Shearwater remains fed to albatross chicks

Fresh tītī remains were found at five of the 50 study nests on 10–11 May 2016. These had been rejected by the chicks, in an area where native, but introduced weka Gallirallus australis could readily scavenge them, thus were likely to have been deposited 1–2 days beforehand. The items were: bolus of shearwater feathers (one nest); the head of a juvenile shearwater and feather bolus (one nest); feet and tarsi (three nests). At nests in two adjacent areas a feather bolus, feet and tarsi were found. Examples were retained in the Te Papa collections (TMP288136–288142).

GPS-tracked albatrosses frequent muttonbirding sites

Seventeen albatross tracks were obtained and the combined set of tracks was used to produce kernels of usage of marine areas (Fig 1.). These kernels showed that most of the birds were feeding chicks using waters within Foveaux Strait, but several also fed at greater distances from the breeding site, some travelling to the Tasman Sea and north of Cook Strait. On average for the 40 foraging trips from the 2016 tracked birds travelled 1290 km (± 1853 km SD), during 2.45 d (± 3.2 d SD), with a range of 224 km (± 222 km SD).

One adult albatross was observed regurgitating tītī remains to its chick on 10 May. This bird was subsequently tracked, and spent time close the tītī islands near to Stewart I. / Rakiura during its 9.6 day trip (Fig 2). It visited five muttonbirding sites, spending 2–110 min within 2 km of the shore on 22 occasions (average 19 ± 28 min, S1 Table). It alternated these periods with visits to the shelf-break west of Stewart I. / Rakiura in bouts of 20–1,175 min (average 208 ± 264 min, n = 20). Visits to the muttonbirding sites were almost always between 06:45 h and 11:00 h. Two other tracked birds passed within 2 km of the muttonbirding sites on their return to Solander I. (Hautere) from feeding further to the east. These birds stayed 6 and 20 minutes at these sites, respectively.

Muttonbirding practice at the tītī islands, Stewart I./Rakuria

Tītī are harvested from 1 April to 31 May annually. During the period when the albatross tracking study was undertaken, harvesting occurred at night-time [26, 27] as fully-feathered juvenile shearwaters emerge from their burrows at this time [11]. The tītī were caught, killed, and hung overnight, then plucked and preserved the following day [26, 27, 28]. The offal including heads, guts, and feathers was discarded in the sea adjacent to the islands from afternoon to evening [28].The exact timing varies between sites, in relation to the number of birds collected and workers processing them [11]. Scavenging seabirds were often seen using the offal [26, 27, 28]. One muttonbirder [26] noted that giant petrels (Macronectes sp.) came ashore frequently during the study period to feed from the offal, and that mollymawks (likely Buller’s albatrosses) congregated further offshore in the bay. Another [28] noted that although both giant petrels and Buller’s albatrosses were present in numbers in May 2016, the giant petrels dominated access to the offal, and kept the albatrosses at a distance. The muttonbirders noted that offal provides food to marine life at these sites [27, 28]. It is assumed that 35% by weight of the tītī is discarded [29], at an average weight of 700 g [16]– 860 g [29], this amounts to between 78 and 120 tonnes of offal annually. This proportion of the whole weight of chicks processed includes the bones, heads, feathers, viscera, feet, all of which were noted in our observations of offal being re-fed to chicks. Based on research into albatross diet [4], it is likely that all discarded parts of the tītī carcasses would be ingested by scavenging albatrosses.

Is muttonbirding offal the likely source of the observed tītī remains?

We examined the evidence that the tītī remains observed at the albatross colony originated from offal, rather than from natural mortality or depredation of juvenile shearwaters following fledging. Wrecks of shearwaters have been reported around New Zealand during their fledging period in May, October and November over several decades [30], and during May 2016, dead and dying shearwater juveniles were observed along southern Stewart Island / Rakiura coastal areas (N. Cobb, pers. comm). While we cannot rule out the possibility that some of the remains observed at the albatross colony originated from the natural mortality of shearwater juveniles at sea, it is unlikely that albatrosses would access these remains on beaches, where many specimens accumulate, as albatrosses are not known to feed on land [31]. In contrast, giant petrels are likely to depredate or scavenge dead and dying birds on the shore, and were observed feeding on tītī offal at the shore line at one muttonbirding site during the study [28]. Had the albatrosses fed on floating moribund tītī at sea, we would expect them to have eaten soft body parts such as muscle and viscera, and not to ingest heads, feathers, tarsi and feet. This is because these elements are firmly attached to a whole carcass, and the whole carcass is too large for these small albatrosses to swallow. It, therefore, seems more likely that the observed tītī remains came from offal which is discarded as discrete body parts into the sea, rather than from the beach wrecked or floating carcasses of juvenile shearwaters.

