Northeastern Naturalist Vol. 24, No. 2 K. Houle, M.D.B. English, J.-P.L. Savard, M.L. Mallory, J.-F. Giroux, and S.R. Craik 2017 165 2017 NORTHEASTERN NATURALIST 24(2):165–172 Food Habits of Flightless American Eiders (Somateria mollissima dresseri) in Québec, Canada Kenny Houle1, Matthew D.B. English2, Jean-Pierre L. Savard3, Mark L. Mallory2, Jean-François Giroux4, and Shawn R. Craik1,* Abstract - We provide the first quantitative study on the diet of Somateria mollissima dresseri (American Eider) undergoing the flightless wing molt. Twenty-nine adult females in wing molt were collected in the St. Lawrence Estuary and Gulf of St. Lawrence, QC, Canada. We identified a total of 510 prey items comprising 6 species of molluscs, echinoderms, and crustaceans from the esophagus and proventriculus of 15 birds. Mytilus edulis (Blue Mussel) was the most important prey for eiders as it occurred in 80% of the gullets and represented nearly 95% of the total aggregate prey mass, confirming the importance of this prey item for American Eiders throughout the annual cycle. The mean length of mussels consumed by eiders in the Estuary was nearly 4 times that of mussels ingested by birds in the Gulf; factors driving this difference remain unknown. Accordingly, we suggest that future work on the foraging ecology of flightess American Eiders examine how the size of mussels consumed by birds may be influenced by the distribution of various mussel size classes available on molting grounds in the St. Lawrence Estuary and in the Gulf of St. Lawrence. Introduction Ducks (Anatinae) undergo a simultaneous molt of remiges, which renders them flightless for a 3–6-week period annually (Guillemette et al. 2007, Viain et al. 2014). Mergini (sea ducks) either gather for the wing molt near nesting sites or undergo a migration to reach distant molting grounds (Salomonsen 1968, Savard and Petersen 2015). While flightless, sea ducks rely largely on their diet to meet nutrient and energetic requirements of feather replacement, and they must rely entirely on swimming to reach productive foraging sites (Fox et al. 2014, Guillemette et al. 2007, Hohman et al. 1992). Foraging capacities may be reduced for those sea ducks that use their wings for propulsion underwater because newly growing wing feathers are fragile (Fox et al. 2014, Viain et al. 2014). Accordingly, flightless sea ducks select habitats with an abundance of accessible, energy-rich foods (Derksen et al. 2015, Fox et al. 2008, Salomonsen 1968). Somateria mollissima dresseri (Sharpe) (American Eider) is one of 4 recognized subspecies of Common Eiders in North America, breeding from the south-central Labrador coast south to Massachusetts (Goudie et al. 2000, Palmer 1976). A large 1Département des Sciences, Université Sainte-Anne, Church Point, NS B0W 1M0, Canada. 2Department of Biology, Acadia University, Wolfville, NS B4P 2R6, Canada. 3 Wildlife Research Division, Science and Technology Branch, Environment Canada, Québec, QC GIJ 0C3, Canada. 4 Département des Sciences Biologiques, Université du Québec à Montréal, Montreal, QC H3C 3P8, Canada. *Corresponding author - shawn.craik@usainteanne.ca. Manuscript Editor: Gregory J. Robertson Northeastern Naturalist 166 K. Houle, M.D.B. English, J.-P.L. Savard, M.L. Mallory, J.-F. Giroux, and S.R. Craik 2017 Vol. 24, No. 2 number of these birds, particularly those breeding throughout Québec, undergo wing molt in the St. Lawrence Estuary (≥15,000 individuals) and along the north shore of the Gulf of St. Lawrence (≥30,000; Rail and Savard 2003, Savard and Lepage 2013). Surveys have identified several important molting sites for flightless female American Eiders in the St. Lawrence Estuary and Gulf region (JWGMCE 2004), with flocks consisting of a minimum of 50–100 females using nearshore areas in coastal bays (S.R. Craik, unpubl. data). Ecological research on Common Eiders has largely been focused on the breeding and wintering seasons, with large knowledge gaps existing during feather replacement when birds are flightless (Goudie et al. 2000). In particular, strategies used by American Eiders to meet the energetic and nutritional demands of wing molt are not well understood (Viain et al. 2015). Identification of resources used throughout the annual cycle is relevant