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The Condor 99~514-519 Q The CooperOmlthological Society 1997

ON THE BEHAVIOR OF THE BLACK SWIFT ’

MANUEL MAR~N Museum of Natural Science and Department of Zoology and Physiology, Louisiana State University, Baton Rouge, LA 70803 and Western Foundation of Vertebrate Zoology, Camarillo, CA 93012, e-mail: [email protected]

Abstract. The behavior of Black Swifts was stud- then most roostedon the cave walls. During incubation ied in southern California from 1990 to 1992. Four and early brooding, an adult always remained at the types of aerial interactions were distinguished: (a) nest, and food transfer between adults was observed group chase, (b) pair chase, (c) pair contact, and (d) during this period. touch and grasp. The latter two interactions can be Key words: behavior, aerial and hostile displays, intraspecificor interspecific.Aerial copulationwas not begging, roosting, Cypseloides niger. observed. Nestlings, from age 18 days onward, and adultsgave hostile or deterrencedisplays by wing-rais- ing. Begging by nestlings was silent but aggressive Swifts as a group are remarkably uniform in shape, toward the adult. Silent begging may be an antipre- which makes them difficult to identify, and their aerial dator strategyfor a speciesthat producesa single chick lifestyle makes them difficult to observe. As a result, per season. Nestlings have a far more conspicuous little is known about their behavior, even for common white facial marking than adults; this may function as species. This is especially true in the New World a target signal to guide food delivery in the dimly lit swifts. nesting conditions and as an aid for the adult to find The subfamily Cypseloidinae contains 12-l 3 spe- the nest when arriving late at night. Adults roostedon cies, most of which are tropical or semitropicalin dis- the nest for the first half of the nestling period and tribution. All are similar in body shape,and most have uniform dark plumage and facial markings, which in some are distinctive. All speciesin the subfamily nest I Received 8 March 1996. Accepted 29 November in sites behind waterfalls, in caves, or deep gorges,on 1996. sea cliffs, and in sea caves. Moisture, inaccessibility SHORT COMMUNICATIONS 515 to terrestrial predators, and darknessor at least deep flew horizontally, and the secondbird approachedfrom shade during most of the day seem to be the rule for below. Next they turned over and clashed or almost nest sites within the group. touched with the feet. Sometimes one bird emitted The Black Swift, Cypseloides niger, is a widely dis- clicking notes. After a few seconds, the higher bird tributed species, found locally from northwestern changedits direction of flight, and the lower bird con- North America through Middle America and the West tinued in horizontal flight. Occasionally,two birds be- Indies (AOU 1983). Despite its wide range, most of came involved in a fourth type of aerial interaction, what is known about the species is limited to infor- “touch and grasp,” in which the birds grasped each mation on distribution and breeding biology (Knot-r other by the feet and tumbled downward for a few 1961, Foerster and Collins 1990). In this paper aerial meters, sometimes several meters, occasionally sepa- interactions,hostile displays, begging behavior, roost- rating just above the vegetation. Subsequently,they ing, and possiblepair bond behavior are described. rejoined the group (if any) above. Both the third and the fourth type are likely aggressiveinteractions. STUDY SITE AND METHODS On four dates in 1992, Black Swifts were observed The biology of the Black Swift was studied in the San in aerial interactionsof the third and fourth types with JacintoMountains in the vicinitv of Idvwild. Riverside Violet-green Swallows (Tuchycinetu thnlassina). Co., southernCalifornia. Most observationswere con- These aerial interactions were observed when the ducted at a single site where an aggregation of 6-7 swifts flew over the top of the pines in a section of the pairs nested every year. From 1990 to 1992, the area canyon where a group of swallows usually foraged. was visited 40 times. Visits were made at l-15 day When a swift approachedthe swallow flock, a swallow intervals, with observation periods ranging from l-3 would chase the swift, and they would graspeach oth- days. The area immediately surroundingthe site was ers’ feet. Sometimes this action was just a brief dash covered by coniferousforest. The nesting site, at 1,500 or a short descent,with birds being graspedby the feet m elevation, was a cave formed by large boulderswith as in the third and fourth interaction types described a substantial flow of water through a side wall and above. On 8 July 1992, I photographedone of these several minor drips in the cave ceiling. Water flowed interspecific interactions (Fig. 1). This type of inter- through the stream year-round, but substantialdiffer- specific aggressionalso occurred intraspecifically(see ences occurred among years and the flow declined below). from beginning to end of summer. More detailed in- Aerial interactionsof the third and fourth types de- formation on the vegetationand site of the area can be scribed above, in which two birds graspedeach other found in Barbour and Major (1977) and Foerster and by the feet while descendingearthward, have been re- Collins (1990). garded as aerial copulations (e.g., Foerster 1987). Whether in swifts these aerial interactionsare actually RESULTS AND DISCUSSION copulations has been controversial (Marin and Stiles AERIAL INTERACTIONS 1992). In other swifts, similar interactionshave been observed or photographed,but not considered copu- For much of the breeding season(May-September) I lations (Francis 1987). Copulation at the nest has been observed the birds in several types of aerial interac- reported for many swifts in other genera, e.g., Apus tions, primarily in the vicinity of the breeding site. I (Moreau 1942, Lack and Lack 1952), Chaeturu (Fi- was able to distinguishfour types of aerial interactions, scher 1958), Collocalia (Medway 1962), Cypsiurus not all related to courtship.The first type, which I des- (Fry et al. 1988), and Streptoprocne (Rowley and Orr ignated as “group chase,” was a horizontal flight dur- 1962). ing which one of the birds led a high-speedchase. The Becausethese aerial interactionscan be interspecif- leading bird (presumably a female) usually was fol- ic, I question whether such behavior involves copula- lowed by 3-6 birds; one or more of the chasing birds tion. It is more likely that aerial interactionsof types emitted soft high-pitched sounds.Although flight was three and four are a form of aggressionrather than occasionally erratic, all birds remained close together sexual behavior. This reasoningmay apply to many if at high speeds, and the group seemed well synchro- not all swifts. nized. Group chasesmay involve the formation of new pairs or pair bond reinforcement. They mainly oc- HOSTILE DISPLAY curred early in the season. The second kind of aerial interaction was “pair The wing-raising display appearsto be used to intim- chase.” Usually, but not always, this began when a idate an opponent or predator. When incubating or pair was moving in a continuous horizontal flight. brooding, adult Black Swifts did not leave the nest, Sometimes this type of chase split off from a group and when I approachedthem closely, they reacted by chase, with two birds separatingfrom the group, and raising the wings. A similar intimidation display seems subsequentlythe birds would make a high-speed dive to be performed by adults in many swift species,e.g., and follow each other closely in erratic flight maneu- Chaeturu SDD. (Fischer 1958. Sick 1959. Snow 1962. vers. During the dive, the birds emitted a high-pitched Thompson1977) Cypseloidei and Strepioprocne spp. call that could be described as a rolling twitter. This (Marfn and Stiles 1992), and Common Swift Apus type of chase was observed most often-from the be- apus (Cramp 1985). In contrastto other swift species, ginning to the middle of the breeding seasonand may in cypseloidinesthis display does not involve soundor function in pair bond establishmentand reinforcement. vocalizations.Although the threat display is most often The third type of aerial interaction was “pair con- observed in adult swifts, this behavior can be per- tact,” which also involved two birds. Usually one bird formed by older nestlings as well. Black Swift nest- 516 SHORT COMMUNICATIONS

