Interspecific Competition in Grallaria Antpittas: Observations at a Feeder

Samuel S. Snow,1,2* Daniel J. Field,2,3,4 and Jacob M. Musser1,2

1 Department of Ecology and Evolutionary Biology, Yale University,P.O. Box 208106, New Haven, CT 06520-8106 USA

*Corresponding author: samuel.snow@yale.edu2 Division of Vertebrate Zoology, Peabody Museum of Natural History, Yale University,

P.O. Box 208118, New Haven, CT 06520-8118 USA3 Department of Geology and Geophysics, Yale University,

P.O. Box 208109, New Haven, CT 06520-8109 USA4Department of Vertebrate Zoology, National Museum of Natural History,

Smithsonian Institution, Washington, DC 20560 USA

ABSTRACT

We describe an aggressive interaction between the extremely rare Jocotoco Antpitta (Grallariaridgelyi) and a close relative, the Chestnut-naped Antpitta (Grallaria nuchalis). Since first discov-ered in 1997, ecological information about the Jocotoco Antpitta has begun to accrue; however,much remains to be learned about this rare species in its limited native range. The observationsdescribed here advance our knowledge of the behavioral ecology of this elusive species and con-tribute more broadly to our knowledge of interspecific competition in antpittas in general. Weshow that in avian systems in which playback experiments fail to detect competition, competitionmay still be prevalent, in this case manifesting in a nonvocal, physically aggressive interaction. Wealso raise new questions about the potential for temporal variation in interspecific competition ingrallariids due to increased resource demands during nestling provisioning.

KEYWORDS

Grallariidae, Jocotoco Antpitta, interspecific competition, playback experiments, endangeredspecies, Tapichalaca

Bulletin of the Peabody Museum of Natural History 56(1):89–93, April 2015.© 2015 Peabody Museum of Natural History, Yale University. All rights reserved. • http://peabody.yale.edu

Introduction

First discovered in 1997, the rare and extremelylocal Jocotoco Antpitta (Grallaria ridgelyi) isamong the least numerous and least studied of theantpittas (Passeriformes: Grallariidae) (Krabbe et al. 1999; Krabbe and Schulenberg 2003).Because of their extremely secretive nature, rela-tively little about general antpitta ecology andbehavior has been documented, beyond what canbe gleaned from individual sightings and thestudy of breeding biology at active nest sites(Krabbe and Schulenberg 2003; Greeney et al.2008; Freile et al. 2010; Greeney 2012). Here wedescribe, for the first time, direct competitiveinteractions between G. ridgelyi and a close rela-tive, the Chestnut-naped Antpitta (G. nuchalis).

In doing so, we contribute to the knowledge of thehabits of a unique, threatened, and little-knownspecies, while also adding to our currently limitedunderstanding of antpitta behavioral ecology.

Intraspecific aggression in the JocotocoAntpitta has been observed only once. F. Men-doza, a park ranger at the Tapichalaca BiologicalReserve (lat 04°30�S, long 79°10�W; elevationapproximately 2,500 m) in southeastern Ecuador,happened to encounter two Jocotoco Antpittasapparently fighting over territory. He first heardthe two birds giving “challenging contact calls”and then, when they emerged onto the trail aheadof him, he described the interaction between thetwo individuals as reminiscent of two domesticfighting-cocks (Gallus gallus gallus): “The strongbill was used for fierce blows, and the victorious

bird furiously followed its enemy, attacking itsback and rump, until both birds disappeared inthe undergrowth again” (pers. comm. in Heinz2002). Similar intense intraspecific aggression inother antpitta species has been previously docu-mented, albeit rarely. For example, a direct fightbetween a pair of wild Rufous Antpittas (Grallariarufula) was observed, which resulted in the birds“rolling down the slope as a ball” (Krabbe andSchulenberg 2003). In captivity at the Bronx Zoo,a male Rufous-faced Antpitta (Grallaria erythro-tis) purportedly killed its female partner in a fight(Bell and Bruning 1976).