to American Eiders, given recent declines in this sea duck in parts of its range (Canadian Wildlife Service Waterfowl Committee 2011). In this paper, we provide the first quantitative information on the diet of flightless American Eiders. Specifically, we studied food items recovered from females undergoing wing molt in the St. Lawrence Estuary and adjacent Gulf of St. Lawrence. Field-site Description We analyzed the diet of after-hatch-year (AHY) female American Eiders in wing molt at 2 sites on the southern shoreline of the St. Lawrence Estuary (Bic [48°22'N, 68°43'W] and Tartigou [48°45'N, 67°48'W]), and at 1 site on the north shore of the Gulf of St. Lawrence (Baie Pontbriand; 50°15'N, 62°31'W). Flocks of AHY female eiders were collected in the subtidal zone, typically 200–300 m from shore. The location of collections may not have reflected patterns of habitat use by eiders because birds may have reacted to our presence prior to being detected. Methods We collected flightless female American Eiders by shooting birds from a stationary boat during 5–10 September 2009. The length of the ninth primary feather from collected females varied between 1 and 125 mm, indicating that each bird was flightless (full-grown ninth primary = 170–175 mm; S.R. Craik, unpubl. data; Viain et al. 2014). We immediately flooded the upper digestive tract with 95% ethyl alcohol to minimize post-mortem digestion and autolysis (Swanson and Bartonek 1970). Within 10 h of collection, prey items in the gullet (esophagus and proventriculus) were removed and preserved in 85% ethyl alcohol. We identified prey to species whenever possible using standard identification literature (Brinkhurst et al. 1975). We counted prey items and measured to the nearest 0.1 g the total wet mass of each prey taxon. For each prey taxon, we calculated (1) frequency of occurrence (%), by dividing the number of gullets that contained the taxon by the total number of gullets with ≥1 prey item, and (2) aggregate percent wet mass, by dividing the total wet mass for the taxon by the total wet mass for all prey taxa. Northeastern Naturalist Vol. 24, No. 2 K. Houle, M.D.B. English, J.-P.L. Savard, M.L. Mallory, J.-F. Giroux, and S.R. Craik 2017 167 Length of each prey item (± 1 mm) was measured with vernier calipers. We calculated mean (± SD) length for each prey taxon and used analysis of variance (ANOVA) to compare the mean length of Mytilus edulis L. (Blue Mussel) ingested by eiders in the St. Lawrence Estuary with that of birds in the Gulf of St. Lawrence. We employed a variance components analysis to determine the amount of total variation in mussel lengths that was attributed to differences between regions (Estuary and Gulf) and among individual eiders. For gullets containing >100 Blue Mussels, we selected 50 mussels randomly for length measurements. Results We collected 29 flightless female American Eiders. Fifteen birds had ≥1 prey item in their gullet and were used for diet analyses. Mean ± SD number of prey items recovered per gullet was 30.3 ± 23.3 (range = 4–72) for 9 birds in the Estuary and 59.6 ± 82.2 (range = 3–205) for 6 birds in the Gulf. Overall, the Blue Mussel was the most common prey identified in molting female eiders; the mollusc occurred in all but 3 gullets and represented just over 90% of aggregate prey mass (Table 1). On average, 44 mussels (± 16.7; mean ± SD) were recovered per gullet, and one stomach contained 205 mussels. All 6 eiders containing at least 1 Strongylocentrotus droebachiensis (O.F. Müller) (Green Sea Urchin) were from the St. Lawrence Estuary. Eiders occasionally contained crustaceans (Cancer irroratus Say [Atlantic Rock Crab]) and other molluscs (Littorina spp. [periwinkles], Nucella lapillus (L.) [Dogwhelk]). The length of nearly 90% of all mussels (n = 199) measured from birds in the St. Lawrence Estuary was between 10.5 and 42 mm (Fig. 1). All but 1 of the 139 Table 1. Frequency of occurrence, length (mm), and aggregate percent wet mass of prey taxa recovered from the gullet (esophagus and proventriculus) of female Somateria mollissima dresseri (American Eider) in wing molt in the St. Lawrence Estuary (n = 9) and Gulf of St. Lawrence (n = 6), Québec, Canada, 2009. Aggregate # of prey Frequency of Length of % wet items from