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238 Southeastern Naturalist Vol. 12, No. 1
Roof-nesting Least Terns Travel to Forage in Brackish/
Elizabeth A. Forys1,*, Arya Poppema-Bannon2, Kristina Krajcik1,
and William A. Szelistowski2
Abstract - Sternula antillarum (Least Tern) are small, migratory seabirds that are listed
as threatened or endangered in every state in which they occur. While their natural nesting
habitat is open beach, Least Terns have adapted to nest on flat tar and gravel roofs,
sometimes many kilometers inland. We used the “fish drop” technique to collect fish that
birds dropped around a roof colony in Pinellas County, FL to assess use of fresh and
saltwater fish species as prey. We collected 37 fish from 12 different species. The most
common (n = 11) was Dorosoma petenense (Threadfin Shad), a species found primarily in
marine/brackish waters, but often stocked in man-made freshwater ponds to supplement
piscivorous fish prey. Eighteen of the remaining 26 fish, encompassing 7 of the remaining
11 species, are marine/brackish species, 5 individuals are freshwater species, and 3 are
freshwater/brackish species. Roof-nesting Least Terns appeared to forage in stormwater
ponds and a lake found near the colony, but they also traveled a minimum of 4.8 km to
forage in marine/brackish waters. Roof nesters may face greater challenges than beach
nesters, because they are potentially exposed to more pollutants in stormwater ponds and
expend more energy and time to travel farther to forage at saltwater sites.
Sternula antillarum Lesson (Least Tern) is a small, colonial-nesting seabird
which breeds along portions of both the eastern and western coasts of North
America and throughout major rivers in the interior United States (Thompson et
al. 1997). Because of past over-hunting, habitat loss, and disturbance on beaches,
Least Terns are listed as threatened or endangered in every state in which they
occur (Thompson et al. 1997). One adaptation the birds have developed is to nest
on tar and gravel roofs (Forys and Borboen-Abrams 2006). In surveys conducted
from 1998–2000 throughout Florida, 83.5% of the total breeding population
nested on roofs (Gore et al. 2007). As of 2011, there were 15 active roof-nesting
colonies of Least Terns in Pinellas County, FL (Florida Fish and Wildlife Conservation
Least Terns are surface plungers that search for small fish from above the
water surface (Eriksson 1985). After capture, they either consume the fish while
flying or carry it to the nest to feed their mate and chicks (Atwood and Kelly
1984). Least Terns consume a wide variety of small fish, and it appears that fish
size (2.5–9.0 cm) is more important to prey choice than type of species (Thompson
et al. 1997). Beach-nesting Least Terns generally feed <2 km from colonies
1Environmental Studies Discipline, Eckerd College, 4200 54th Avenue South, St. Petersburg,
FL 33711. 2Marine Science Discipline, Eckerd College, 4200 54th Avenue South, St.
Petersburg, FL 33711. *Corresponding author - firstname.lastname@example.org.
2013 SOUTHEASTERN NATURALIST 12(1):238–242
2013 E.A. Forys, A. Poppema-Bannon, K. Krajcik, and W.A. Szelistowski 239
until chicks have fledged (Atwood and Minsky 1983). While nearly all Florida
roof colonies are located in coastal counties, some roofs are >10 km from saltwater
(Gore et al. 2007), indicating that roof nesters either travel farther than
beach nesters or forage in other bodies of water. The purpose of our study was to
determine foraging location by identifying species of fish that roof-nesting Least
Terns brought back to the colony.
Study Area and Methods
This study was conducted at a large Least Tern colony (≈200 nesting pairs) located
on the tar and gravel roofs of two adjacent warehouses in an industrial park
in Pinellas County, FL, approximately 8 km from the Gulf of Mexico (Fig. 1).
To determine which fish were brought to the colony, we used the “fish drop”
technique (Atwood and Kelly 1984). Fish brought to feed a mate or chick are occasionally
dropped by accident or because they are unsuitable for consumption
Figure 1. Location of the roof-colony warehouses (W) in relation to bodies of water suitable
for Least Tern foraging in Pinellas County, FL. Lake Seminole and the stormwater
ponds (shaded in dark grey) are freshwater. Boca Ciega Bay ranges from brackish (<5
ppt) in its upper reaches near Lake Seminole to nearly marine (>31 ppt) in the lower portions
of the Bay adjacent to the Gulf of Mexico.
240 Southeastern Naturalist Vol. 12, No. 1
because of size or presence of spines. Dropped fish were collected from the pavement
surrounding the warehouses daily from 2–23 June 2010 and 1–18 June 2011
while the colony was feeding primarily unfledged chicks. The age of the chicks
varied throughout the sample period. Dropped fish were fixed in 10% formalin
solution and stored in 60% ethanol.
Potential sources of these fish included five freshwater retention ponds (located
0.4–1.0 km from the colony), a 289-ha freshwater lake (Lake Seminole, located 0.8
km from the colony), and a nearby estuarine bay (Boca Ciega Bay, located 4.8 km
from the colony). We looked for Least Terns foraging at these locations twice per
week from 2–23 June 2010. Observations were made a few meters from the shore
to avoid disturbance using binoculars (Swift 10 x 42), lasted 30 minutes, and were
made between peak foraging hours of 0700–0900.
