Burkett 



Chapter 22 

 Table 5 Size of prey items for hatching-year and nestling Marbled Murrelets 



Food Habits and Prey Ecology 



a Carter (1984); 16 June - 6 July 1980, n = 144 fish observed. 



" Sizes of prey estimated while held by murrelets in their bills when on the water 



c Ralph and others (1990); observation during mist-netting operation, 3 July 1989. 



d Simons (1980); observation of a feeding at a ground nest. 



e Sealy (1975c); from 6 newly-fledged murrelets collected between 10 July and 4 August 1971. 



f Mahon and others (1992); murrelets observed on the water in the evenings, 6 June - 8 August 1991 . 



nestlings. Energy values of seabird prey items have been 

 little studied (Hislop and others 1991), but a gross comparison 

 from some closely related species reveals marked differences 

 in food energy, protein, and total lipids (table 6). Especially 

 when considering the feeding of nestlings at inland sites, 

 optimal foraging theory would predict that the largest and 

 most energy-rich food items would be brought to the nestlings. 

 This would be adaptive by reducing energy demand on the 

 adults, and by increasing the chances of a successful fledging. 

 However, prey availability and competition with other seabirds 

 also affects prey selection. The small size of the murrelet 

 also limits its prey load, and a long flight time inland with a 

 heavy prey load would be energetically costly and would 

 subject the bird to an increased period of vulnerability to 

 inland predation. Prey also loses water during transport by 

 seabirds. Montevecchi and Piatt in Hislop and others (1991) 

 simulated transport of capelin by tying fish to a drying rack 

 mounted on a pick-up truck which was driven at 60 km/h. 

 After one hour, weight loss averaged 9 percent for male 

 capelin and 1 1 .5 percent for females. 



A detailed analysis of variation in the calorific value 

 and total energy content of the lesser sand eel (A. marinus) 



and other fish preyed on by seabirds was conducted in north 

 Scottish waters by Hislop and others (1991). They found the 

 calorific values and body weights of sand eels larger than 10 

 cm showed marked seasonal trends, and thus the total energy 

 content of a sand eel of given length in summer was 

 approximately double the spring value. Calorific values of 

 Atlantic herring also varied from month to month, but seasonal 

 cycles were less obvious. Seasonal cycles in fat content and, 

 consequently, in calorific value are generally associated with 

 the annual reproductive and feeding cycles of the fish, and 

 tend to be greater among the larger, mature members of the 

 population. Since different species of fish spawn at different 

 times, their condition cycles are out of phase to some extent. 

 And, since herring spawn in different waves, their condition 

 is not uniform at any one point in time. 



Hislop and others (1991) concluded that because fish 

 demonstrate intraspecific length-related and seasonal changes 

 in calorific value and energy content, it is unwise to generalize 

 about the relative food values of different prey species to 

 predators. They noted that sand eels have maximum calorific 

 values intermediate between those of gadoids and clupeoids. 

 Of interest, Hislop and others also noted that juvenile sand 



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



