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Unusually Large Wintering Flock of Common Loons Foraging in the Gulf of Mexico
Darwin Long IV and James. D. Paruk

Southeastern Naturalist, Volume 13, Issue 4 (2014): N49–N51

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N49 2014 Southeastern Naturalist Notes Vol. 13, No. 4 D. Long IV and J.D. Paruk Unusually Large Wintering Flock of Common Loons Foraging in the Gulf of Mexico Darwin Long IV1 and James. D. Paruk1,* Abstract - Gavia immer (Common Loon) utilizes freshwater lakes for nesting and breeding, but winters off both US coasts and the Gulf of Mexico in marine habitats. They are primarily piscivorous. In the winter, Common Loons feed predominately as single individuals, but they occasionally form small groups or flocks. The groups likely facilitate greater feeding efficiency. The largest previously reported foraging aggregation of wintering Common Loons was approximately 200 individuals. We report an extremely large wintering flock of >600 loons foraging 17 km from the southern coast of Mississippi. More work is needed to understand the prevalence of larger groups of overwintering loons as well as temporal and spatial factors that might predict their occurrence. Introduction. Gavia immer Brunnich (Common Loon) exhibit variation in foraging behavior across their winter range (Vlietstra 2000). In Rhode Island, for example, they are primarily solitary and occasionally form small groups (<25 individuals; Daub 1989, Ford and Gieg 1994). In North Carolina and Florida, they may form large groups of 50 to 200 individuals (Jodice 1993, Vlietstra 2000). Geographic differences in foraging behavior within a species are often attributed to variation in resource availability (Cairns 1987, Schoener 1971). Common Loons are primarily piscivores, and their spatial and seasonal distribution is likely tied to the shifting abundance of their small fish prey (Ford and Gieg 1994, Vlietstra 2000). Small-fish abundance and distribution are driven by shifting tide lines, salinity gradients, and water depth and clarity (Lee 1987, Haney 1990, McIntyre 1978, Vlietstra 2000). Other avian piscivores—Morus bassanus L. (Northern Gannet), Pelecanus occidentalis L. (Brown Pelican), Phalacrocorax auritus Lesson (Double-crested Cormorant), Larus spp. (gulls), and Sterna spp. (terns)—are often found in association with larger Common Loon foraging flocks (>15 individuals; Vlietstra 2000). Observations. On 25 Feb 2011, at 14:56 CT, we encountered an extremely large aggregation of Common Loons foraging 17 km off the Mississippi shoreline between Ship Island and Cat Island (30°13'3.0894''N, 89°2'14.1972''W). The weather was clear, the wind was southwest at 5km/h or less, and the water was calm with waves less than 0.5 m in height. The flock occupied an area approximately 1.5 km wide and 3 km in length. The lead author counted visible individuals in one single 360 degree pass by boat. The inter-bird spacing of Common Loons was approximately 2–3 m apart; thus, counting individuals was fairly straightforward. The aggregation consisted of smaller, loosely associated groups that sometimes temporarily separated by swimming a short distance away, then rejoined the larger flock. As we made our count, some Common Loons were diving underwater, so we estimated 20% to 25% of the aggregation were below the surface as we counted. We counted 500–550 individuals and estimate the Common Loon foraging flock consisted of 600–687 individuals. The great majority of the birds were actively foraging, but a few individuals were preening. We heard both hoot calls and short wails; the Common Loons were likely calling to retain overall cohesion among the group (McIntyre 19 78). 1Biodiversity Research Institute, 276 Canco Road, Portland, ME 04103. *Corresponding author - jim. paruk@briloon.org. Manuscript Editor: Wylie Barrow Notes of the Southeastern Naturalist, Issue 13/4, 2014 2014 Southeastern Naturalist Notes Vol. 13, No. 4 N50 D. Long IV and J.D. Paruk Discussion. To our knowledge, this is the first report of such a large foraging group of wintering Common Loons; our count exceeded the previously reported maximum winter aggregation by approximately 400–500 individuals (Jodice 1993, Vlietstra 2000). This unusually large assemblage was foraging and diving in unison, much like Double-crested Cormorants, as first described by Bartholomew (1942). We suspect this Common Loon aggregation was likely