occurrence of prey item (mm) mass of Taxon all birds prey itemA Mean SD Range prey item Mytilus edulis (Blue Mussel) 494 80 11.4B 8.4 2.0–42.0 93.2 Stronglylocentrotus droebachiensis 7 40 15.5C 3.3 12.0–20.0 3.5 (Green Sea Urchin) Cancer irroratus (Atlantic Rock Crab) 2 13 21.0 1.4 20.0–22.0 1.0 Littorina spp. (Periwinkle) 3 13 3.8 1.3 2.5–5.0 0.1 Nucella lapillus (Dogwhelk) 1 7 35.0 - - 2.4 Polychaeta 1 7 27.5 - - 0.1 Unidentified gastropod shell fragment 2 13 - - - - Total 510 APercent of eider gullets containing item. BMean, SD and range for Blue Mussel based on a sample of 338 mussels. For gullets containing >100 mussels, 50 mussels were selected randomly for length measurements. CDiameter of tests (mm). Northeastern Naturalist 168 K. Houle, M.D.B. English, J.-P.L. Savard, M.L. Mallory, J.-F. Giroux, and S.R. Craik 2017 Vol. 24, No. 2 mussels measured from eiders in the Gulf were ≤10 mm in length (Fig. 1). Blue Mussels ingested from eiders in the St. Lawrence Estuary (mean ± SD = 18.1 ± 7.0 mm) were nearly 4 times longer than those ingested by birds in the Gulf of St. Lawrence (5.0 ± 1.7 mm; F1,336 = 469.4, P < 0.001). The majority (81%) of the total variation in mussel lengths was attributed to differences between the St. Lawrence Estuary and Gulf of St. Lawrence. Discussion Our study is the first to document food habits of flightless American Eiders. The sample of eiders collected that contained at least 1 food item in their gullet was small (n = 15), and it had limited temporal (early September 2009) and spatial (3 study sites) replication. Despite this, we found that most birds fed almost exclusively on Blue Mussels. The Blue Mussel is a dominant prey item for pre-breeding, brood-rearing, and wintering Common Eiders in eastern North America (Cantin et al. 1974, Cottam 1939, Goudie et al. 2000, Guillemette et al. 1992), and strong selection for Blue Mussel has been observed among wintering eiders (Goudie and Ankney 1986). It is unknown whether or not flightless eiders preferred mussels over other types of prey, because the availability of potential food items at molting sites has yet to be addressed. Green Sea Urchins, for example, may be particularly important to at least some molting eiders in the St. Lawrence Estuary, given that 6 of 9 (67%) birds collected in the region contained at least 1 urchin. Nevertheless, our results, combined with eider diet information from outside the wing molt period, Figure 1. Frequency distribution of Mytilus edulis (Blue Mussel) lengths recovered in the gullets of 15 flightless female Somateria mollissima dresseri (American Eider) collected in the St. Lawrence Estuary (n = 9) and in the Gulf of St. Lawrence (n = 6), Québec, Canada, 2009. Northeastern Naturalist Vol. 24, No. 2 K. Houle, M.D.B. English, J.-P.L. Savard, M.L. Mallory, J.-F. Giroux, and S.R. Craik 2017 169 suggest that Blue Mussels are among the most important prey for female American Eiders throughout the annual cycle. Sea ducks may meet the energetic and nutritional demands of feather replacement by increasing foraging effort and/or consuming foods that favor energy gain (Hohman et al. 1992, Pethon 1967). The use of highly productive environments may be particularly important for flightless Common Eiders given that they exhibit reduced foraging effort early during wing molt when newly growing wing feathers are fragile (Viain et al. 2015). Diets of molting eiders are largely unknown, save for work on flightless Somateria spectabilis (L.) (King Eider) in Western Greenland (Frimer 1997) and Common Eiders in Norway (Pethon 1967). Both studies found that adult male and female eiders during their flightless wing molt ingested a variety of relatively high-energy prey items, including molluscs and crustaceans (Frimer 1997, Pethon 1967). Blue Mussels are an abundant food source for Common Eiders in the St. Lawrence Estuary and Gulf (Cantin et al. 1974, Guillemette and Himmelman 1996), and eider distribution in this region is associated with that of Blue Mussels (Diéval et al. 2011). Foraging on mussel reefs may be relatively efficient for flightless eiders due to large concentrations of the stationary prey, and consequently birds may spend relatively little time foraging for other hard-bodied prey such as urchins or crabs (Guillemette