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COSEWIC Assessment and Update Status Report on the Ancient Murrelet in Canada

Biology

General

The Ancient Murrelet is a small marine bird and is fairly generalized physically for an auk, having a bill that is neither specialized for fish nor plankton, and a functional morphology intermediate between specialized wing-propelled divers such as murres and the less specialized auklets (Gaston 1994a). It is unique in being the only seabird that has fully precocial chicks that go to sea without ever having been fed on the nest (Gaston 1994b). It also has a unique vocal repertoire that allows it to communicate effectively during its nocturnal colonial visits (Gaston 1994a).


Reproduction and Survival

In Canada, Ancient Murrelets breed only in the Queen Charlotte Islands. Timing of breeding varies across its range, with an observed 6-day delay in egg laying for every 1ºC decrease in mean April sea-surface temperature near colonies (Gaston 1992). Generally, birds arrive at the colony in March with median clutch completion dates being between mid-April and early May (Gaston and Jones 1998). Timing of breeding varies between colonies and may be dependent on when the risks to adults from predation are least, and the availability of food for chicks is highest (Gaston 1997). Each breeding pair produces one clutch per year of 2 eggs each, 6 – 10 days apart, with no replacements (Gaston 1994a).

Ancient Murrelet parents share incubation duties equally (Gaston 1992) but do not feed their chicks before leaving the burrow. The chicks leave the burrow at night, 1‑4 days after hatching (Sealy 1976; Shibaev 1978; Jones et al. 1990) and are raised entirely at sea (Litvinenko and Shibaev 1987). Parents feed the young for about 1 month, by which time they are fully grown and feathered (Litvinenko and Shibaev 1987).

Ancient Murrelets first breed at 3-4 years of age (Gaston and Jones 1998). Where there are no introduced predators, breeding pairs rear an average of 1.5 young per year to the stage of colony departure (Vermeer and Lemon 1986; Gaston 1992). Most family groups seen at sea after departure from their colonies are made up of two adults and two chicks. So, despite the precocial departure, survival of young during the first few days after deserting the nest site appears to be high (Gaston 1992). However, body condition and date of chick departure influences survival. Based on rates of recapture, chicks that leave the colony at 26 g or less (at any time) survive less well than heavier birds; birds leaving after the median date of departure survive best (Gaston 1997). Mean annual survival rate of adult Ancient Murrelets is 77%, relatively low for an alcid (Gaston 1990), but breeding success is high. Based on data from Reef Island, adult birds breed for an average of 4.5 years (Gaston 1994a).


Physiology

There is limited information on the physiology of Ancient Murrelets. However, there are traits common to alcids that provide some information. Auks store fat in a thin subcutaneous layer over much of the body, rather than in pockets as is the case in most other birds (Gaston 1992). Auks also have heat exchange mechanisms in their legs (rete mirabile) to limit heat loss (Gaston 1992) and for their size, have high basal metabolic rates; an adaptation for keeping their bodies warm in colder climates. In order for auks to store oxygen while diving, they have a high blood volume and high levels of myoglobin (Gaston 1992).

Chicks are born at a mean weight larger than in other auks (15% of adult body weight versus 11%), with legs that are almost adult size. They also have very large fat stores at hatching (Duncan and Gaston 1988) used for energy reserves and possibly insulation. At hatching, chicks contain 13 g fat, but lose 40% of this before leaving the burrow (losing 2 g per day while in burrow). The remaining reserves are essential for the long, steady swim that they will have to undertake once they reach the water. What also helps the bird once they reach the water is their ability to thermoregulate immediately (Gaston and Jones 1998).


Movements/Dispersal

Ancient Murrelets in Canada breed exclusively in the Queen Charlotte Islands off the British Columbia coast from April to June. Once breeding is complete, they leave the vicinity of their breeding colonies. Family groups from colonies in Hecate Strait remain in the Strait for several weeks (Duncan and Gaston 1990) while some appear off the Goose Islands in Queen Charlotte Sound during the same period (Guiguet 1953). By August, Ancient Murrelets are uncommon in Hecate Strait and in September they virtually disappear from British Columbia waters (Gaston 1992). Their movements during this season are unknown (Campbell et al. 1990; Gaston 1994). Large numbers of Ancient Murrelets appear in inshore waters off Vancouver Island by late October, where they remain until mid-February (Wahl et al. 1981; Campbell et al. 1990). A small number of birds move into the Bering Sea to winter, while others are known to remain within the Canadian breeding range (Gaston 1994a). Smaller numbers occur during the same season in waters out to the edge of the continental shelf off Washington, Oregon and California (Ainley 1976; Balz and Morejohn 1977; Briggs et al. 1987). By March, they appear in large numbers in Hecate Strait and begin visiting their colonies prior to egg laying.

