Grant’s Storm Petrel

Starting in the late 1980s, the young Portuguese scientist Luís Rocha Monteiro studied mercury pollution in the marine environment of the Azores. After a few years, he recruited the help of seabirds. From mercury concentrations in their plumage and regurgitations of food, he was able to infer the health of the rest of the ecosystem. Among the birds he studied were band-rumped storm petrels Oceanodroma, which had only recently been proven to breed in the Azores.

Monteiro soon discovered that there were two populations in the islands, one present in the hot season (March to September) and the other in the cool season (August to March). During the early autumn, adults of the cool-season population could often be found in the very same burrows where nestlings of the hot-season population were still growing up. Striking differences in their mercury levels led Monteiro to believe that the two populations might differ in their feeding ecology, and subsequent research showed that they also differed in morphology and egg dimensions. Hot-season birds were smaller in most body measurements, particularly weight, but had longer tails, and proportionally longer wings. A detailed paper was published, raising the exciting possibility that hot- and cool-season populations in the Azores might be cryptic species (Monteiro & Furness 1998). Tragically, Monteiro never had the opportunity to follow the story through. He died in a plane crash on the island of São Jorge in December 1999, when he was still in his late 30s.

Eight years after his death, the dust is still far from settling over Monteiro’s discovery, and the number of different types of band-rumped storm petrel in the North Atlantic has risen to four. They form a classic example of a group of birds whose diversity went unnoticed due to similarity in plumage, despite differences in measurements, the timing of their annual cycle, and DNA. For us it has been a real adventure obtaining sound recordings of each band-rumped and listening to vocal differences. These are so clear that they readily support any appeals for an increased level of conservation for each one. Collectively, they can be referred to as ‘band-rumped storm petrels’. Madeiran Storm Petrel O castro is the hot-season breeder of the Madeiran archipelago, Selvagens and the Canary Islands. Cape Verde Storm Petrel O jabejabe breeds only in the Cape Verde Islands. Monteiro’s Storm Petrel is the hot-season breeder of the Azores, named after its discoverer Luís Monteiro; a formal description of this new taxon will be published shortly (Bolton et al 2008). Grant’s Storm Petrel (not yet formally described) is the cool-season breeder of the Azores, Berlengas, Canary Islands, Madeiran archipelago and Selvagens. This name was suggested by Steve Howell in conversation, in memory of the late Peter Grant who died in 1990. Although I never met Peter, some of the birders who have inspired me most over the years had him as their mentor, and his example led Mark Constantine to found The Sound Approach. Grant was best known for his pioneering work on gull identification, but since we may have to wait a long time for a new gull to be discovered, we felt it was appropriate to suggest naming another exciting seabird in his honour.

present as breeding bird from August to March in the Azores, Berlengas, Canary Islands, Madeiran archipelago and Selvagens.

The first I read about Monteiro’s exciting discovery was when George Sangster wrote a paper about band-rumped storm petrels in Dutch Birding (Sangster 1999). He summarised everything then known about Western Palearctic populations, and indicated that at least one new cryptic species of storm petrel was awaiting formal description. The most interesting part for me was when George wrote that, although differences had been discovered in areas as diverse as breeding phenology, DNA, egg size measurements, and moult, nothing had yet been written about sounds.

One of the photographs accompanying George’s article had been taken by Manuela Nunes, a friend I had lost touch with. The realisation that Manuela was now working on band-rumped storm petrels encouraged me to contact her again. In September 2003, she kindly arranged for me to make sound recordings in the Berlengas, Portugal, where her boyfriend Carlos Santos was working as a ranger. Manuela put me in touch with the director of the park, António Teixeira who, together with Colm Moore, had discovered that band-rumped were breeding there (Teixeira & Moore 1983). Manuela was not able to come along, because she was busy setting up a huge project protecting marine areas for seabirds. Since I would be visiting a strictly protected part of the archipelago, where nobody is allowed to go alone, she found two volunteers to come with me, both of whom were called João.