Albatrosses change their foraging zone when offal is available

We explored whether there were changes in the albatrosses foraging behaviour between periods with and without harvest, reported in the literature. Tracking studies of breeding Buller’s albatross from the 1990s [19, 20] showed that while 5–20% of tracked birds fed near to the tītī islands during incubation, the proportion increased to 40% during the first few weeks of chick-rearing. This change in feeding activity is likely to represent a combination of a) the need to feed chicks often early in the brood period, with adults reducing their foraging ranges at this time, and b) the opportunity to exploit offal from muttonbirding practice, which occurs in the central part of the range of this albatross species, within 120 km of the nesting sites for both populations.

The timing of albatross hatching coincides with the provision of offal

The mean hatch date for Buller’s albatross on 2 April [32]occurs at the same time as the tītī harvest starts each year, 1 April. The provision of offal increases through the first month of the albatross chick-rearing as the rama hunting period, when most tītī are harvested, occurs from 25 April– 30 May [33]. Thus the timing and availability of offal is likely to be a food resource upon which albatrosses from nearby colonies can rely when raising young chicks [34], recognised as the most constraining stage in the 5-month long chick-rearing period for albatrosses.

Offal feeding albatross spent its foraging trip visiting muttonbirding sites

In addition to observing tītī remains at the albatross colony, one of 17 Buller’s albatrosses tracked in May 2016 made 22 searches near tītī harvest sites during a period that offal from muttonbirding was available at these particular sites, with active muttonbirding activity at these islands in 2016 (Fig 2). This bird fed a tītī head to its chick, along with tītī feathers. Two other GPS-tracked birds flew close to active muttonbirding sites during the study, but stayed for short periods only (less than 20 mins each). The behaviour of albatrosses and other carrion feeding birds at the muttonbirding sites observed by workers on the harvest corroborates the information from the GPS tracking, that Buller’s albatross frequented the sites, but remained at some distance from the shore [28].

Buller’s albatross diet

The use of tītī remains as a food by Buller’s albatross has not been noted previously, although 3% of diet samples analysed in the 1990s contained bird feathers [35]. However, these authors [35] noted that “bird feathers of non-penguin origin were present in eight May samples, sometimes in large numbers, and suggested scavenging on carcasses”. The timing in the season of these observations in the 1990s coincides with the availability of carcasses and offal from the tītī harvest.

Albatrosses and petrels are known as adaptable scavengers, feeding from a variety of natural and anthropogenic food sources, as well as capturing live prey [4]. For example, in past decades, they were frequently seen around meat-works effluent outfalls [7, 36, 37] until improvements in waste-treatment practices eliminated this food source from the 1960 to 1980s [7]. It is therefore not surprising that they would make use of the offal from tītī harvest, but our observations highlight the inter-dependence of marine species and traditional practices in a hitherto undocumented manner.

Offal from muttonbirding is comprised of the viscera, wings, tails, heads, feathers, tarsi and feet of the tītī [29]. These elements of the carcass can provide important nutrients and minerals if digested. Buller’s albatross are known to ingest fish guts and frames from fishing vessels, as well as feeding on more natural prey such as fish and squid, and invertebrates such as crustacean and salps [35]. Although offal may seem to be of little nutritive value, it contains the building blocks of seabird muscle, bone and feather, and is presented in a readily digestible parcels, with dissected body parts that could be easily swallowed whole by an albatross. It, therefore, seems likely that an albatross would readily eat offal of this type, and derive benefit from it.

Population implications of albatross feeding on tītī offal

The provision of tītī offal for Buller’s albatrosses may be contributing to the overall population growth of the species’ two breeding populations, which are subject to a number of threats to their population viability, such as mortality in trawl and longline fisheries [38, 39]. The food supplied by muttonbirding offal may have benefited the two populations of Buller’s albatross over time, as it is within easy reach (less than 120 km from the two breeding populations) during the short trips required to feed chicks immediately after hatching. The population of albatrosses has increased at around 3% p.a. since surveys began in 1969. Although undocumented, the populations may have been recovering from human exploitation during the sealing era in the 1800s at both of the breeding sites. Sealers were stationed or marooned at both breeding sites over several years [40], and it is highly likely that they harvested the albatrosses for food during this period. Thus, the tītī harvest may have assisted in the population rebuild over many decades prior to scientific surveys of the populations. Buller’s albatrosses feed chicks substantially on trawl discards (42% - 60% by weight of diet samples) [35]. While fisheries waste has no-doubt contributed to population increases, recent modelling studies showed that the negative populations effects of occasional fisheries mortality has not affected the viability of the populations [39]. Feeding on tītī offal is another important factor to consider in the evolution of the albatross populations. This food source varies with El Niño Southern Oscillation [41], a fact which may amplify any climate-related changes in chick survival for the albatrosses.

The intensity of muttonbird harvest is reducing over time as the community of muttonbirders changes [9, 11], and will also vary with climatic conditions and long-term declines in shearwater populations [42]. As a result, the food provided to albatrosses and other marine scavengers from tītī offal may decrease. Our findings show that while muttonbirding is recognised for its value in maintaining cultural traditions, it may also be important in sustaining this threatened species.