et al. 1992). Small mussels require little handling prior to ingestion; they are swallowed whole underwater, while larger molluscs generally have thicker shells and they require more energy to handle and digest (Goudie et al. 2000, Jorde and Owen 1988). The available energy from Blue Mussels per gram of live tissue is greater than that for some other hard-bodied prey (e.g., urchins; Guillemette et al. 1992) and when the shell is removed is greater than soft-bodied prey (e.g., amphipods; Petersen 1981). Eiders may select small mussels over larger individuals to maximize the amount of energy (tissue) ingested relative to shell intake (Bustnes and Erikstad 1990, Hamilton et al. 1999). Mussels consumed by flightless American Eiders in our study were generally small, despite a large discrepancy between lengths of mussels consumed by females in the Estuary (mean of 18 mm) and those from birds in the Gulf (mean of 5 mm). Eider foraging habitat (e.g., use of intertidal and/or subtidal areas) and the distribution and abundance of various mussel size classes available to molting birds at our sites in the Estuary and Gulf were not assessed, so it remains unclear as to whether or not flightless eiders generally preferred small mussels and avoided larger size classes of the prey. Alternatively, flightless eiders may prefer the most common mussel sizes available to them on molting grounds. Laursen et al. (2009) found that the length of Blue Mussels taken by wintering Common Eiders in the Wadden Sea corresponded to the most abundant mussel size class growing on beds during their study. We suggest that future work on the foraging ecology of molting American Eiders examine how (1) the choice of prey items is influenced by habitat use and (2) the size of mussels consumed by eiders interacts with the distribution and abundance of various mussel size classes available on molting grounds in the St. Lawrence Estuary and Gulf of St. Lawrence (Guillemette et al. 1996). Northeastern Naturalist 170 K. Houle, M.D.B. English, J.-P.L. Savard, M.L. Mallory, J.-F. Giroux, and S.R. Craik 2017 Vol. 24, No. 2 Estimates indicate that ≥45,000 American Eiders undergo an ~6-week wing molt in the St. Lawrence region (Rail and Savard 2003, Viain and Guillemette 2016). Many of the molting birds are females congregated in flocks of at least 50 to 100 birds (S.R. Craik, unpubl. data). Given these concentrations of flightless female eiders and their dependence on Blue Mussel, we suggest that consumption of mussels on particular reefs by these birds is significant. It is suspected that Blue Mussel is also an important dietary component for male Common Eiders during wing molt (Pethon 1967). Foraging male eiders would thus exacerbate any local effects of mussel stock depletion by females where molting distributions of the two sexes overlap. On the north shore of the Gulf of St. Lawrence, the number and distribution of wintering male and female Common Eiders in a flock is correlated with local mussel availability (Guillemette and Himmelman 1996), indicating that eiders deplete mussel stocks evenly across reefs (Guillemette et al. 1996). Wintering Melanitta perspicillata (L.) (Surf Scoter) have been documented depleting mussel beds in a relatively short period of time (Lacroix 2001). Given the flightless nature of the wing molt, molting American Eiders are more restricted in their distribution than during other periods of the annual cycle (Derksen et al. 2015). It would be reasonable to predict that flightless eiders crowd into productive feeding patches and subsequently deplete Blue Mussel stocks to a certain level (Guillemette and Himmelman 1996) because flocking behavior would improve the ability of flightless birds to sample and to find mussel beds, and to increase feeding efficiency (Pulliam and Caraco 1984). A study examining the potential relationship between (1) flightless American Eider distribution and density and (2) Blue Mussel availabilities on specific reefs in the St. Lawrence Estuary and Gulf would provide novel insight into how flightless sea ducks meet the energetic and nutrient demands of wing molt. Also, it may be pertinent from a conservation perspective to protect heavily used mussel beds. 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