The breeding biology of the gadfly petrels Pterodroma spp. of the Pitcairn Islands: characteristics, population sizes and controls

This paper reports the breeding biology and nesting seasons of the gadfly petrels which nest on the four islands of the Pitcairn group, Pitcairn, Henderson, Oeno and Ducie. The species currently breeding are Murphy's petrel Pterodroma ultima , Kermadec petrel P. neglecta , Herald petrel P. heraldica and Henderson petrel P. atrata. Of these, Murphy's petrel is the most numerous; an estimated 250000 pairs bred on Ducie, which is probably the major breeding station of the species. Novel basic breeding data for Murphy's petrel are presented. Incubation spells, averaging 19.3 days, are exceptionally long for a petrel. Phoenix petrel P. alba appears to have ceased to breed on the Pitcairn Islands since the 1922 surveys of the Whitney Expedition. Nesting success was low on Henderson Island during the study. For all four breeding species, less than 20% of eggs laid yielded fledglings. Failure occurred at the early chick stage and observations indicated that it was due to predation by Pacific rats Rattus exulans. Although rats are present on Ducie, predation was apparently less severe there. The situation on Oeno may be intermediate. I consider how the populations of Henderson are maintained in the face of this intense predation. The Murphy's petrel population may be sustained by immigration from Ducie while the Herald and Henderson petrel populations could be undergoing a long-term decline on Henderson. It is not clear how the Kermadec petrel population is maintained. The conservation implications of these findings are discussed.     

Phylogenetic relationships in Pterodroma petrels are obscured by recent secondary contact and hybridization

The classification of petrels (Pterodroma spp.) from Round Island, near Mauritius in the Indian Ocean, has confounded researchers since their discovery in 1948. In this study we investigate the relationships between Round Island petrels and their closest relatives using evidence from mitochondrial DNA sequence data and ectoparasites. Far from providing clear delimitation of species boundaries, our results reveal that hybridization among species on Round Island has led to genetic leakage between populations from different ocean basins. The most common species on the island, Pterodroma arminjoniana, appears to be hybridizing with two rarer species (P. heraldica and P. neglecta), subverting the reproductive isolation of all three and allowing gene flow. P. heraldica and P. neglecta breed sympatrically in the Pacific Ocean, where P. arminjoniana is absent, but no record of hybridization between these two exists and they remain phenotypically distinct. The breakdown of species boundaries in Round Island petrels followed environmental change (deforestation and changes in species composition due to hunting) within their overlapping ranges. Such multi-species interactions have implications not only for conservation, but also for our understanding of the processes of evolutionary diversification and speciation.     

Influence of reproductive success on breeding frequency in four southern petrels

Patterns of breeding frequency of White-headed Pterodroma lessoni, Great-winged Pterodroma macroptera, White-chinned Procellaria aequinoctialis and Grey Petrels Proceharia cinerea have been studied at Kerguelen and Crozet Islands. The White-headed Petrel (summer breeder) followed an essentially biennial cycle with only 13% of successful individuals breeding again the next year. On the other hand, the Great-winged Petrel (winter breeder) was mainly an annual breeder with 78% of successful birds breeding again the next year. Both White-chinned (summer breeder) and Grey Petrels (winter breeder) showed a higher breeding frequency than Pterodroma species, with 91% and 93%, respectively, of successful birds returning to breed the following season. These contrasting patterns may be related to the timing of energetic constraints encountered during the interbreeding period, principally the moult and the restoration of breeding condition. It is suggested that these post-breeding constraints are more marked in winter and in oceanic waters, leading to breeding frequencies being lower than in summer and in neritic waters. The intermediate patterns of breeding frequency found in the Great-winged Petrel (mainly annual) and the White-headed Petrel (mainly biennial) show that among Procellariiformes there is a continuum between annual and strictly biennial species.     

Breeding of the Trindade Petrel Pterodroma arminjoniana on Round Island, Mauritius

Round Island (19°51'S, 57°47'E), an uninhabited volcanic islet lying 14 miles NNE of the Mauritian mainland, provides locally important breeding grounds for four species of seabirds; the Trindade Petrel Pterodroma arminjoniana , the Wedge-tailed Shearwater Puffinus pacificus , the Red-tailed Tropic Bird Phaethon rubricauda and the White-tailed Tropic Bird Phaethon lepturus. The landing conditions on the island are notoriously difficult and prior to 1975, visits by naturalists had been limited to short stays, a few days at the most. Accounts of the natural history of the island have been made by Vinson (1965), Bullock (1977) and Bullock & North (1984). This paper gives the results of studies on the breeding biology of the Trindade Petrel during two visits to Round Island from 27 July to 2 September 1978, and from 30 July to 18 August 1982.     