Sightings of Ancient Murrelets have been recorded in Hecate Strait and the Queen Charlotte Sound in January and April through July, but not in September (Morgan 1997). They are present in Dixon Entrance in April and May; they have been reported in July, but are absent by October (Morgan 1997). Though it appears at times that most birds disperse south of the Queen Charlotte Island (Sealy 1976), some remain close to their colonies (Morgan 1997). Bird densities are highest during breeding season. The highest summer densities occur at the outer edge of the continental shelf, between Langara and Frederick islands. In the fall, sightings are restricted to the edge of the Shelf, west of Cartwright Sound, while winter sightings have been restricted to Hecate Strait, between Skidegate Inlet and Cape St. James (Morgan 1997). Ancient Murrelets are regularly sighted on Christmas Bird Counts in Masset and Rose Spit (Morgan 1997).

Visits by birds to other colonies do occur when colonies are close. Birds visiting non-natal colonies were found at Reef and East Limestone islands, located 6 km apart. However there was no evidence that these birds were breeding on their non-natal island. Whether birds were also visiting Lyell Island, located 15 km from Reef Island and 20 km from East Limestone Island, could not be determined but it may be possible (Gaston and Adkins 1998).

Only one banding recovery of a Canadian Ancient Murrelet has been away from the Queen Charlotte Islands: a sub-adult/immature found dead on a beach in Washington State (Gaston 1994b). The origin of the birds that occur off southern British Columbia in winter is not known, but the large numbers involved suggest they are from the Queen Charlotte Islands.

Of all the alcids, the Ancient Murrelet is one of the species most prone to vagrancy (Gaston and Jones 1998). There have been sightings on lakes in the British Columbia interior and eight records from the Canadian Prairie provinces (Sealy et al. 2001). Several records also exist for the Ancient Murrelet in Mexico (Erickson et al. 1995) and Great Britain (Waldon 1994).


Nutrition and Interspecific Interactions

The diet of the adult Ancient Murrelet is made up primarily of large zooplankton and fish. Composition of the diets varies by season, age, location and availability (Sealy 1975; Vermeer et al. 1985; Gaston 1994). Before breeding in late March and early April, the birds are known to eat Euphausia pacifica, while Thysanoessa spinifera becomes a dominant part of their diet after breeding begins (Sealy 1975). Fish (including larval and juvenile fish) dominated the diet of the birds in June. Later in the season, they also feed on fish, primarily Pacific sand lance, (Ammodytes hexapterus) and rockfish (Sebastes spp.) but also flatfish (Pleuronectidae), juvenile shiner perch (Cymatogaster aggregata) and greenlings (Hexagrammos spp.) (Sealy 1975).

Information on winter diet is scarce. Gaston et al. (1993) found that the entire winter diet of Ancient Murrelets off southeastern Vancouver Island consisted of Euphausia pacifica (Harfenist 2003). In November, juvenile herring (Clupea harengus) dominated their diet.

There is also very little information on Ancient Murrelet chick diet. The stomach contents of 8 young collected at sea contained only sand lance (Gaston 1992). Results of an earlier study from Langara Island showed that the stomachs of subadults contained sand lance and euphausiids (Sealy 1975).


Behaviour/Adaptability

While Ancient Murrelets can be found breeding in all types of forest, potential recolonization following forest destruction may take a long time (Gaston 1994b). At Limestone Island, a large area cleared by a wildfire was still not being used by murrelets 20 years later, although alder had regenerated strongly.

The reduction in area occupied by Ancient Murrelets at colonies on Langara and Lyell islands seems to be a response to heavy predation pressure by rats (Gaston 1994b). Recovery from such declines may require a critical number and density of active burrows to attract recruits. If that is the case, then recovery of extirpated colonies is unlikely, especially when population declines through the entire breeding range are common.

Pre-breeding birds have been trapped on islands other than their natal site, which suggests that some birds disperse away from their natal colony to breed (Gaston 1990). For instance, only 3% of pre-breeders trapped at East Limestone Island were banded as chicks, although about 50% of departing chicks were banded (Gaston 1990; Gaston and Adkins 1998). Subsequent DNA analysis at George and East Limestone Islands also suggests that birds move between colonies (Pearce et al. 2002). This ability to move between colonies may provide it with a mechanism to adjust to increasing pressure from human disturbance or predators, if appropriate habitat is nearby.

Finally, Ancient Murrelets are highly sensitive to disturbance at nest sites, causing pairs to abandon their burrow and eggs (Gaston et al. 1988). As pairs do not lay replacement eggs, the impact of abandonment is high and is considered the greatest cause of reproductive failure for these birds (Gaston and Jones 1998).