Burketl 



Chapter 22 



Food Habits and Prey Ecology 



marine protein contributions, because nitrogen isotope ratios 

 in the muscles of fish species in coastal lakes may overlap 

 with those of marine fish (Hobson 1 990). Lower trophic- 

 level fish such as fingerling salmonids also overlap with 

 marine invertebrates. Thus, nitrogen isotope analysis may 

 be better suited than carbon to delineating the trophic levels 

 of murrelets and other seabirds. The results of this analysis 

 (Hobson 1990) showed Marbled Murrelets in the middle of 

 a spectrum (10 species) from Dovekies (Alle alle) to Pigeon 

 Guillemots; the Marbled Murrelet was between the Ancient 

 Murrelet and the Common Murre. This isotopically 

 intermediate position is consistent with the results of the 

 studies described above which document murrelet 

 consumption of invertebrate prey as well as marine fish. 

 The trophic-level approach also has the value of being less 

 biased against the soft-bodied invertebrates which are not 

 easily detected in conventional studies. 



A further analysis of the variability of stable nitrogen 

 isotopes in wildlife showed that tissue can be enriched because 

 of fasting or nutritional stress (Hobson and others 1993). 

 Thus, studies using analysis by stable nitrogen isotopes to 

 infer diet or trophic position must take into account the 

 nutritional history of the individual specimen. Fasting should 

 not be a factor for the murrelet because both sexes incubate 

 the egg and feed the nestling, but nutritional stress could 

 affect the results in a year of severe prey shortage. 



Ecological Studies and Anecdotal Information 



Alaska 



Food habits of the murrelet were described by Bent 

 (1963). "The food of the marbled murrelet seems to consist 

 largely of fish which it obtains by diving in the tide rips and 

 other places where it can find small fry swimming in schools." 

 It appears he derived this information from observations 

 contained in Grinnell (1897) and Grinnell (1910) (table 1). 

 In the summer of 1 8%, during a visit to Sitka Bay, Alaska. 

 Grinnell (1897) noted. "Small fish caught by diving seemed 

 to be the standard article of food, but dissection of the 

 stomachs also showed remains of some small mollusks. A 

 shoal of candle-fish [Thaleichthys pacificus] was sure to 

 have among its followers, besides a cloud of Pacific kittiwakes 

 [Rissa sp.]. several of the Murrelets" (table 1). Grinnell 

 (1910: 366) noted fish as a prey item in a collected specimen 

 and during an observation by Joseph Dixon of a foraging 

 murrelet. but the species offish were not recorded (table I). 



Observations at the first documented ground nest of a 

 murrelet indicated capelin as a food source for the nestling 

 (Simons 1980) (table 1). An adult murrelet delivered a 

 single fish about 8 cm long. Simons (1980) noted, "The 

 fish appeared to be a capelin (Mallotus sp.)..." [emphasis 

 added]. This observation would appear valid given the 

 documented importance of capelin in murrelet diet in 

 Alaska (Sanger 1983). Simons (1980) also noted that the 

 pattern of weight gain was variable from days 2 to 12, and 

 he suggested the possibility of multiple feedings. He 

 concluded that predation and the distribution of the food 



resource were important selective agents acting upon 

 ground-nesting murrelets. 



British Columbia 



Food habits of "water fowl" during die spawning season 

 of herring in the vicinity of Departure Bay, British Columbia, 

 were studied between 1928 and 1930 (Munro and Clemens 

 1931). Four male murrelets were collected in late February 

 to mid-March, and the stomachs contained Cymatogaster. 

 larval fish, mysids, and schizopods (table 1). The archaic 

 group schizopoda included euphausiids and mysids because, 

 superficially, the members of these two orders appeared so 

 similar. These two groups are now separated into different 

 tribes based on characteristics of the carapace and the 

 distinguishing luminescent organs of the euphausiids (Hardy 

 1965: 171-172). 



The results from Munro and Clemens ( 193 1 ) differ from 

 the winter results of Carter (1984) and Vermeer (1992), in 

 that identifiable herring are absent (table 7). This difference 

 could be due to the small sample size. Alternatively, it could 

 result from differences in availability of herring age classes 

 and in herring distribution relative to murrelets, and differences 

 in the magnitude and duration of the herring spawn between 

 the three study areas (McAllister, pers. comm. ). A number 

 of herring stocks aggregate close to the spawning area for 

 some time before actually moving on to the grounds to 

 spawn (Lambert 1987). 



An anecdotal account of murrelet diet by Guiguet was 

 published in 1956. He spent many summers on zoological 

 exploration in coastal British Columbia and stated that the 

 murrelet "...eats small Crustacea such as euphausid [sic] 

 shrimps, and fishes such as the sand launce [sic]...." He also 

 described watching murrelets foraging off the Queen 

 Charlotte Islands in July 1946 and noted, "all were feeding 

 on sand launces [sic]...." When darkness had almost 

 descended that day, the murrelets disappeared inland to the 

 west. Guiguet (1956) noted, "All of them were 'packing 

 feed' in their bills, and the silvery sand launce [sic] showed 

 up in the darkness" (table 1). 



Between 6 June and 8 August 1991, Mahon and others 

 ( 1992) conducted 27 at-sea surveys to determine the composition 

 and density of mixed feeding flocks. They observed 126 feeding 

 flocks, 100 of which contained only murrelets and Glaucous- 

 winged gulls (Larus glaucescens). Murrelets were seen to 

 feed on schools of sand lance by driving the fish to me surface. 

 First-year sand lances were the only prey identified in feeding 

 flocks (table 7). In the evenings, murrelets were seen holding 

 larger sand lance. Pacific herring, and shiner perch as prey for 

 nestlings (table 7). The nestling prey items closely match the 

 juvenile diet reported by Sealy ( 1 975c), and two of the nestling 

 items, herring and sand lance, reported by Carter (1984) and 

 Guiguet (1956), respectively. 



Additional anecdotal information on nestling food habits 

 in British Columbia comes from a nest which was monitored 

 in summer 1993 (Jones and Dechesne 1994). Sand lance was 

 noted as a prey item for the nestling (table I). 



USDA Forest Service Gen. Tech. Rep. PSW-152. 1995. 



231 



