Procellariiformes and foraging

Seabirds exploit ocean resources at many trophic levels (Schreiber & Burger, 2002).

Procellariiform diet varies between species and includes a variety of food types such as; fish, cephalopods and small marine invertebrates (Marchant & Higgins, 1990; Warham, 1990). Foraging is generally limited to the upper two to three metres of the ocean surface, although some shearwater species have been observed foraging at depths of 20m (Warham, 1990). Feeding styles have been described based on the methods used to acquire food items - surface seizing, for example, is used by 80% of 103 petrel species (Warham, 1996). Other feeding methods include surface diving, pursuit diving, pursuit plunging, dipping and hydroplaning (Warham, 1996).

The spatial distribution and behaviour within the Procellariiformes reflects food supply (Monaghan, 1996; Schreiber & Burger, 2002). Petrels can cover long distances whilst migrating; for example, sooty shearwaters (Puffinus gresius) can travel more than 15,000 km over a two to three week period to reach summer feeding grounds (Hedd, Montevecchi, Otley, Phillips, & Fifield, 2012). During chick rearing, petrels forage in closer proximity to their breeding colonies (Warham 1996). The ranges are however likely to be dependent on

regional oceanic productivity and food availability (Rayner, et al., 2008; Stahl & Sagar, 2000; Weimerskirch, 2007). The white-chinned petrel (Procellaria aequinoctialis) of South

Georgia forages on average 610 km from the colony during chick rearing (Phillips, Silk, Croxall, & Afanasyev, 2006) and the maximum recorded range for the cook’s petrel

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The life stage of an individual is also important to its spatial locality; for example breeders, non-breeders and recent fledglings are often found in different areas and at different times of the year (Warham, 1996).

Petrels can store sub-dermal fat and stomach oils, which assist with insulation and can be utilised when provisioning opportunity is limited (Warham, 1990). Consequently, petrels can survive for long periods of time without food; an advantage as food can be patchy in

concentration and distributed over thousands of kilometres (Warham, 1990; Weimerskirch, 2007). Fat storage also supports fasting adults during long incubation periods; for example, the Galapagos albatross (Diomedea irrorata) incubates for 19–22 consecutive days (Harris, 1973) and the medium sized grey-faced petrel (Pterodroma macoptera gouldi) sits for an average of 17 days without provisioning (Imber, 1976; Warham, 1990). The digestive tract of petrels is different from other birds in that they have no crop; instead the proventriculus, located at the bottom of the oesophagus, is thick, glandular and folded, forming a large bag. This anatomical feature allows petrels to carry large meals back to their chicks (Brooke, 2004). The oil found in the proventriculus of petrels is very energy rich, with 40 kJ/g compared to 4–8 kJ/g of fresh prey (Brook, 2004). Generally, birds that forage further from the breeding colony return with greater ratios of oil, while those foraging in close proximity deliver more fresh prey (Brooke, 2004).

Procellariiform chicks are provisioned by both parents. Feeding is done soon upon parental arrival at the burrow and for many species adults will return to sea after spending only minutes in the nest (Warham, 1990). The frequency at which chicks are provisioned varies between species as well as with the growth stage of the chick (Warham, 1990). Chicks also have high lipid stores and reach weights often exceeding adult body weight by 40–70% (Warham, 1990; Brooke, 2004). Obesity is a conspicuous feature of chick growth in Procellariiformes and is thought to act as a buffer against inconsistencies in provisioning (Brooke, 2004; Hamer & Hill, 1997; Hamer, Lynnes, & Hill, 1999; Phillips & Hamer, 1999). Chicks, like the adults, can also withstand periods of fasting. This is done by entering a state of torpor characterised by a cold and lethargic appearance (Warham, 1990). For example, fork-tailed storm petrels (Oceanodroma. furcata) show frequent declines of 10 ˚C from mean body temperatures of 37.4 ˚C and in once case a chick was recorded with a body temperature of just 10.6 ˚C (Boersma, 1986). Chicks cope with fasting by slowing down their metabolism (Ochoa-Acuna & Montevecchi, 2002; Richdale, 1965; Warham, 1990). This ability to cope

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with fasting is likely an adaptation in response to widely distributed food resources and irregular provisioning (Boersma, 1986).

4.1.3 White-faced storm petrels

The smallest of the Procellariiformes are the cosmopolitan storm petrels. Storm petrels maintain a pelagic existence except for nocturnal visits ashore to provision chicks during breeding (Marchant & Higgins, 1990). Belonging to the sub-family Hydrobatidae,

Pelagodroma marina maoriana, commonly known as white-faced storm petrels (specifically referred to as WFSP), are an endemic to New Zealand subspecies.

Observed as either gregarious or solitary at sea, storm petrels forage by picking food items - planktonic crustaceans, molluscs and small fish - from the water’s surface (Marchant & Higgins, 1990). The majority of WFSP food is obtained from the uppermost centimetres of the sea surface (Warham, 1990) by employing methods of aerial dipping and contact dipping, pattering and less frequently surface seizing and surface plunging (Marchant & Higgins, 1990). WFSP are recognised for their distinctive flight patterns when feeding as they hop using both feet along the sea surface with horizontally spread wings (Plate. 4.1) (Marchant & Higgins, 1990).

The diet of WFSP analysed from 22 regurgitations samples from birds of the Chatham Islands, comprised predominantly crustaceans and fish, representing 70% and 30% of weight respectively (Imber, 1981). Crustaceans included Copepods (Calanus tonsus), two species of Stomatapods, Amphipods (7 species) and, Euphausiids (6 species), as well as immature crabs and Decapod larvae (Imber, 1981). One regurgitation sample collected from WFSP on Whero Island mostly consisted of Euphausiids, possibly squid meat, two lenses and barnacle larvae (Richdale, 1943a). Fish up to 6.5 cm long have been recorded in P. marina (Warham, 1990).

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WFSP breed during the austral summer and migrate to tropical eastern Pacific waters during the austral winter (Imber, 1982; Leveque, Bowman, & Billeb, 1966; Spear & Ainley, 2007). Both WFSP parents share incubation, with rotational shifts averaging four to five days (Richdale, 1943b). During this time the incubating bird does not leave the nest nor does the other parent provide provisioning. Chicks are fed by incomplete regurgitation - usually at night by one or both parents, although provisioning may occur during the day while chicks are young and being brooded (Marchant & Higgins, 1990; Richdale, 1965).