Ecology

Interspecific aggression in all antpittas, and in theJocotoco Antpitta in particular, has rarely beendirectly documented. However, there is consider-able circumstantial evidence to suggest that theJocotoco Antpitta and its close relative the Chest-nut-naped Antpitta (Krabbe et al. 1999) may com-pete for resources and territory. The JocotocoAntpitta is found on steep slopes of the Andes, inwet montane forest with a low canopy and exten-sive bamboo cover between 2,300 and 2,680 m inelevation. This species primarily exploits the lowerpart of the forest (below 4 m), and is often seenon the ground (Ridgely and Greenfield 2001;Heinz et al. 2005; Greeney 2013). The Chestnut-naped Antpitta occurs in a more extensive geo-graphical range (and a greater elevational range of2,000 to 3,000 m), but it occupies nearly identicalhabitat, and is found in sympatry with the Joco-toco Antpitta where the latter species exists(Krabbe et al. 1999; Ridgely and Greenfield 2001).Interestingly, both Grallaria ridgelyi and G.nuchalis (along with several others in this genus)have been reported to engage facultatively in feed-ing associations, periodically following army antswarms, as well as large mammals (includinghumans) that are crashing through vegetation(Greeney 2012). Stomach content analysis of G.ridgelyi and G. nuchalis has revealed nearly iden-tical diets, including millipedes, beetles, ants, andarthropod larvae (Krabbe and Schulenberg 2003).

In spite of these ecological similarities, previ-ous work has failed to show any associationbetween the two species. Heinz (2002) hypothe-sized that Grallaria ridgelyi and G. nuchalis maycompete for resources, with implications for

determining the range and distribution of theJocotoco Antpitta (G. ridgelyi). However, Heinz(2002) concluded that there was no direct sup-port for this hypothesis: he documented signifi-cant territorial overlap between the two species,and a distinct lack of response by each species tothe territorial calls of the other. Anecdotally, onlymild aggression between the Jocotoco Antpittaand other birds at worm-feeders has been previ-ously reported. Worm-feeders are a recentlydeveloped phenomenon driven by the eco-tourism industry as reliable means for tourists to see generally furtive species such as antpittas.The increasing prevalence of these feeding sta-tions may encourage the association of antpittasand other vermivorous birds (Greeney 2012).Greeney (2013) suggested that in such settingsthe Jocotoco Antpitta is “apparently not aggres-sive towards the similarly-sized Chestnut-napedAntpitta...” In contrast to these findings, ourobservations provide the first concrete evidencefor interspecific competition between G. ridgelyiand G. nuchalis.

Materials and Methods

The events described here took place on themorning of 19 March 2013 in the vicinity of aworm-feeder adjacent to the Tapichalaca Biolog-ical Reserve (see Greeney and Juiña 2010 for adetailed site description). The Fundación de Con-servación Jocotoco (www.fjocotoco.org) estab-lished the worm-feeder discussed here in 2005,specifically to attract Jocotoco Antpittas (Woodset al. 2011).

We arrived at the feeder at 1006 h ECT(Ecuador Time) accompanied by a guide from theFundación de Conservación Jocotoco. During ourvisit we observed two adult Jocotoco Antpittas, asingle Chestnut-naped Antpitta, and direct com-petitive interactions between these two species(Figure 1). The first adult Jocotoco Antpitta wasalready present at the feeder on our arrival. Itbegan collecting earthworms that were regularlytossed onto a stone platform by the guide. A sec-ond Jocotoco Antipitta adult soon joined the first,and they alternated gathering worms from thefeeder until the supply was exhausted at 1048 h(see Figure 1B). A typical feeding cycle consisted ofthe antpitta ingesting some worms, followed by aperiod of gathering worms to store in the bill or

Bulletin of the Peabody Museum of Natural History 56(1) • April 201590

Interspecific Competition in Grallaria Antpittas • Snow et al. 91

perhaps crop, and then leaving for several minutesbefore returning and repeating this process. Ourcompanion from the Fundación Jocotoco, whowas familiar with these local birds, indicated thatthese two adults were a mated pair carrying wormsto a nestling nearby, which we did not directlyobserve. Throughout this interval the two Joco-toco Antpitta individuals often called to oneanother with the characteristic hoo-coo call, towhich they owe their common name (Ridgely andGreenfield 2001; see Figure 1A). We noted thatone individual would always call first, followedwithin seconds by a response with the same callfrom the second individual. This hoo-coo or ho-

grrr call has been described as a territorial chal-lenge-seeking call (Heinz 2002). Our observationssuggest that this call may also be used as a contactcall within pairs.