feeding on a large school of fish. Support for this conclusion includes: a) the group moved approximately 2.75 km against the wind and water current, b) loons dove in a continuous fashion without pause or rest, and in random formation, and c) we occasionally observed fish catapulting out of the water ahead of diving loons, at times causing the water to roil with activity. Vlietstra (2000) reported that Common Loons observed in multispecies groups foraged more than those in monospecific groups. She also noticed that formation of multispecies groups was independent of tide stage (low or high) or time of day. We observed this large flock between islands where tidal exchange currents or upwelling likely influenced the abundance of fish in the area. This situation suggests that formation of extremely large rafts of Common Loons is opportunistic and fundamentally based upon the movements of prey, rather than being based on water depth or sea condition (as appears to be the case for smaller rafts of less than 20 individuals; Vlietstra 2000). Winter foraging ecology in Common Loons is poorly understood (Evers et al. 2010). Although Common Loons typically occupy inshore waters, they may range up to 100 km offshore across the continental shelf (Haney 1990, Kenow et al. 2002, Lee 1987). The flock we observed was approximately 17 km offshore, providing more support for the suggestion that Common Loons in the Gulf of Mexico may forage many kilometers from shore. Also, it is important to emphasize that this observation took place in mid-winter and was not as large an aggregation of Common Loons as is typically observed in the Gulf of Mexico prior to spring migration (Kratter 2009). Future research involving surveys of broad-scale Common Loon movements coupled with data on regional shifts in water salinity and species-specific fish movements would improve our understanding of factors influencing intraseasonal resource use by this species. Acknowledgments. We thank our boat captain Ray Schmitt and the Saltgrass Outdoors staff for providing logistical support, and I. Stenhouse and K. Taylor for providing helpful comments on an earlier draft of the manuscript. These data were collected during a national resource damage assessment and have been approved for publication by the US Fish and Wildlife Service. Literature Cited Bartholomew, G.A., Jr. 1942. The fishing activities of Double-crested Cormorants of San Francisco Bay. The Condor 44(1):13–21. Cairns, D.K. 1987. Seabirds as indicators of marine food supplies. Biological Oceanography 5:261–271. Daub, B.C. 1989. Behavior of Common Loons in winter. Journal of Field Ornithology 60:305–311. Evers, D.C., J.D. Paruk, J.W. McIntyre and J.F. Barr. 2010. Common Loon (Gavia immer). Number 313. In A. Poole (Ed.). The Birds of North America. Cornell Lab of Ornithology, Ithaca, NY. Available online at http://bna.birds.cornell.edu/bna/species/313/. Accessed 10 October 2013. Ford, T.B., and J. Gieg. 1994. Winter behavior of the Common Loon. Journal of Field Ornithology 66:22–29. Haney, J.C. 1990. Winter habitat of Common Loons on the continental shelf of the southeastern United States. Wilson Bulletin 102:253–263. Jodice, P. 1993. Distribution of wintering loons in the northeastern Gulf of Mexico. Pp. 172–193, In L. Morse, S. Stockwell, and M. Pokras (Eds.). Proceedings of the 1992 North American Loon Conference: The loon and its ecosystem. Bar Harbor, ME. US Fish and Wildlife Service, Concord, NH. 247 pp. N51 2014 Southeastern Naturalist Notes Vol. 13, No. 4 D. Long IV and J.D. Paruk Kenow, K.P., M.W. Meyer, D.C. Evers, D.C. Douglas, and J. Hines. 2002. Use of satellite telemetry to identify Common Loon migration routes, staging areas, and wintering range. Waterbirds 25(4):449–458. Kratter, A.W. 2009. Crossing the isthmus: Overland spring migration of Common Loons (Gavia immer) in Alachua County, FL. Florida Field Naturalist 37(1):8–15. Lee, D.S. 1987. Common Loons wintering in offshore waters. Chat 51:40–42. McIntyre, J.W. 1978. Wintering behavior of Common Loons. Auk 95:396–403. Schoener, T.W. 1971. Theory of feeding strategies. Annual Review in Ecology and Systematics 2:369–404. Vlietstra, L.S. 2000. Local variation in foraging strategies employed by wintering Common Loons. Pp. 25–34, In J. McIntyre and D.C. Evers (Eds.). Proceedings of the 1997 North American Loon Conference: Old history and new findings. Holderness, NH. North American Loon Fund, Holderness, NH. 115 pp.