Breeding ecology of the White-headed Petrel Pterodroma lessoni on the Kerguelen Islands

The breeding season of the White-headed Petrel Pterodroma lessoni on Mayes Island off the Kerguelen archipelago was studied between December 1985 and February 1987. The birds returned to land in November, and the chicks hatched at the end of January and fledged in May. These incubation and fledging periods are among the longest known in the family Procellariidae. Successful birds normally bred every two years, which has not previously been reported for any member of this family. The breeding ecology is compared to that of other gadfly petrels from sub-antarctic and tropical waters, and the breeding frequency is discussed.     

Year‐round spatial movements and trophic ecology of Trindade Petrels (Pterodroma arminjoniana)

Trindade Petrels (Pterodroma arminjoniana) are vulnerable gadfly petrels that breed on the remote Trindade Island, located ~1100 km off the Brazilian coast. Little is known about their spatial ecology, and their trophic ecology has only been described for the breeding season. We tagged four Trindade Petrels with global location sensing loggers (GLS) from October 2013 to November 2014 and sampled the blood and feathers (innermost primary and the eighth secondary) of 14 individuals to evaluate their year‐round spatial and isotopic ecology. We examined individual distributions, habitat use and suitability, activity, and isotopic values during the breeding, migration, and non‐breeding periods. Trindade Petrels used areas in the southwest Atlantic Ocean (between 10°N and 50°S in latitude) during the breeding season. They migrated through pelagic waters of the tropical Atlantic to the northwest Atlantic, where they spent the non‐breeding season. Trindade Petrels used mostly tropical to subtropical waters in areas of intermediate to high wind speeds and low marine productivity. Individuals spent more time foraging at night than during the day. During the breeding season, birds in northerly areas had higher carbon‐13 values, and birds that used more pelagic areas foraged on prey at a higher trophic level (higher nitrogen‐15 values) than those in more southern and coastal areas. Isotopic values during the breeding, migration, and non‐breeding periods differed, possibly due to differences among individuals in their at‐sea distribution throughout the year. We confirmed the non‐breeding distribution of Trindade Petrels, which was previously known only from vessel sightings and stranded birds. Our results also suggest a strong temporal segregation in the at‐sea distribution and trophic ecology between two groups of individuals, which might indicate the existence of two separate breeding populations.     

Species‐specific foraging strategies and segregation mechanisms of sympatric Antarctic fulmarine petrels throughout the annual cycle

Determining the year‐round distribution and behaviour of birds is necessary for a better understanding of their ecology and foraging strategies. Petrels form an important component of the high‐latitude seabird assemblages in terms of species and individuals. The distribution and foraging ecology of three sympatric fulmarine petrels (Southern Fulmar Fulmarus glacialoides, Cape Petrel Daption capense and Snow Petrel Pagodroma nivea) were studied at Adélie Land, East Antarctica, by combining information from miniaturized saltwater immersion geolocators and stable isotopes from feathers. During the breeding season at a large spatial scale (c. 200 km), the three species overlapped in their foraging areas located in the vicinity of the colonies but were segregated by their diet and trophic level, as indicated by the different chick δ¹⁵N values that increased in the order Cape Petrel < Southern Fulmar < Snow Petrel. During the non‐breeding season, the three fulmarines showed species‐specific migration strategies along a wide latitudinal gradient. Snow Petrels largely remained in ice‐associated Antarctic waters, Southern Fulmars targeted primarily the sub‐Antarctic zone and Cape Petrels migrated further north. Overall, birds spent less time in flight during the non‐breeding period than during the breeding season, with the highest percentage of time spent sitting on the water occurring during the breeding season and at the beginning of the non‐breeding period before migration. This activity pattern, together with the δ¹³C values of most feathers, strongly suggests that moult of the three fulmarine petrels occurred at that time in the very productive high Antarctic waters, where birds fed on a combination of crustaceans and fish. The study highlights different segregating mechanisms that allow the coexistence of closely related species, specifically, prey partitioning during the breeding season and spatial segregation at sea during the non‐breeding season.     

