ing site (Ht•glund and Alatalo 1995). large territories--are sometimescalled explod- In this study,we addressedquestions related ed leks (Emlen and Oring 1977), as opposedto to the lek-like mating systemand functionof classicalleks, where malesare tightly clumped. the conspicuousdisplay exhibitedby a little- Males may chooseto display at sitesassociated studied,small, Neotropical bird, the Blue-black with resourcesused by females.For example, Grassquit (Volatiniajacarina). This common epaulatedbats court femalesfrom within ter- specieshas a geographicalrange extending ritories with resources females need, but do not from Mexico throughoutmost of SouthAmer- controlaccess to theseresources as a way to ob- ica. It typically occursin open savannagrass- tain females (Bradbury 1981, 1985). In cases where resources are located within male terri- lands,usually involvingsome degree of human disturbance.During the breeding season,adult tories,it is possiblethat femalechoice is based males are easily distinguishedfrom brownish upon the quality of resources,and not upon quality of the male. Thus, differencebetween a females and juveniles by their glossy black lekking speciesand one with territorial polyg- plumages and white wing underparts.They yny may be difficult to discern,for in certain are easily spottedin the field due to their dis- species,such as some hummingbirds(Stiles tinctivedisplay, which consists of repeatedver- tical flights of about 50 cm, from elevated perches.During theseshort vertical flights, the 3Present address: Ecology, Evolution and Conser- vation BiologyProgram/314, University of Nevada, males exposethe white underwing area and Reno, Nevada 89557-0013,USA. E-mail: jaimeida@ emit a shortbuzzing vocalization("teez-wee"). unr. nevada.edu Males are generally found executingtheir dis- 4Address correspondenceto this author.E-mail: playsin a clusteredmanner, quite similarin ap- [email protected] pearanceto leks. 4O4 April 2001] Matingand Displays in Grassquits 405 Very little information is available concern- surementsof each individual (weight; and bill, tar- ing the Blue-blackGrassquit's mating systemor sus,right wing, and tail lengths). Behavioral observations.--Observations were con- the males' attention-callingdisplay. A descrip- ducted between the second week of November and tion of the display is provided by Sick (1997), third week of April, almostalways between 0600 and and early observationssuggested that the dis- 1100h (GMT-3 h). After this time period, displaying play could be related to pair formation (Alder- activity markedly declined until late afternoon, ton 1963). Some experimental work concerning when displayingresumed in a more subduedform. the acousticpart of the display was done by Observations were made with binoculars, at distanc- Wilczynski et al. (1989). The display rate is ap- es ranging from 15 to 20 m from grassquits,which proximately12 to 14 leapsper minute (Carval- did not disturb their displaying. Display perches ho 1957) at a speed of 1.13 m/s (Weathers used by males were marked with an identifying tag 1986). On the basis of very short field obser- to allow the demarcation of territorial boundaries. vations (3-4 days), Murray (1982) and Webber Other observationsincluded agonisticdominance in- teractions among males, feeding, courtship, and (1985) suggestedthat the Blue-blackGrassquit nesting. Three categoriesof parental care were con- had a lek mating system,solely on the basisof sidered: (1) visitation, which is the number of times the fact that males display in aggregations. the adult entered the nest; (2) permanence,which is Bradbury (1977, 1981, 1985) specified four the time the adult spentinside the nest;and (3) prox- criteria for distinguishing traditional lekking imity, which is the time the adult spentwithin a 2 m speciesfrom those using alternative lek-like radius of the nest. Observations also included 5 min mating systems.These criteria are: (1) no pa- focalsamples of bandedmales to recordthe number ternal care; (2) the aggregationof males in an of display leaps executed(at least two samplesper arena, to which females come for mating; (3) male). To analyze how number of displaying males the arena contains no resources for females varied within the breeding season,we calculated a display sightingrate, that is, the number of displays other than the males themselves(e.g. no food, countedin a week divided by the total time spent in nesting sites, water, etc.); and (4) females are the field that week (number of displays divided by free to choosea mate when visiting the arena. sum of minutes in the field). That rate reflects the fre- In this study we analyzed male territorial dis- quency with which grassquitswere encounteredin persion, habitat preferences,and nesting be- display,because the time spentin the field involved havior of the Blue-blackGrassquit to see how both focal observationsand time spentsearching for well Bradbury's(1977, 1981, 1985) criteria for individuals. A different measure assessed the vari- characterizinglekking behaviorwere satisfied. ation in intensity with which males displayedalong the season,and that was calculatedby taking into ac- countonly the 5 min focalperiods, disregarding time METHODS spent between focal observations(number of dis- plays divided by sum of minutes of focal observa- Study site and generalmethods.•Our study took tions). We also registered the causesof display inter- placeat an experimentalfarm belongingto the Univ- ruption, which could be due to factors considered ersidadede Brasflia(Fazenda •gua Limpa)located neutral, suchas leaving the territory without appar- approximately30 km from Brasfiia,Brazil (15ø56'S ent reason and interruption by external elements and 47ø55'W). The study site comprised an area of (suchas carsor people on the road), or agonisticfac- typical "cerrado" vegetation,which varied from a tors, suchas the intrusion of other males.Displaying low savannagrassland sparsely covered with bushes maleswere occasionallyinterrupted by other males, to a denser form of "cerrado", consistingof grass- often causingthe resident to approachthe intruder land, bushes, and small trees. This area was divided and, sometimes,to pursue him. When a female ap- into a grid (100 x 50 m) of 50 quadrants of 100 m2 proacheda male,he continuedhis displaywhile get- each, with the 66 intersectionpoints marked with ting closer to the female. Copulation was only ob- identifying flags. The grid was boundedon one side servedonce because most of the time, after courtship by a dirt road, by a cultivatedarea on its secondside, was initiated, the pair flew to the ground beneaththe a small cliff on the third side,and shrubbygrassland vegetationand was lost to sight. on the fourth side. Vegetationstructure and resource availability.--We as- The study was conductedwithin the rainy season, sessedlocal vegetationstructure, which could be as- from September1995 to April 1996. From mid-No- sociatedwith suitability of the area for nesting,and vember 1995 until early April 1996, we captured 66 also availability of food (grasslandseeds) in territo- male and 38 female grassquitswith mist netswithin ries identified within the grid. That was done by ex- the study plot, marked them with unique combina- tending two perpendicular tapelines