Heermann's Gull Larus heermanni
Version: 2.0 — Published April 9, 2020
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Diet and Foraging
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Main Foods Taken
A generalist predator feeding on a wide variety of small fish such as Pacific sardine (Sardinops sagax), northern anchovy (Engraulis mordax), and Pacific mackerel (Scomber japonicus), smelt (Osmeridae), and other species (83, 107, 10, 3, 4, 11, 5). Additionally, feeds on pelagic crabs, euphasids, shrimps and other crustaceans, amphipods, small mollusks, squid, lizards, and insects (108, 109, 110, 74, 111, 50, 10, 3, 4, 11, 5). At breeding colonies, may prey on eggs of Elegant Tern (Thalasseus elegans) and Royal Tern (T. maximus); also may prey on eggs of congeners and rarely conspecifics (112, 19). Also, scavenges with other gulls along shoreline and beaches and follows some fishing boats for fishermen's refuse and carrion (31, 101, EV). Seaside individuals seem to be feeding mainly on Pacific sand crabs (Emerita analoga) (J. Chin unpublished data).
Microhabitat for Foraging
Forages primarily over the ocean surface usually within 8 km of land (85). Favored feeding grounds are kelp beds located 46–91 m from shore. Also, forages on intertidal and rocky shorelines (108, 110, 74). During the nesting season may forage close to nesting islands or in nearby bays if fish shoals occur in those areas.
Food Capture and Consumption
From Burger (111). Foraging methods in California include picking up food from the ground (pick up) and water surface (surface dip). To obtain food below water while swimming, jumps up and then dives down into water to secure item (jump plunge). When flying, plunges into water in pursuit of fish (surface plunge). Surface plunge (75%) and surface dip (25%), however, formed bulk of the observed foraging methods.
Along the Pacific Northwest coast, often hovers over breakers to catch smelt from the crests of waves as these fish head to spawn along beaches in August (107). When herring swim in schools near surface, approaches the school from behind in flight making quick, repeated dips into the school. As soon as the herring return to the surface after initial attack, these gulls make a wide circuit and return to pursue the school from behind (113).
Congregates around feeding flocks of birds (e.g., cormorants, boobies, pelicans) and marine mammals (e.g., seals, sea lions, sea otter [Enhydra lutris]). Kleptoparasitizes feeding California sea lions (Zalophus californianus) by seizing scraps of meat (16). In addition, a wide range of birds are actively pursued in jaeger-like style to pirate their catches, including Blue-footed Booby (Sula nebouxii), Ringed Kingfisher (Ceryle torquata), Bonaparte’s Gull (Chroicocephalus philadelphia), Elegant Tern, and Royal Tern (114, 115, 116, 117, 118). However, Brown Pelicans are most often kleptoparasitized. Heermann’s Gulls initiate a kleptoparasitic attempt by flying directly toward a diving pelican. After the pelican plunge dives and bobs up to the surface, these gulls position on either side of the pelican and attempt to dislodge fish directly from gular pouch (113, 83, 100). Tershy et al. (119) observed 96 kleptoparasitic attempts on Brown Pelicans by 89 adult and 7 immature Heermann’s Gulls in the Gulf of California, Mexico. Adult gulls were attracted to 26.1% (70/268) of the dives by adult Brown Pelicans but only to 4.2% (19/453) of dives by immatures. In contrast, all 7 kleptoparasitic attempts by immature gulls were on immature Brown Pelicans. Thus, adult gulls appear to preferentially select and attack adult Brown Pelicans as they are more likely to be successful in obtaining prey for any given dive. This feeding method is so typical that the indigenous Comcáac culture from the coastal state of Sonora, Mexico, has a traditional song-story related to it (120).
Major Food Items; Quantitative Analysis
Velarde et al. (10) conducted detailed studies of the diet during the nesting season, from April through June on Isla Rasa, Gulf of California, Mexico between 1983 to 1992, and further studies until 2012 (3, 4, 11, 5). Pacific sardine and northern anchovy, were the dominant food items. Contents of regurgitations by year (10) revealed the following: 1983—Pacific sardine (97%, n = 43) and northern anchovy (3%, n = 1); 1984—Pacific sardine (64%, n = 73) and northern anchovy (36%, n = 42); 1989—Pacific sardine (13%, n = 2) and northern anchovy (87%, n = 13); 1990—northern anchovy (100%, n = 36); 1991—northern anchovy (98%, n = 90) and others (2%, n = 2); 1992—Pacific sardine (2%, n = 1), northern anchovy (96%, n = 48), and others (2%, n = 1). Based on correlation analyses in this and latter works, the proportions of sardines and anchovies in diet were positively correlated with the total commercial landing of sardines and anchovies in the following fishing season; thus, these findings suggest that variation in diet from year-to-year is correlated with availability of the pre-recruits of these prey species which will be later available to the fishing fleet. Additional data to support these findings was a negative correlation between proportion of sardines in the diet versus the proportion of anchovies in diet and total commercial landing of anchovies.