FIGURE 1. An interspecific aerial interaction at moment of separationbetween Black Swift (larger bird) and Violet-green Swallow. lings from age 18 days onward reactedto my presence Collins 1968, Oniki et al. 1992; White-throated Swift by raising the wings (Fig. 2). This behavior also has Aeronaures saxarulis, Marin unpubl. data; and Lesser been reported by MarIn and Stiles (1992) for older Swallow-tailed Swift Punyprilu cayennensis. Hav- nestlings of two Cypseloides species, White-chinned erschmidt 1958). In the two-egg clutch species, such Swift C. cryptus and Spot-fronted Swift C. cherriei. as Streptoprocne spp., begging calls are soft (Mat% This display apparentlyoccurs in some nestlingsof the and Stiles 1992). In species with one-egg clutches, Common Swift (A. upus; Lack and Lack 1952). In begging is silent (Ma& and Stiles 1992; present New World swifts, wing-raising by nestlingshas been study). These observationsare consistentwith Harper’s reported only in specieswith one-egg clutches. (1986) prediction that begging intensity should de- In the Black Swift, only twice I observed the body- crease with a decrease in clutch size. However, Har- uplifting or upright threatsreported for nestlingsof the per’s model predicts little or no begging for broods of Spot-fronted Swift (Cypseloides cherriei) by Mar-m one chick. At least in the New World swifts, the lack and Stiles (1993). of sound productionis replacedby a very violent beg- ging response. This might be explained in terms of FOOD-BEGGING BEHAVIOR reproductive costs and benefits. Predator attraction Nestling Black Swifts begged silently for food when would be particularly costly in such specieswith long adults approached the nest; however, nestlings were incubation and nestling periods, because at least in aggressivetowards the parents. Nestlings uplifted the higher latitudes there would be no time to replace a outstretchedwings in the same manner as the hostile grown nestling. display. The few observations of adult Black Swifts All adults of cypseloidines with one-egg clutches feeding young differ as to whether the young vocalize. have uniformly dark plumage with distinctive facial Some authorsdo not report any nestling vocalizations, markings. In young cypseloidines,a conspicuousbare e.g., Hunter and Baldwin (1962) and Michael (1927). area is presenton the forehead for about I8 days after whereas others report some vocalizations, e.g., Smith hatching. At hatching, and for the first few days of (1928) and Murphy (1951). I did not hear nestlings life, this area is bright pinkish in coloration. In the vocalize, but they did raise the wings with violent Black Swift, the bare spot was present until white moves when fed. feathers emerged on the throat and lores to cover it In altricial birds, begging is associatedwith arrival with white-tipped feathers. Thus, at all ages there is of parents and is usually noisy. Among New World some light-colored or pale area around the face. swifts, begging is noisy and harsh in specieswith large What is the significanceof the bare spot and white clutches(e.g., Chaerura spp., Fischer 1958, Sick 1959, feathering on the facial area? Once the bare spot is SHORT COMMUNICATIONS 517

