Download POPULATION SURVEY OF THE INTERIOR LEAST TERN

POPULATION SURVEY OF THE INTERIOR LEAST TERN
ON THE MISSISSIPPI RIVER
CAPE GIRARDEAU, MISSOURI
TO
VICKSBURG, MISSISSIPPI
2003
By
URS Corporation in association with
Dyersburg State Community College
Kenneth H. Jones
Associate Professor of Biology
July 2003
Population Survey of the Interior Least Tern
On the Mississippi River from
Cape Girardeau, Missouri, to Vicksburg, Mississippi
2003
by
URS Corporation in association with
Dyersburg State Community College
Kenneth H. Jones
Associate Professor of Biology
Dyersburg State Community College
1510 Lake Road
Dyersburg, Tennessee 38024
On and between July 13 and July 17, 2003, URS Corporation, with subcontractor
Dyersburg State Community College, conducted a survey of interior Least Tern (Sterna
antillarum athalassos) nesting colonies along the Mississippi River. This annual survey is
performed for the Memphis District Army Corps of Engineers and assists the U.S. Fish and
Wildlife Service with its census in determining population size and distribution of the interior
Least Tern. This survey was the 19th consecutive annual count since the surveys were initiated in
1985 by the Army Corps of Engineers. The survey area extended 576 miles from Cape
Girardeau, Missouri (Upper River Mile 52) to just below Vicksburg, Mississippi (River Mile
430) (Figure 1). In prior years three types of surveys were performed: towboat, aerial and small
boat (Figure 2). The towboat surveys were discontinued in 1995 as were the aerial surveys in
1997 when it became obvious that of the three, the small boat survey yielded the higher, more
accurate counts. Since 1997 the surveys have all been conducted by Dyersburg State
Community College and Kenneth Jones, Associate Professor of Biology. Other observers and
crew members of this year’s survey included Larry Griffin, Wanda Jones, Mike Thron, John
Rumancik, Michael Heckethorn, Roger Taylor and Patricia Taylor.
This year’s survey yielded a total count of 8,082 Least Terns and 68 nesting colonies.
Sixty colonies were identified based on the presence of eggs and/or chicks (Tables 1 and 2).
Eight colonies were identified based on the presence of a large number of nest scrapes, adult
behavior and other indicators of likely reproductive effort such as wetting breast feathers or
intense mobbing of intruders. The largest colonies were located at Island #84 Dikes (RM 533)
and Densford Bar (RM 754) with 729 and 700 terns. The smallest colonies were at Island #86
Dikes (RM 519) and Cracraft Lower Dikes (RM 510), both with just 7 birds.
Numbers of adult and juvenile Least Terns were recorded on data sheets developed by the
Corps and based on the Cornell University Bird Registry Form. One data sheet was used for
each observation with the following noted: date, time, river mile, latitude/longitude (decimal
degrees determined from GPS), number of adult terns, presence of juveniles, chicks and eggs,
locations of sand bars, substrate, distance from vegetation and type, weather conditions, nearest
river gage readings, and other notes on bird behavior, signs of predation or other disturbance.
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Colony locations were marked on 1998 Flood Control and Navigation Maps of the Mississippi
River, 61st edition, USACE. Data sheets and colony locations are found in Appendix A.
The average number of adult terns per colony was 112, and 7 was the smallest at Island
#86 Dikes (RM 519) (Appendix A-II-100) and Cracraft Lower Dikes (RM 510) (Appendix A-II103). The numbers of eggs per nest ranged from 1 to 4 and many nests were associated with
debris wrack lines or in close proximity to small, random pieces of drift. Many adults were
observed utilizing other habitats often miles away from the nearest colony locations. These
habitats included both still and running water of all depths in the channel, behind dikes, in back
chutes, tributary mouths, and fast current against revetted banks on the outside of river bends.
The small boat survey was initiated July 13, 2003, 12 days after the river stage at
Memphis, Tennessee fell below 12.0 feet (Table 3). The reason being that at higher river stages,
many of the more optimal sand and gravel bars for tern nesting habitat are under water. This
year, by the time the survey could be initiated, there was a 10 ft. rise or bump in the river stage
due to heavy rains in the Ohio and Upper Mississippi River Valleys. The stage at Memphis was
at 6.2 ft. on July 7, rose to 12.3 on July 15 (when that section of the river was counted) and
crested July 18 at 15.9 ft. The survey proceeded downstream during daylight hours without
interruption from Cape Girardeau, Mo., to Vicksburg, Ms., at an average pace of 115 miles per
day.
Upon spotting a likely colony site, the boat would approach the bar as close as shallow
water levels would allow, cut power, and slowly drift with the current. Two or more observers
would count birds using 7x35 or 8x40 binoculars. One observer counted terns resting along the
waterline, another observer counted birds in the air. After this preliminary count, the boat would
beach and one or more observers would walk toward the perceived center of the colony to find
evidence of nesting activity. The presence of eggs or chicks was considered confirmation of
nesting activity. Intense mobbing, wetting of breast feathers, and the presence of fresh scrapes
were also considered to indicate an active nest colony at the 8 sites where eggs or chicks could
not be confirmed. Once a colony was located, several counts were made, both by observers
stationed at the boat and those penetrating the colony. The highest number of birds sighted by
any one observer was recorded as the total number for the site. The number of older, fledgling
juveniles, when observed, was also noted, though during this year’s survey no such observation
were made. However, it is usually impossible to discriminate between flying juveniles and
adults when determining total colony counts at long ranges.