MORPHOMETRICS AND BREEDING BIOLOGY OF THE WHITECHINNED PETREL PROCELLARIA AEQUINOCTIALIS AT SUB-ANTARCTIC MARION ISLAND

Berruti, A., Cooper, J. & Newton, I.P. 1995. Morphometrics and breeding biology of the Whitechinned Petrel Procellaria aequinoctialis at sub-Antarctic Marion Island. Ostrich 66: 74–80.

Aspects of adult morphometrics and the breeding biology of the summer-breeding Whitechinned Petrel at sub-Antarctic Marion Island, southern Indian Ocean, are given, based on a study conducted in 1980/91, along with additional observations on breeding success made in 1990191. It is concluded that Whitechinned Petrels at Marion Island are similarly sized and breed in a similar manner to other studied populations of the nominate race. The eradication of cats at Marion Island in 1991 should now lead to a slow population recovery.     

Systematics of the Soft-plumaged Petrel Pterodroma mollis (Procellariidae): new insight from the study of vocalizations

The taxonomic status of the populations of the polytypic Soft-plumaged Petrel Pterodroma mollis is controversial. Previous taxonomic studies used morphological characters (measurements and colouration) and univariate statistics. In this study, I add the use of multivariate statistics on morphological (biometry and colouration) and behavioural characters (vocalizations) of the petrels. Vocalizations are especially relevant in nocturnal burrowing petrels (such as the Soft-plumaged Petrel) and act as behavioural isolating mechanisms. The analysis, based on 535 calls from seven of the 11 known breeding localities of the species, showed that physical parameters of the call did not overlap at all between breeding localities in the northern and southern hemispheres but did so within each hemisphere. Conversely, the morphological analysis, based on morphometries from ten localities, displayed wide and significant intra-hemisphere variation, whereas there was no consistent difference between northern and southern parts of the breeding range. Thus analysis of the two sets of characters gave contradictory results. Calls, owing to their role in species-specific recognition in these birds, should be given priority over other characters for species separation. I suggest that only two species should be recognized, Pterodroma mollis and Pterodroma feae, with three subspecies each. I also examined behavioural affinities between P. mollislfeae groups and other gadfly petrels and found that P. mollis/feae were closest to Pterodroma cahow, whilst Pterodroma hasitata on the one hand and Pterodroma phaeopygia and Pterodroma externa (which may comprise two species instead of one) on the other belong to other groups of gadfly petrels.     

Migratory movements and wintering areas of Leach's Storm‐Petrels tracked using geolocators

Accumulating evidence suggests that Atlantic populations of Leach's Storm‐Petrels (Oceanodroma leucorhoa) are experiencing significant declines. To better understand possible causes of these declines, we used geolocators to document movements of these small (～50‐g) pelagic seabirds during migration and the non‐breeding period. During 2012 and 2013, movement tracks were obtained from two birds that traveled in a clock‐wise direction from two breeding colonies in eastern Canada (Bon Portage Island, Nova Scotia, and Gull Island, Newfoundland) to winter in tropical waters. The bird from Bon Portage Island started its migration towards Cape Verde in October, arrived at its wintering area off the coast of eastern Brazil in January, and started migration back to Nova Scotia in April. The bird from Gull Island staged off Newfoundland in November and then again off Cape Verde in January before its geolocator stopped working. Movements of Leach's Storm‐Petrels in our study and those of several other procellariiforms during the non‐breeding period are likely facilitated by the prevailing easterly trade winds and the Antilles and Gulf Stream currents. Although staging and wintering areas used by Leach's Storm‐Petrels in our study were characterized by low productivity, the West Africa and northeastern Brazilian waters are actively used by fisheries and discards can attract Leach's Storm‐Petrels. Our results provide an initial step towards understanding movements of Leach's Storm‐Petrels during the non‐breeding period, but further tracking is required to confirm generality of their migratory routes, staging areas, and wintering ranges.     