Peniche, about an hour’s drive north of Lisbon, is the nearest port to the islands. From there, Carlos took us by zodiac to Berlenga, the main island, 13 km out to sea. The next morning we continued to the Farilhões, a few tiny islets 7 km to the northwest. Just 3 km beyond the main islet, the sea floor plummets to a depth of over 1000 m. Farilhão Grande is really just a big, steep rock. A dilapidated, dry stone path winds its way up to a small lighthouse on top, 94 m above sea level. The slopes that are a little less steep and eroded are covered in a thick mat of Ice Plant Carpobrotus edulis, a succulent from South Africa. In several countries with Mediterranean climates, this invasive species has become a serious problem, but Grant’s Storm Petrels love nesting underneath it. 

Our trip was timed to coincide with a late September new moon, shortly before the Grant’s Storm Petrels would lay their single egg. Manuela had assured me there would be a high level of vocal activity, and she was right. The first seabirds we heard after dark were Cory’s Shearwaters Calonectris borealis, but it was not long before the Grant’s took over the show. From about 45 minutes after sunset, their chatter calls could be heard frequently as they looped and fluttered over the steep slopes, while others purred quietly underground (CD2-40). 

If we compare sounds of Grant’s and Leach’s Storm Petrels, a clear difference can be heard between their chatter calls, often given in flight by both species. Leach’s chatter calls peter out with a bouncing ball effect, while Grant’s become louder towards the end. The last four notes of their chatter calls usually have a stereotypical rhythm, with notes that are l o n g -short, l o n g -short. Think of Grant’s’ breeding season, and you might hear a rhythm like AU-tumn, AU-tumn in the end of the call. This characteristic rhythm also distinguishes Grant’s from other band-rumped storm petrels breeding in the North Atlantic.

If we compare sounds of Grant’s and Leach’s Storm Petrels, a clear difference can be heard between their chatter calls, often given in flight by both species. Leach’s chatter calls peter out with a bouncing ball effect, while Grant’s become louder towards the end. The last four notes of their chatter calls usually have a stereotypical rhythm, with notes that are l o n g -short, l o n g -short. Think of Grant’s’ breeding season, and you might hear a rhythm like AU-tumn, AU-tumn in the end of the call. This characteristic rhythm also distinguishes Grant’s from other band-rumped storm petrels breeding in the North Atlantic.

Male and female differences in chatter calls have been proven for some populations of band-rumped storm petrels (James & Robertson 1985e, Smith & Friesen 2007), and we can assume that they apply to Grant’s Storm Petrel as well. As in Leach’s Storm Petrel, male and female chatter calls are similar in their general form, timing and emphasis of notes, but differ in timbre. In males, the notes are clear, whereas in females they are harsh, as is the case with many other petrel species. Strictly speaking, vocal sexing in Grant’s should be regarded as provisional, until it has been confirmed by other means. In CD2-41, you can hear chatter calls of males in flight. In CD2-42, most of the louder callers are females in flight, perhaps attracted to the purring of a male in a nearby burrow.

Like most of their closest Northern Hemisphere relatives, band-rumped storm petrels purr. However, their purring differs from British Hydrobates pelagicus, Mediterranean H melitensis and Leach’s Storm Petrels in the way it is constructed. In all these storm petrels, purring is interrupted at regular intervals by a more far-carrying sound. In British, Mediterranean and Leach’s this is a simple monosyllabic breath note, whereas the four band-rumped petrels have an exhaled flourish sounding like several notes from their chatter call. You can tell Grant’s from the other band-rumped by the l o n g -short rhythmic pattern of the flourish. Just as in chatter calls, the timbre tells you the sex. Try listening to the flourishes in CD2-43. They sound clear and not hoarse, so the caller can be presumed to be a male. 

In the recordings I made on Farilhão Grande, there was much variation in the purring of males, especially in the length of the phrases. I once recorded a Grant’s Storm Petrel phrase of 37 seconds, measured from flourish to flourish. The bird then carried on with much shorter phrases of just a few seconds. Lengths also vary between individuals. In CD2-44, one of the males has phrases twice as long as the other’s.

