Black-footed albatross (Phoebastria nigripes) COSEWIC assessment and status report: chapter 6

Biology

Black-footed Albatrosses have been studied extensively at their nesting colonies in the Hawaiian archipelago since the first comprehensive population surveys took place in the 1950s, although intermittent censuses took place at certain colonies as early as the first decade of the 20th century (Rice and Kenyon 1962a,b) and banding efforts occurred from the 1940s (Yocum 1964; Cousins and Cooper 2000). Behaviour at sea is also relatively well-known as Black-footed Albatrosses have been included in numerous multi-species surveys and studies of seabirds and other marine predators in subsequent decades (e.g., Ainley et al. 1995; Hunt et al. 2000; Burger 2003). The species has also been the subject of a number of more recent projects using satellite telemetry and other remote monitoring technologies to determine marine habitat use (e.g., Fernández and Anderson 2000; Fernández et al. 2001; Hyrenbach and Dotson 2001; Hyrenbach et al. 2002; Shaffer et al. 2005; Suryan pers. comm. 2005). The Black-footed Albatross was the focus of several recent demographic modelling exercises to determine population status and trends in the context of fisheries-related mortality (Cousins and Cooper 2000, Lewison and Crowder 2003; Wiese and Smith 2003, Niel and Lebreton 2005; and cf. Tuck et al. 2001; Mills and Ryan 2005). Most information on the Black-footed Albatross is from studies conducted on birds from the Hawaiian Islands population or on birds tagged or observed at sea; the latter most likely come from the Hawaiian population.

Life cycle and reproduction

Adults usually breed at the age of seven or eight, with some birds breeding as early as five or six years of age. A very small number return to colonies at two to three years of age but individuals have not been observed on the colony at all during the first two years of life. Most (63%) breeding birds nest two years in a row but less than half do so for three consecutive years (Whittow 1993; USFWS 2005a). The nest is constructed by both adults and consists of a shallow hollow scraped in loose sand, often augmented with vegetation. Black-footed Albatrosses are monogamous, with a pair bond that remains intact until the death or disappearance of a mate. Breeding birds return to the colony in late October and lay a single egg between mid-November and early December. If an egg is lost, relaying does not occur that year (Whittow 1993; USFWS 2000). Like other Procellariiformes, incubation is prolonged, a life history feature related to the slow growth of the embryo (Warham 1990). Both sexes incubate the egg for approximately 66 days until hatching occurs between mid-January and early February. The chick is brooded or attended continuously for about 19 days, guarded intermittently for another 10 days and then visited only for provisioning (Whittow 1993).

During the brood stage (>18 days) adults begin extended foraging trips of up to 28 days in length, ranging over thousands of ocean kilometres, from Hawaii to the west coast of North America, and engaging in area-restricted food searching behaviours (Fernández et al. 2001; Hyrenbach et al. 2002). Chicks are fed a diet of flying fish (Exocoetidae) eggs and stomach oil derived from squid (USFWS 2000), reaching an asymptotic mass of about 3,600 g (Sievert and Sileo 1993). The nestling period lasts 140–150 d and in June, adults leave the colony and chicks fledge without assistance in June or July (Whittow 1993; USFWS 2005a). Although Black-footed Albatrosses are highly philopatric (USFWS 2005a), young birds do not return to their exact natal nest site to breed. Some individuals may later nest on nearby islands (Rice and Kenyon 1962a; Whittow 1993). Established breeders generally nest within four to six metres of a former nest site, but some individuals have been recovered on islands >1000 km away from their original colony (Whittow 1993). Historically, Black-footed Albatross colonies were more widespread than they are today (Rice and Kenyon 1962b; USFWS 2005a), so the current availability of nesting habitat is presumably not a limiting factor for these populations.

Although hybrids of Black-footed and Laysan albatrosses occur (Whittow 1993), “the amount of interbreeding is almost negligible” (Fisher 1972).

Physiology, foraging and nutrition

Both breeding and feather moult are energetically demanding, and like most bird species, Black-footed Albatross do not moult and breed at the same time (Tickell 2000). Instead adult and juvenile birds undergo moult at sea during the post-breeding dispersal, replacing 20–90% of their feathers each year (Langston and Rohwer 1995, Edwards and Rohwer 2005). At the same time, Black-footed Albatrosses also gain fat deposits in preparation for the upcoming breeding season (Fring and Fring 1961): foraging success and prey availability clearly have the ability to affect an individual bird’s physiological readiness to breed in the upcoming season, and successful breeders are more likely to skip nesting in the subsequent year than are failed breeders (Langston and Rohwer 1995).

Total energy expenditure over the breeding season (250 d) for the colony of Black-footed Albatross at French Frigate Shoals was calculated to be 8.019 ´106 kj, and birds consumed an estimated total of 929 tonnes of food to meet this energy requirement. Hunt et al. 2000 estimated that as much as 16,730 tonnes of prey were consumed by Black-footed Albatrosses each summer within the Western and Eastern Tropical zones of the North Pacific.

