An analysis of vocal communication in young brown noddies (Anous stolidus)

-Nestlings and young fledglings of the Brown Noddy (Anous stolidus) produce three structurally different vocal signals. The postures of the chicks and the contexts in which these signals are produced differ for each call. Nestlings produce all three calls within one day after hatching. One is given during pipping, when the chick is moving on or near the nest, or when it is isolated from the nest. The second is given by begging chicks. The third is given when an intruder approaches. The repertoire is composed of frequency-modulated tonal elements and broad-band bursts of sound. Although changes occur in the temporal and frequency patterns of the calls during the nestling and fledgling stages, there is little resemblance to the adult repertoire during these periods. Received 4 January 1985, accepted 14 November 1985. ONE of the difficulties encountered when studying vocalizations of young birds is distinguishing between differences in the calls associated with different contexts and changes related to maturation. Despite work on the development of song in passerine birds, little is known of the ontogeny of general vocal repertoires for nonpasserines, and the development of vocal communication of only a few has been studied: domestic fowl (Gallus gallus; Collias and Joos 1953), the American Coot (Fulica americana; Cosens 1981), the European Quail (Coturnix coturnix; Schleidt and Shalter 1975), and the Aldabra White-throated Rail (Dryolimnas cuvieri aldabranus; Wilkinson and Huxley 1978). Although the Laridae have been a favored group for the study of communication in adult birds, less is known of the ontogeny of their vocal communication. Moynihan (1959b) examined the function of hostile behavior in captive young Franklin's Gulls (Larus pipixcan) and Ring-billed Gulls (L. delawarensis) and offered a scheme for the development of the adult calls. Davies and Carrick (1962) noted changes in the vocalizations of very young Great Crested Terns (Sterna bergii) prior to the onset of chick recognition by adults. Impekoven (1971) examined motivational control of vocal communication in young Common Black-headed Gulls (L. ridibundus). Other workers, such as Cullen (1957) in Black-legged Kittiwakes (Rissa tridactyla) and Cullen and Ashmole (1963) in Lesser Noddies (Anous tenuirostris), noted the signaling behavior of the young in more general studies of the breeding biology of a species. The importance of the context in which a signal is used was stressed by Smith (1977) in his analysis of messages and meanings. The "message" is information encoded in the signal that is descriptive of the signaler and independent of the context in which it is given. It can be determined by examining the features common to all contexts in which it is used. The "meaning" to the receiver is based on the specific details of the context, such as location, identity, sex, and age class of the signaler, other individuals present, prior experience, concurrent activities, and season. Meaning can be interpreted from the response of the recipient to the signal. Although its relationship among larids remains uncertain, the Brown Noddy (Anous stolidus) has been considered to be the tern most closely related to the gulls based on the observed behavior of this species (Moynihan 1959a). Detailed analyses of the form and usage of the vocal signals of both adults and young will increase our understanding of the evolution of communication behavior in this family. STUDY AREA AND METHODS Observations and recordings were made in a colony of approximately 15,000 adult Brown Noddies on Bush Key, Dry Tortugas, Florida during four breeding seasons from 1979 through 1982. In a selected area of the colony, I securely marked active nests, color-banded adults at these nests, and recorded hatching dates of nestlings and individually banded them. I observed the behavior of known-age nestlings throughout the nestling and early fledgling periods, concentrating on the vocal and visual sig351 The Auk 103: 351-358. April 1986 This content downloaded from 157.55.39.35 on Thu, 01 Sep 2016 05:46:15 UTC All use subject to http://about.jstor.org/terms 352 DIANE E. RisKA [Auk, Vol. 103 TABLE 1. Numbers of individuals and calls analyzed for each age group. No. No. of calls of indianalyzed Age viduals per indiCall (days) analyzed vidual Total Harsh Cheep 1-10 5 5 25 11-20 6 5 30 21-30 5 5 25 44-62 2 5 10 Screech 1-10 9 3 27 11-20 10 3 30 31-40 9 3 27 49-61 4 3 12 Frequency1-10 6 5 30 modulated 11-20 5 5 25 Cheep 21-30 5 5 25 44-62 5 5 25 nals used by them during this period of development. I recorded vocalizations with a Uher Report L5000 tape recorder at 19 cm/s with a Sennheiser ME88 directional condenser microphone. Video recordings were made with a Sony (AVC-3450) black-and-white portable camera and a JVC (CR-4400U) portable cassette recorder, with the Sennheiser microphone. The microphone was placed at one nest or two adjacent nests to allow me to hear the low-amplitude vocalizations of the young. I recorded contextual information and details of the nature of the behavior, along with information about individual identity and age, and examined features common to the contexts in which each call was given. I made spectrograms of recorded calls on a Kay Elemetrics Corporation Sona-Graph 6061B, using both a 45-Hz (narrow) and a 300-Hz (wide) band-pass filter, and digitized these on a Tektronix 4081 Graphic System tablet (Tektronix, Inc. 1978) with a density of 1,550 cross-wires/cm2. The advantages of this technique over the traditional grid overlay are that the grid of the digitizing tablet is finer, and the computer reads and records the x-y coordinates directly. I measured the following parameters using spectrograms produced with the narrow filter for frequency measurements and with the wide filter for time measurements: frequency range; frequencies of most intense sound energy band at onset, peak, midpoint, and offset; note length (measured from onset to offset); internote interval (measured from offset of one call to onset of next by the same individual). Low-amplitude calls of nestlings were difficult to separate from the loud background noise of the colony during all stages of the nestling period. Therefore, I selected recordings of known individuals of known age (110 days, 11-20, 21-30, 31-40, 44-62, 49-61) with low background noise for the analysis. Numbers of inA B