One individual Chestnut-naped Antpitta firstappeared in the area at 1053 h, attracted by whis-tles from the Fundación Jocotoco guide. Itapproached tentatively from the opposite side ofthe clearing relative to the Jocotoco Antpittas at amoment when no Jocotoco Antpittas were pres-ent. The Chestnut-naped Antpitta remained at theempty feeding platform until 1055 h, anddeparted quickly when a Jocotoco Antpittaarrived (see Figure 1C).

FIGURE 1. Jocotoco Antpitta, Grallaria ridgelyi, and Chestnut-naped Antpitta, G. nuchalis, at a worm-feeder atthe Tapichalaca Reserve, southern Ecuador, on 19 March 2013. A, Adult G. ridgelyi performing characteristichoo-coo call, 1034 h ECT. B, Adult G. ridgelyi collecting the last remaining worms from the feeding platform, 1048h ECT. C, Adult G. nuchalis approaching empty feeding platform, 1055 h ECT, with feathers ruffled in responseto approach from G. ridgelyi. D, Adult G. nuchalis standing at empty feeding platform at 1104 h ECT, immedi-ately before being chased away by an adult G. ridgelyi.

At 1059 h, again when no Jocotoco Antpittaswere visible, a Chestnut-naped Antpitta appearedand approached tentatively, taking a few steps andthen pausing for several seconds, becoming erectand alert before again proceeding forward anotherfew steps. It advanced in this way until it arrived atthe feeder at 1103 h (see Figure 1D). At 1104 h, aJocotoco Antpitta emerged from the ground veg-etation and began to run toward the Chestnut-naped Antpitta, which quickly retreated. TheJocotoco Antpitta did not pursue the Chestnut-naped Antpitta out of sight, and the chase ended assoon as the Chestnut-naped Antpitta disappearedinto the underbrush at the edge of the clearing.

The third and final interaction between thetwo species occurred when the Chestnut-napedAntpitta returned to find a Jocotoco Antpitta atthe empty feeding platform at 1109 h. At theappearance of the Chestnut-naped Antpitta, theJocotoco Antpitta became visibly agitated, ruffledits feathers, and chased the Chestnut-napedAntpitta far from the clearing at high speed, fol-lowing it up a slope and out of sight. It was unclearwhether both or just one of the adult JocotocoAntpittas engaged in this aggressive behavior.

Discussion

This aggressive interaction is a curious windowinto the ecology of an extremely rare and little-known bird. In contrast to previous findings, theJocotoco Antpitta may indeed engage in resourcecompetition with other closely related, sympatricGrallaria antpittas. Though playback experimentshave failed to reveal interspecific aggressionbetween these two species, we have observeddirectly that they do interact aggressively, with thelarger Jocotoco Antpitta appearing to represent thedominant combatant (Heinz 2002). It could be thatin certain contexts playback experiments may sim-ply be inconclusive, failing to reveal physical con-flict over food resources, as opposed to mere vocalconflict. In this case we have shown that aggres-sive interactions between G. ridgelyi and G.nuchalis apparently consist of silent chasing andposturing rather than vocalizing. These would nat-urally be overlooked in a playback experiment.

Perhaps more curious is the contrast betweenour findings of violent interspecific aggressionbetween Grallaria ridgelyi and G. nuchalis andthose of previous observations at this feeder and

others (Woods et al. 2011; Greeney 2013). TheJocotoco Antpittas we observed were purportedlyprovisioning a nestling at the time. It may be thatcompetition for food is at its highest during thenestling provisioning period, and that previousreports derive from nonbreeding periods. If thisis the case, it could indicate previously underap-preciated temporal variation in interspecific com-petition in grallariids. Future research may focuson whether sympatric antpittas stagger breedingperiods so as not to overlap during this potentiallydemanding time.

Acknowledgments

We would like to thank our friends at the Fun-dación de Conservación Jocotoco and the Tapicha-laca Biological Reserve for their admirable workto conserve these fascinating birds. We would alsolike to thank Dr. Kristof Zyskowski of the YalePeabody Museum of Natural History and Dr.Harold Greeney of the Yanayacu Biological Sta-tion for reviewing this manuscript and provid-ing many helpful comments. The authorsacknowledge the following sources of funding:Yale University (Snow, Musser, and Field); NationalSciences and Engineering Research CouncilCanada Graduate Scholarship D and SmithsonianPredoctoral Fellowship (to Field); and NationalScience Foundation Graduate Research Fellow-ship under grant no. DGE-1122492 (Snow andMusser).

Received 30 May 2014; revised and accepted 19December 2014.

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