FIGURE 2. Wing-raising by young Black Swift at 27 days of age used as threat display when approachedat nest. This display was given from age 18 days onward. A similar threat was used toward adults during food delivery.

covered, the white feathering that replaces it becomes ROOSTING conspicuous,much more so than in the adults (Fig. 3). On eight occasions,I recorded the roosting habits of This suggeststhat both the initial bare spot and the the Black Swift in the cave. During incubation,usually subsequentwhite feathering around the face may act one bird roosted on the nest, and its mate sat or as a signal for food transfer to the nestling, because perchedto one side with hunchedbody and sometimes these areas would reflect light in the dim nesting en- with ruffled feathers.When young nestlings were pres- vironment. The pale areas might serve two purposes: ent, both adults were observed roosting on the nest. to direct the adultsto the nestling’s mouth and to direct As the nestling became older (> 20 days), only one the adult to the nest, especially when arriving late at adult roosted occasionallyon the nest, and the second night. The nestlingswere very active when the parents bird roosted on an adjacent wall (also see Murphy arrived. Thus, a pale patch also may elicit a behavioral 1951). During the latter part of the nestling stage, both responseto stimulate food transfer,which involves re- adultsroosted together on the cave walls with hunched gurgitationof the bolus. Some supportingevidence for bodies and sometimes with ruffled feathers. When this hypothesis is: (I) the pale spots stand out in the roostingon the walls, the birds clung to a small ledge, dimly illuminated nesting site, (2) from about ?4of the with their feet placed at the level of the upper breast nestling period onward, the chicks were fed late at with wings usually hanging loosely. Sometimes, the night, 21:OO-23:00 and later. and (3) feeding was in- bent tail-end, but not the tips of the rachises,was used frequent, so successfuldelivery of the bolus is critical for support by spreading it like a fan. This behavior (the food boluses are usually a single glue-like mass correlateswith rachisesthat protrude less in the Black of insects). Swift than in other cypseloidine swifts. The roosting 518 SHORT COMMUNICATIONS

FIGURE 3. Conspicuous white facial markings of older (> 25 days) nestlings that may aid adult to find nestling and as a target to deliver the food.

postureof C. niger is similar to other cypseloidinespe- I am grateful to R. Corado, L. Lyon, D. MacLean, ties, except the tips of the rachisesare evidently used J. Schmitt, C. Sumida, and W. Wehtje for their com- less for support (Mat% and Stiles 1992). panionshipand help in the field. This paper benefited greatly from commentsby J. M. Fitzsimons, L. E Kiff, OTHER BEHAVIORS W. D. Koenig, D. E Lane, J. V. Remsen, L. A. Whit- When a bird was incubating, it usually sat motionless tingham, and an anonymous reviewer. The Western when approached.Its first reaction was to droop its Foundation of Vertebrate Zoology provided partial wing over the side of the nest, covering the egg or funding for the field work and preparationcosts. AVI- recently hatched chick. This reaction usually occurred NET provided some of the necessaryfield equipment. before wing-raising. On many occasions during late incubation and early nestling period, the birds sat so LITERATURE CITED quietly while covering the nest that when approached they did not move but instead pecked at my fingers AMERICAN ORNITHOLOGISTS ’ UNION. 1983. Check-list and raised the wings. When the young were a few days of North American birds, 6th ed. American Or- old, the adultsdeparted from the nest when I was with- nithologists’ Union, Washington,DC. in I m or so of the nest. BARBOUR, M. G., AND J. MAJOR [EDS.]. 1977. Terres- Several times during incubation and the early part trial vegetationof California. John Wiley & Sons, of brooding, I observed one adult giving food to an- New York. other. Only two prior accounts of food transfer be- COLLINS,C. T 1968. The comparativebiology of two tween adults exist (Michael 1927, Murphy 1951). species of swifts in Trinidad, West Indies. Bull. When I observed food transfer,the pair usually rotated Florida State Mus. I 1:257-230. incubationor brooding chores, and so perhapsthis be- CRAMP, S. [ED.]. 1985. The birds of the Western Pa- havior reinforces the pair bond. leartic. Vol. IV. Oxford Univ. Press, Oxford. SHORT COMMUNICATIONS 519

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