Long Call Displays of Sympatric South Polar and Brown Skuas

Abstract

Several aspects of the skua display were examined to determine their species specificity. Acoustical parameters were examined for 70 calls of 24 South Polar Skuas (Catharacta maccormicki) and 61 calls of 17 Brown Skuas (C. lonnbergi) at Palmer Station, Antarctica. I found no significant species differences in composition (notes/call), duration, or note duration. Despite large range overlaps, the species differed significantly in four parameters. On average, South Polar Skua notes had a faster repetition rate, lower pitch, lower average frequency, and more harmonics than those of Brown Skuas. Differences among individuals were apparent for all acoustic parameters, but again were most marked for note repetition rate, pitch, average frequency, and number of harmonics. Film analysis of 109 calls made by over 30 South Polar Skuas and of 44 calls made by 12 Brown Skuas revealed that South Polar Skuas tended to lean farther back at some point in the display. About 25% of the birds, however, gave displays that some previous researchers considered atypical for these species, and more than 36% of the birds showed intra-individual variation in display postures. Variation limits the value of these visual and acoustic aspects of the display for individual and species recognition of skuas at Palmer. The value of these parameters for answering evolutionary and taxonomic questions about skuas, however, has yet to be assessed. Skuas (Stercorariidae) have been the subject of taxonomic controversy at both the generic and specific levels for well over a century (Pietz 1984). I follow Brooke (1978) in separating skuas (Catharacta) from jaegers (Stercorarius) on the basis of derived juvenal plumage characteristics. Within the genus Catharacta, the taxonomic picture is more complicated; morphological features do not provide clear bases for making some taxonomic decisions. Currently, at least five groups are recognized as either species or sub-species. These include the Great Skua (C. skua) of the North Atlantic, the Chilean and Falkland skuas (chilensis and antarctica, respectively) of southern South America, the Brown Skua (lonnbergi) of the sub-Antarctic, and the South Polar Skua (maccormicki) of the Antarctic. Based on morphological characteristics, some authorities lump the Falkland, Brown, and Great skuas and assign full species status to both the Chilean and South Polar skuas. Hybridization has been documented, however, between Chilean and Falkland skuas (Devillers 1978), and between Brown and South Polar skuas (Watson et al. 1971, Parmelee et al. 1977) in their respective zones of sympatry. Pending resolution of their taxonomy, I will refer to them all as separate species. Overlaps in size and plumage characteristics among geographically distant populations of these groups have been recognized (Hamilton 1934, Murphy 1936). As a result, there is interest in using other traits for species identification, and for evaluating evolutionary and taxonomic relationships in this genus. Some biologists (e.g., Watson 1975, Devillers 1978, Jouventin and Guillotin 1979) have suggested that acoustic and visual aspects of the skua's long call display (Moynihan 1955) may help to distinguish species. The display, as described by num rous authors (e.g., Stonehouse 1956, Perdeck 1960, Moynihan 1962, Burton 1968, Spellerberg 1971, Devillers 1978), is by far the most visible and audible display of those in the skua's behavioral repertoire. It is also one of the most common, employed for territorial advertisement as well as for greeting mates. Moynihan (1955) considered this display to be homologous with the and oblique po tures of gulls. Visual components typically associated with the skua's include ing raising (Perdeck 1960) and the bent n ck (Moynihan 1962), as well as the oblique. The display resulting from the integration of th se components is sometimes referred to as he long complex, and several variations have been noted in the sequencing of components (Perdeck 1960, Spellerberg 1971). With respect to its multiple functions, the skua's is analogous to that of many