Response of breeding European Storm Petrels Hydrobates pelagicus to habitat change

Mainly through trampling and manuring, ground-nesting seabirds induced significant habitat changes both on vegetation cover and soil in one of the largest French colonies of European Storm Petrel Hydrobates pelagicus, Habitat deterioration led to a high level of erosion and the collapse of many former Rabbit Oryctolagus cuniculus burrows previously occupied by breeding Storm Petrels. The loss of burrows accelerated in recent years since Great Cormorants Phalacrocorax carbo bred on the islet with growing numbers. The main consequence of this disturbance was at first shifting of breeding Storm Petrels from burrows to rocky sites, reflecting some behavioral plasticity to buffer environmental variability. But over 18 years, a significant decrease in breeding numbers of Storm Petrels was recorded and attributed to continuous nest site destruction. Thus, other behavioral responses were also suspected, such as temporary non-breeding or emigration of birds that have to find a new nest site. Such a problem of heavy erosion and loss of nesting habitat could induce serious detrimental effects on burrowing seabirds breeding in a limited number of colonies.     

Predation risk and moonlight avoidance in nocturnal seabirds

Unlike most seabird families, the vast majority of small petrel species are nocturnal on their breeding grounds. Further, they reduce markedly their activity when the light level increases. Moonlight avoidance might be a consequence of reduction in foraging profitability, as bioluminescent prey do not come to the sea surface on bright nights. Alternatively, petrels may avoid colonies during moonlit nights because of increased predation risk. We studied predation on petrels by Brown Skuas Catharacta antarctica lönnbergi at Kerguelen, and the influence of moonlight on behaviour of both skuas and petrels, to test the 'predation risk' hypothesis. On the study area, Brown Skuas hunt at night and prey heavily upon the Blue Petrel Halobaena caerulea and the Thin-billed Prion Pachyptila belcheri . Predation risk was higher on moonlit nights, as skuas caught more prey, and particularly more Blue Petrels when the light level increased. Nightly intakes of Blue Petrel and Thin-billed Prion by skuas was related to colony attendance of non-breeders rather than that of breeders. Biometry of prey also suggested that skuas caught a higher proportion of non-breeding birds than was present at the colonies. Predation risk was thus greater in non-breeders and on moonlit nights. Colony attendance by non-breeding Blue Petrels and Thin-billed Prions was also reduced during moonlit nights. Vocal activity, which is mainly by non-breeders, was also drastically reduced when the light level increased in the species suffering the highest predation rate. Our results supported the 'predation risk' hypothesis, although the 'foraging efficiency' and the 'predation risk' hypotheses are not mutually exclusive: the former might explain the moonlight avoidance behaviour of breeding, and the latter that of non-breeding individuals.     

Seabird associations in Pacific equatorial waters

Seabirds were counted along the same cruise tracks in the eastern tropical Pacific (10°øN-10°øS and 140°ø-90°øW) each spring and autumn for 8 years, 1984–1991. During spring, three species groups were apparent among eight species seen in five or more years: (1) Leach's Storm-petrel Oceanodroma leucorhoa and Wedge-rumped Storm-petrel Oceanodroma tethys , which feed on neuston by surface seizing; (2) Black-winged Petrel Pterodroma nigripennis and White-winged Petrel Pterodroma leucoptera , which feed on neuston and micronekton by surface seizing; (3) Juan Fernandez Petrel Pterodroma externa , Wedge-tailed Shearwater Puffinus pacificus and Sooty Tern Sterna fuscata , which feed on micronekton driven to the surface by foraging tuna. During autumn, only one group was identified from the seven species seen in five or more years: Leach's Storm-petrel associated with either Wedge-rumped Storm-petrel or Juan Fernandez Petrel and/or White-winged Petrel. Annual variability in assemblage composition was more extreme in the autumn and thus was similar to variation in physical and biological factors linked to the El Niño-Southern Oscillation cycle.     

Hardly hard-hearted: heart rate responses of incubating Northern Giant Petrels (Macronectes halli) to human disturbance on sub-Antarctic Marion Island

Guidelines for visitors to sub-Antarctic Marion Island recommend 15 and 100 m minimum approach distances for breeding Northern (Macronectes halli) and Southern (Macronectes giganteus) Giant Petrels, respectively. Using artificial eggs containing FM transmitters, we measured the heart rate responses of incubating Northern Giant Petrels to pedestrian approaches. The mean resting heart rate was 80 beats per minute. Heart rates increased upon the detection of a person approximately 40 m away, and continued to increase during the approach to 5 m. Maximum increases over resting heart rate in response to natural disturbances and human approach were 97 and 204%, respectively. Northern Giant Petrels appear at least as sensitive to human disturbance as their congenerics. While low-key disturbance is unlikely to affect this solitary breeder as severely as it would the colonial Southern Giant Petrel, improved protection from disturbance could be achieved by restricting human passage through breeding colonies of Northern Giant Petrels to defined paths.     