Food Selection and Storage
According to some studies, fish species seem to be selected according to their availability in the environment (10, 3, 4, 11, 5, 6), suggesting that Heermann's Gull can be used as a reliable indicator of prey abundance. In some of these studies, where diet analyses of this species have been compared to those of California Brown Pelican (Pelecanus occidentalis californicus) and Elegant Tern (Thalasseus elegans), similar trends in prey composition have been found for all 3 species, strongly suggesting that all are taking prey according to their relative abundance in the environment. These studies have shown that the composition of the diet of these seabirds may aid in the management of the fish they prey upon, particularly the Pacific sardine. Breeding adult Heermann´s Gulls, as is common to other gull species, store food in their crop, which will later be used, in the case of males, to feed the female during the courtship period or, in both sexes, the food that will be fed to their chicks during the chick rearing period.
Nutrition and Energetics
Metabolism and Temperature Regulation
From Ellis and Frey (14). Measurements of oxygen consumption and body temperature (Tb) at different ambient temperatures (Ta) were used to develop a metabolic profile. Basal metabolism (Hb) was 122.6% of that predicted for a gull weighing 383 g (n = 5). Across a range of Ta (~22–37°C), mean Tb was 41.2°C.
From Bennett and Dawson (12). Oxygen consumption, heart rate, and thermal tolerance were measured in week-old embryos. Heart rate and oxygen consumption were temperature independent between 30–40°C and averaged 120 beats/min and 1.87 cm3 O2/(egg-hr), respectively. Below 30°C, however, heart rate became strongly temperature dependent; heart stopped beating when embryos cooled between 7–13°C but recovered when embryos rewarmed even after 1 h at 6°C. Similarly, heart stopped beating when embryos heated above 40.0–41.6°C but recovered when cooled. However, heart beat did not recover after embryos were exposed to 43°C for 1 h. This thermal dependence of embryonic function over the 30–40°C range minimizes disruptions associated with variations in temperature during incubation. Although unattended eggs at night may chill to temperatures which cause cessation of heart beat, such exposure is not lethal if the eggs are rewarmed. In contrast, exposure to radiant heat during the day can rapidly raise egg temperature to lethal levels. Thus, diurnal incubation aids in shading the eggs to prevent overheating and death of the developing embryo.
There is behavioral regulation of temperature in adult birds, particularly during incubation when they are unable to leave the nesting site and ambient temperatures are high (19), where raising of feathers of different parts of the body aids in the formation of an air space, isolating the bird from solar radiation (see Breeding: Parental Care). Also, incubating adults will stand over the eggs or very small chicks during the hottest time of the day, in order to protect them from solar radiation and, at the same time, allow the circulation of air between them and the eggs or chicks. Chicks of all ages seek cover from solar radiation during the hottest hours of the day under the shade of the parent attending the nest (121).
Drinking, Pellet-Casting, and Defecation
As with most medium-sized gulls, drinks both salt and fresh water by dipping bill into water and lifting head to swallow. Excessive salt excreted into each nostril from nasal glands (74).
No information on pellet-casting.
One short study observed defecation in relation to nesting substrate, nest density, age, and associated behaviors on Isla Rasa (122). This study found that defecation was higher in the valley areas, where nesting density was higher, versus rocky hills, where nest density was lower. Also, chicks had higher defecation frequency because, unlike adults that left the area to feed, bathe, etc., chicks stayed permanently in the nesting territory while adults tended to move away from the territory to defecate. Under certain circumstances, adult defecation rates were associated with aggressive behaviors; in the valley area, defecation was higher for males than in the rocky hills. In the rocky hills, females had a higher defecation rate associated with aggressive behaviors. Because these birds feed on fish, their feces known as guano, is very rich in nitrates and phosphates. Guano accumulates in large amounts in the nesting areas (mainly islands) and this resource was intensely sought after in the past. Guano extraction constituted an important industry in Mexico (7) when it was used mainly to produce fertilizers and gunpowder; at present, guano mining is no longer profitable and it is not practiced any more in the area.