In CD2-45, you can hear a duet between a male and a female Grant’s Storm Petrel both purring in a burrow. The female is the caller more to the left, with the harsher flourishes, and the male is the slightly louder or closer caller to her right with a clearer voice. This male is somewhat less clear sounding than most, but still recognisably different from the female. As you can hear, they usually take turns to flourish. These two birds are clearly communicating with each other. 

James & Robertson (1985e) described a third call in the Selvagens, which they called the ‘high call’. Like the equivalent ‘screech call’ of Leach’s Storm Petrel, it is the same in both sexes, and tends to be heard during aggressive encounters. When I recorded the high calls in CD2-46, a heated situation had developed. The male in the middle had been giving purring calls from a stone dyke for some time. Two females in flight were attracted to his calls. One by one they landed and scurried into the dyke. Another bird inside was not at all pleased to receive them, and gave a long series of high calls. I suspect this was a female who had got there first, perhaps the mate of the purring male. The high calls became harsher towards the end, and eventually one of the birds bumped into a microphone. 

Farilhão Grande was estimated to hold some 200-400 breeding pairs in 1994-95, based on capture-recapture techniques. The laying season is spread over a couple of months, with most eggs probably laid in early October and hatching in November, though recently hatched chicks have been found from 6 October through to 28 January. On this latter date, the majority of chicks were close to fledging (Granadeiro et al 1998b). 

Grant’s Storm Petrels also breed in autumn and winter in the Azores, the Canary Islands, Madeira (including the Desertas) and the Selvagens. Genetic evidence supports the idea that cool-season populations from the Canary Islands northwards are essentially the same storm petrel (Smith et al 2007). In all these archipelagos, cool-season breeders have the same calls as the examples from the Berlengas presented here. Published recordings include chatter calls from the Canary Islands (Moreno 2000) and the Desertas (background of CD1-10 & CD1-11), both from late September, and sonagrams of diagnostic purring calls from the Azores in October (Bolton 2007). 

In the USA, band-rumped storm petrels are seen regularly in the Gulf of Mexico and the Gulf Stream from April until September. Some of these are presumably Grant’s Storm Petrels, based on the dates they are present as well as their primary moult and tail shape. To date the only concrete proof is a recovery of a bird ringed by Luís Monteiro on Praia islet, Graciosa, Azores, on 25 September 1993. It was captured by a fisherman south of Pensacola, Escambia County, Florida, USA, on 27 April 1998. He handed it in to a wildlife sanctuary, who arranged to release it into the Gulf of Mexico the next day, but unfortunately the bird died before it could be freed. It was identified as a male and preserved as a study skin (Woolfenden et al 2001). 

Haney (1985) found band-rumped storm petrels to be particularly attracted to cold eddies on the fringes of the Gulf Stream. In the centre of these eddies, colder water welling up from the depths is 10 to 100 times more nutrient-rich than adjacent Gulf Stream waters, full of zooplankton, small fish, squid and other tasty morsels that band-rumped are specialists at exploiting. Off North Carolina, presumed Grant’s Storm Petrels arrive in mid- to late May, increase through June and are commonest in July and early August. Their numbers tail off in mid- to late August. Primary moult tends to be completed in late July and early August, when adults may move back to their breeding grounds. Lingering birds may be pre-breeders, or Cape Verde Storm Petrels (Steve Howell pers comm).

Until recently, populations breeding in various parts of the Atlantic and the Pacific were thought to be so similar that band-rumped storm petrels were not considered to show any significant geographical variation (eg, Cramp & Simmons 1977). In 1962, R G Allan devoted a 21-page paper to the breeding biology of band-rumped near Ascension Island in the South Atlantic. Bannerman & Bannerman (1966) were so impressed with his work that they wrote: “It does not seem that Dr Allan has left much for anyone else to discover!”. They could hardly have been more wrong.