Results and Discussion
Least Terns were observed foraging at least once at all sites surveyed, although
it is possible that some birds observed were from other roof or ground
colonies. We collected a total of 37 fish from 12 species (Table 1). The most
common (n =11) was Dorosoma petenense (Threadfin Shad), a species found
primarily in marine/brackish waters but often stocked in man-made freshwater
ponds to supplement piscivorous fish prey (DeVries et al. 1991). Eighteen of
the remaining 26 fish, encompassing 7 of the remaining 11 species recorded,
are primarily marine/brackish species, 5 individuals are freshwater species,
and 3 individuals are common in both brackish and freshwater. This study is
the first published record of 5 of these species being used by Least Terns (Table
1; Thompson et al. 1997, Zuria and Mellink 2005). Fish size ranged from
30–95 mm, which is generally within the size range of fish ingested by Least
Terns (Table 1).
Table 1. Fish dropped by Least Terns at a warehouse roof colony in Largo, FL. Fish sizes are expressed
in standard length (SL). # = number collected. Size range given in mm SL.
Scientific name Common name # range Primary habitat
Dorosoma petenense (Günther) Threadfin Shad 11 35–65 Marine/brackish/freshwater
Hyporhamphus unifasciatus Halfbeak* 7 74–95 Marine/brackish
Mugil cephalus L. Striped Mullet 3 47–60 Marine/brackish
Anchoa mitchilli (L.) Bay Anchovy 3 50–54 Marine/brackish
Lepomis macrochirus Rafinesque Bluegill 3 42–52 Freshwater
Brevoortia sp. Menhaden 2 49–51 Marine/brackish
Poecilia latipinna (Lesueur) Sailfin Molly* 2 50–52 Freshwater/brackish
Micropterus salmoides (Lacépède) Largemouth Bass 2 43–64 Freshwater
Lagodon rhomboides L. Pinfish* 1 42 Marine/brackish
Sardinella aurita Valenciennes Spanish Sardine* 1 62 Marine/brackish
Leiostomus xanthurus Lacépède Spot* 1 50 Marine/brackish
Gambusia holbrooki Girard Eastern Mosquitofish 1 30 Freshwater/brackish
*Indicates species not previously recorded as being used by Lea st Terns.
2013 E.A. Forys, A. Poppema-Bannon, K. Krajcik, and W.A. Szelistowski 241
Research in 2008 and 2009 at Lake Seminole found all these freshwater and
freshwater/brackish species to be present, but did not find any of the marine/
brackish species (Champeau et al. 2009, Pouder et al. 2010). These saltwater
species are dependent on marine waters at some point in their life cycles and
therefore would not have been captured in isolated freshwater bodies such
as Lake Seminole or the stormwater ponds. The closest location where Least
Terns could have obtained these fish is Boca Ciega Bay, found 4.8 km from the
nesting sites. Marine/brackish species were brought to the colony on at least
12 separate days and during both nesting seasons, suggesting that foraging in
marine/brackish areas was a common practice for roof nesters.
Compared to Least Terns examined in other studies, roof nesters traveled
farther to obtain food between chick hatching and fledging and likely spent more
time away from their mates and young. Atwood and Minsky (1983) observed
California Least Terns feeding within a 2-km radius from the beach colony until
chicks had fledged. Interior Least Terns foraged up to 12 km away from their
colony during the egg-laying period and when the chicks were fledging, but
stayed close to the colony while brooding and feeding young chicks (Schweitzer
and Leslie 1996). Interior Least Terns nesting on spoil piles—an artificial habitat—
foraged 1.5 km or more, but still remained closer than the distance birds
traveled in our study (Wilson et al. 1993). Further research should investigate
if the additional energy and time needed to forage farther away decreases nest
productivity and/or increases mortality of adult Least Terns and should include
multiple roof colonies.
It appears there are advantages to foraging in Bay and Gulf waters rather than
in much closer freshwater ponds and Lake Seminole. Least Terns elsewhere have
shifted their foraging locations in response to prey abundance (Zuria and Mellink
2005). Additionally, if turbidity is lower in marine waters or if fish swim closer
to the surface, these conditions would favor foraging by Least Terns because of
their surface-plunging feeding technique (Eriksson 1985).
Least Terns foraging in freshwater ponds and lakes could be ingesting fish
with elevated levels of heavy metals and pesticide byproducts (Allen et al. 1998).
Future research should also examine the potential impact of contaminants on
roof-nesting Least Terns.
Many volunteers of St. Petersburg and Clearwater Audubon helped with this project,
including R. Hanley, J. Hood, B. and S. Jenks, M. Korosy, and W. Meehan. M. Flock
of the Pinellas County Department of Engineering and Environmental Services and B.
Pouder of the Florida Fish and Wildlife Conservation Commission provided access to
data on Lake Seminole fishes. This research was funded in part by grants from the Natural
Science Summer Research Program of Eckerd College.
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