Lifetime reproductive success in common buzzard, Buteo buteo: from individual variation to population demography

We studied annual and lifetime reproductive success (LRS) of both sexes of common buzzard Buteo buteo in eastern Westphalia, Germany. We followed a bottom‐up approach starting from individual breeding attempts, over lifetime reproductive success to derive population demography. Annual breeding performance and survival followed a quadratic relationship with breeding experience; individuals starting their breeding career were less likely to survive and breed successfully than birds of intermediate breeding experience. According to an analysis of selection gradients, both the opportunity and intensity of selection peaked in the early stages of the breeding career. The distribution of both LRS and another fitness measure, λ, was highly skewed, with ca 17% of adult birds producing 50% of fledglings in both sexes. Besides breeding life span and number of breeding attempts, habitat quality and plumage morph were significant predictors of LRS. There were strong differences in LRS and λ between the plumage morphs in both sexes: intermediate pigmented buzzards were much more successful than either dark or light ones. There was no significant difference between buzzard cohorts either in LRS or λ, nor did these fitness measures differ between individuals starting their breeding career at different conditions of food availability. Based on individual life histories, we formed a transition matrix and analysed its properties to study the population as a whole. This analysis showed that the population growth rate was close to unity (0.906, bootstrapped 95% confidence limits: 0.834 and 0.962). Analysis of reproductive values and elasticities further emphasised colour morph differences: the contribution of intermediate individuals to population growth greatly exceeded that of dark or light individuals. Thus most phenomena on all levels from individual breeding attempts over lifetime reproductive success to population demography can be explained by the fitness differences between the colour morphs with the intermediate morph maintaining the current population renewal potential.     

Geographical variation in the White-tailed Tropicbird Phaethon lepturus, with the description of a new subspecies endemic to Europa Island, southern Mozambique Channel

Geographical variation in measurements and colouration among populations and subspecies of White-tailed Tropicbirds Phaethon lepturus , including birds from Europa Island (southern Mozambique Channel] was examined worldwide. Two groups were distinguished: the 'large subspecies' (lepturus and fulvus from the Indian Ocean, catesbyi from the western Atlantic Ocean) and the 'small subspecies' (ascensionis from the central and eastern Atlantic Ocean, dorotheae from the Pacific Ocean, and the birds from Europa Island). No clinal variation was found in the Indian Ocean, the birds from Europa Island being the only 'small' ones. This population also had a high frequency of golden morphs, a feature that does not exist elsewhere in the western Indian Ocean. These results indicate that Europa's population is isolated from all nearby colonies in the Indian Ocean, and does not belong to any of the two previously known subspecies of the area. It also differs from the birds of the two small subspecies by the frequency of the colour morphs and the distribution. Consequently, we propose to treat this population as a previously undescribed subspecies, endemic to Europa Island, for which we propose the name Phaethon lepturus europae. Geographical isolation of Europa Island and oceanic conditions in the Mozambique Channel are discussed to explain the isolation of this population.     

The impacts of introduced House Mice on the breeding success of nesting seabirds on Gough Island

Invasive species are the main threat to island biodiversity; seabirds are particularly vulnerable and are one of the most threatened groups of birds. Gough Island, a UNESCO World Heritage Site in the South Atlantic Ocean, is an Important Bird and Biodiversity Area, and one of the most important seabird colonies globally. Invasive House Mice Mus musculus depredate eggs and chicks of most seabird species on the island, but the extent of their impact has not been quantified. We used field data and bootstrapped normal distributions to estimate breeding success and the number of surviving chicks for 10 seabird species on Gough Island, and compared estimates with those of analogous species from predator‐free islands. We examined the effects of season and nest‐site location on the breeding success of populations on Gough Island, predicting that the breeding success of Gough birds would be lower than that of analogues, particularly among small burrow‐nesting species. We also predicted that winter‐breeding species would exhibit lower breeding success than summer‐breeding species, because mice have fewer alternative food sources in winter; and below‐ground nesters would have lower breeding success than surface nesters, as below‐ground species are smaller so their chicks are easier prey for mice. We did indeed find that seabirds on Gough Island had low breeding success compared with analogues, losing an estimated 1 739 000 (1 467 000–2 116 000) eggs/chicks annually. Seven of the 10 focal species on Gough Island had particularly high chick mortality and may have been subject to intense mouse predation. Below‐ground and winter breeders had lower breeding success than surface‐ and summer‐breeders. MacGillivray's Prion Pachyptila macgillivrayi, Atlantic Petrel Pterodroma incerta and Tristan Albatross Diomedea dabbenena are endemic or near‐endemic to Gough Island and are likely to be driven to extinction if invasive mice are not removed.     

