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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
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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.
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