Principal food items for Black-footed Albatrosses breeding in Hawaii consist of squid and flying fish eggs, followed by fish and deep-water crustaceans such as large mysids and isopods (Harrison et al. 1983; Whittow 1993; USFWS 2005a). Black-footed Albatrosses feed at one-third to one full trophic level higher than Laysan Albatross (Gould et al. 1997, J. Bisson, unpubl. data, 2005). This, along with a combination of ship-following behaviour, daytime foraging, and the presence of offal (fishing vessel discards as well as naturally occurring dead squid, fish, marine mammals and birds) in the diet indicate that scavenging represents an important mode of obtaining prey (Gould et al. 1997, 1998; Fernández and Anderson 2000; Harfenist et al. 2002). Birds primarily feed by seizing food items from the surface of the water; in one study, Black-footed Albatross fitted with immersion loggers spent an average of 90.8% of their time at sea flying. Most foraging, as indicated by immersion events, took place during the day (Fernández and Anderson 2000, Hyrenbach et al. 2002). Black-footed Albatross have low levels of rhodopsin in their eyes (4 density units/g, as opposed to 20 density units/g for Barn Owls Tyto alba) and it is assumed that their night vision is relatively poor (Fernández and Anderson 2000).

Behaviour

Black-footed Albatrosses are scavengers, feeding at the surface, and are well known ship-followers (summarized in Hyrenbach 2001) that are drawn to fishing vessels, where they scavenge for discarded fish, squid and offal. This behaviour makes them particularly vulnerable to drowning in demersal and pelagic longlining, and in high seas driftnet fisheries (see Limiting Factors and Threats).

Black-footed Albatrosses form small to large flocks at sea. Wahl and Heinemann (1979) recorded a maximum flock size of 250 birds foraging at a factory trawler. Morgan et al. (1991) reported that in spring, most observations of Black-footed Albatrosses were of single birds or of small flocks typically numbering less than 10 birds. During the summer months (July–August), the species was more gregarious, with flocks of 20 to 40 birds commonly seen and a maximum group size of 180 birds observed rafting on the water (Morgan et al. 1991).

Attraction to vessels as well as rafting behaviour could also make the Black-footed Albatross especially vulnerable to mortality from chronic or catastrophic oil spills, as has been suggested for rafting flocks of Pink-footed Shearwater (Puffinus creatopus; COSEWIC 2004).

Survival and recruitment

Black-footed Albatross are long-lived birds, with an average lifespan of 12–40 years (USFWS 2000). The oldest known Black-footed Albatross was at least 43 years in 2004 (USFWS 2005a), although the species undoubtedly attains a greater age: the age record for the sympatric and closely related Laysan Albatross is at least 51 years, and that bird was brooding a healthy chick at the time of band reading (Walker 2003; Robbins pers. comm. 2005). For the purposes of assessing species’ status, generation time has been estimated from 18.67 (BirdLife International 2004a,b) to 20 years (Lewison and Crowder 2003; NatureServe 2006). Niel and Lebreton (2005) modelled the mean generation time for this species, under optimal conditions, as 17.09 years.

As a K-selected species (see Limiting Factors and Threats), the stability of the Black-footed Albatross population is highly vulnerable to factors that affect adult survivorship. There are no published estimates for adult survival in Black-footed Albatross but it is estimated at mean=0.923, ranging 0.81–0.994, based on data collected in the 1960s (Cousins and Cooper 2000, p. 49). Even under ideal conditions on the colony and at sea, it is estimated that the total Black-footed Albatross population cannot grow at a rate greater than 5.9% per annum so any additional source of mortality in excess of that amount (ca. 8,900 individuals/year, based on recent estimates of population size) will result in a population decline (Niel and Lebreton 2005). Cousins and Cooper (2000) estimated a similar maximum mortality threshold of 10,000 birds per year, including mortality from natural sources and without incorporating indirect mortality effects such as widowing and lower net reproduction.

Predation

In Hawaiian waters, tiger sharks (Galeocerdo cuvier) depredate juvenile Laysan and Black-footed albatrosses when they fledge from their natal colonies. These sharks take an estimated 10% of albatross fledglings each year (Wake Forest University 1999). Introduced rats (Rattus spp.) killed nestlings at Kure and Midway Atolls in the past but these rodents have now been eradicated. Polynesian rats (R. exulans) on Kure Atoll killed both adult and juvenile Laysan Albatross but they have only been documented as predators of Black-footed Albatross nestlings (Kepler 1967; Moors and Atkinson 1985; Whittow 1993). On Torishima (Izu Islands, Japan), Steller’s Sea Eagles (Haliaeetus pelagicus) are occasionally seen taking Black-footed Albatross chicks (USFWS 2005c), and rats are present on the island.

Dispersal/migration

Adult Black-footed Albatrosses are concentrated around their colonies during the egg-laying, incubation and chick guard periods (November – February; Whittow 1993; Cousins and Cooper 2000). Non-breeders and failed breeders start to leave the nesting grounds in April, and between June and September the remaining adult birds disperse northeast across the Pacific Ocean into transitional and sub-arctic waters that are cooler and more productive than those adjacent to the colony. Fledglings disperse primarily west of 180 degrees, and subsequent summers and winters are spent mostly in the eastern temperate north Pacific (Robbins and Rice 1974; Whittow 1993; Gould et al. 1998; Hyrenbach et al. 2002). See also sections on Distribution and Habitat Requirements – Marine habitat.

Diseases and parasites

Avian pox (Avipoxvirus) has been found in Laysan Albatross chicks at Midway Atoll and it also occurs, albeit rarely, in Black-footed Albatross nestlings. In Laysan Albatross chicks the disease generally appears to be self-limiting, i.e., most chicks eventually recover if they continue to be fed (Fefer pers. comm. 2006; Sileo pers. comm. 2006). Mortality of Black-footed Albatross chicks has been documented from nocardiosis (infection with the soil bacterium Nocardia asteroids) and from chigger infestation (Whittow 1993): three species of chigger and one species of tick have been reported from Black-footed Albatross nests (Whittow 1993; Ushijima et al. 2003). Of the six species of chewing lice (Pthiraptera) recorded on Black-footed Albatross, three occur on no other host species.

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