Spatio-temporal overlap between the at-sea distribution of Southern Giant Petrels and fisheries at the Patagonian Shelf

The interactions between seabirds and fisheries pose significant threats for the seabird species such as incidental capture. In contrast, several species of seabirds meet part of their energetic requirements through the use of fisheries discards. Knowledge about the relationship between at-sea distribution of Procellariiformes and fisheries is a key tool in marine ecosystem management. We analysed the spatio-temporal relationship between the areas used by 16 satellite-tracked breeding adults of the Southern Giant Petrel and fisheries distribution and catch at the Patagonian Shelf. We also determined the time spent by adults in different marine jurisdictions. Results indicated a marked spatio-temporal association between birds and fisheries, mainly trawlers. The Southern Giant Petrels concentrated their foraging effort over Argentinean waters. The use of an abundant and predictable food source provided by the fisheries discards may be one of the factors affecting the dynamics of the Southern Giant Petrel populations in Patagonia, Argentina.     

Origins, phytogeny and taxonomy of the gadfly petrels Pterodroma spp

Taxonomic characters of gadfly petrels (Procellariidae: Pterodroma spp.) are reviewed and the genus is redefined. The structure of the upper intestines, which have become helicoidally twisted to varying degrees in most species, is an important character not hitherto used. Results of a phylogenetic study of the genus based mainly on the development of helicoidal intestines agree substantially with findings from studies of the Mallophaga parasitizing these petrels. One species, the Kerguelen Petrel Lugensa brevirostris, previously classified in Pterodroma, is shown to have closer affinity with some fulmar genera. The Tahiti Petrel Pseudobulweria rostrata and its rare or extinct relatives, formerly placed in a superspecies of Pterodroma, are more closely related to the Procellaria and Bulweria petrels, and may be derived from the link between the subfamilies Procellariinae and Fulmarinae. From consideration of the fossil record, anatomical characters and Mallophaga, a phylogeny of the Procellariidae is proposed which supports its monophyletic origin. Gadfly petrels apparently descended from the fulmars through the ancestral lines of Snow Petrels Pagodroma, the Kerguelen Petrel and finally the Blue Petrel Halobaena, which may have given rise separately both to gadfly petrels and to the prions Pachyptila. A late Pliocene origin of Pterodroma in the vicinity of southern New Zealand is possible. The genus Pterodroma is divided into four subgenera, representing four connected radiations, and 29 species are recognised. In probable chronological order, Proaestrelata (new subgenus) contains five species and is restricted to the Pacific Ocean, Cookilaria comprises six species restricted as breeders to the South Pacific, Hallstroma has seven species mainly in the tropics of the Pacific, Indian and South Atlantic Oceans, whereas Pterodroma includes 11 species which extend as breeders from the southwest Pacific west to the South Atlantic and into the North Atlantic. Ecological, physical and physiological adaptations in the evolution of gadfly petrels are discussed.     

The vocal systems of two nocturnal burrowing petrels, the White-chinned Procellaria aequinoctialis and the Grey P. cinerea

I studied the calls of White-chinned Petrels Procellaria aequinoctialis and Grey Petrels P. cinerea at the Prince Edward Islands in the Southern Ocean. White-chinned Petrels gave two calls. One, the Wheezy call, was usually given from within breeding burrows, was given in response to playback of strange (= non-mate) calls, and probably served a burrow defence function. The other call, the Rattle call, was uttered roughly as often on the ground as from within the burrow. This call was mostly given by males and probably served as sexual advertisement. The one identified call type of the Grey Petrel was given in the same circumstances as the Wheezy call of the White-chinned Petrel.
In both species there was evidence of mate recognition of calls. Birds tended to remain silent to the playback of the mate's call, whereas they responded vocally to strange calls.
This vocal system, where both sexes share two call types but where sexual dimorphism in call is absent, has not been described before for petrels. Possible reasons for the variation in petrel vocal systems are discussed.