As with previous surveys, observations within the colony were performed as quickly as
possible (< 15 minutes) to minimize the disturbance time and limit the exposure of any chicks
and eggs to the intense summer heat. Though mobbing behavior was often intense upon the
observers first entering the colony, it ceased almost immediately as soon as the observers
retreated. The highest counts usually occurred during the initial waterline count or upon the first
few minutes of mobbing when most, if not all, of the adults in the colony would join in to
dissuade the intrusion. Nesting adults, nest scrapes, nests with eggs and chicks were recorded as
well as other features of the habitat. Eight thousand eighty-two (8,082) Least Terns were
recorded at 115 locations (Table 1) (Figure 1). Of these, 68 sites were identified as nesting
colonies, 17 as resting bars only and 30 observations were of birds over feeding areas or in
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transit along the river. Of the identified nest colonies, 8 had neither chicks nor eggs, 2 had
chicks but no eggs, 48 had eggs but no chicks, and 10 had both chicks and eggs present within
the colony. No fledgling juveniles were noted in this year’s survey.
As with previous years, the typical tern nest colony was located on an unattached sand
and/or gravel bar usually associated with a dike field. Isolation and remoteness appeared to be
the main selection criteria. The larger colonies were hundreds of acres in size, while some of the
smallest were just a few acres. Most of the sand and gravel bars containing colonies were
associated with dike fields, however 8 were located on bars away from dikes, either on the
outside of bends, below large islands or more often point bars on the inside of bends without
dikes. As a result of the rising river stages mentioned earlier, many of this year’s colonies and
resting areas were located on very small, barely emergent sand bars often just a few acres or less
in size. And many sites noted during previous surveys were completely submerged. However,
in some ways, this condition also made the counts easier as birds were more concentrated and
easier to see. As before, the least likely sand bars to be utilized by nesting terns were those that
were directly attached to the riverbank. Only 4 colonies met this criteria.
The locations of the colonies as indicated on the 1998 Navigation Maps (Appendix A-I)
should be viewed as only approximations based on river mile markers visible from or nearby the
sites. Since many sand and gravel bars do move or change shape often, such features on the
maps do not necessarily correspond to the actual conditions at the time of the survey (hence the
occasional colony indicated in open water).
For exact colony locations the GPS
latitude/longitude coordinates (decimal degrees) are the most accurate and should be referenced
as occurring at the water’s edge on the channel side of the colony (except when specified
otherwise on the data sheets). Most colonies were located on the high centers or distributed
evenly across the bars showing no preference for river vs. chute sides.
The colonies’ proximities to the nearest water (least distance from any nest with eggs to
the waterline) varied from 0 to 1,000 feet and averaged ~245 feet. Though some colonies were
densely populated, with scrapes occurring within a few dozen feet of each other, many more
colonies were extremely dispersed with nests occurring hundreds of feet apart and the colony
extending as far as a mile or more across the larger sand bars. Major vegetation was absent or
far removed (>300 feet) from all but 19 colony sites. Vegetative species included grasses,
sedges, and various unidentifiable forbs in a few cases, but 15 sites did have substantial stands of
sapling Cottonwoods (Populus deltoids) and/or Black Willows (Salix nigra). The other colonies
with vegetation had only very sparse, recently emerged grasses and herbs in small patches within
the colony. Most open water within 2 miles of active colonies appeared to be utilized by
foraging terns. These sites included swift current on revetted outside bends, slack water between
dikes, chutes, mouths of tributaries and even mid-channel currents.
Predation on terns, their eggs or chicks was noted at 5 colony sites. Coyotes were
responsible for 2 incidents, one at Refuge Dikes, RM 528.5 (Appendix-II-95) and one at
Kentucky Bend Bar, RM 518 (Appendix-II-101). A domestic dog accompanied by humans
apparently caused chick mortality at Wrights Point Dikes, RM 819.5, a popular recreation site
just across from Ed Jones Boat Ramp (Appendix-II-31). In other incidents, Mississippi Kites
were observed pursuing adult Least Terns at Cedar Point, RM 759 (Appendix-II-41), Great Blue
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Herons preying on eggs or chicks at Kangaroo Point Dikes, RM 647 (Appendix-II-66), and
unknown avian predation on an adult Least Tern at Densford Bar, RM 754.5 (Appendix-II-42).
The presence of coyote or dog tracks was noted at 17 colonies and still lends strong support to
the contention that coyotes and/or feral dogs are one of the more common predation threats to
nesting Least Terns along the Lower Mississippi River. Of 68 colonies, 10 had evidence of
recent intrusion by humans using boats or ATVs to access the sites.
Other potential predators, or their sign, seen within colonies during this year’s survey
included Great Blue Heron (Ardea herodius), Egrets (Casmerodius, Egretta, Bubulcus), Ringbilled Gull (Larus delawarensis), Common Crow (Corvus brachyrhynchos), Turkey Vulture
(Cathartes aura), Mississippi Kite (Ictinia mississippiensis), Bald Eagle (Haliaeetus
leucocephalus) and Great Horned Owl (Bubo virginianus). In one colony at Nebraska Point
Dikes, RM 807 (Appendix-II-33) a dead chick was observed but no cause could be determined.
Other bird species observed in close proximity to Least Tern colonies included Canada
Goose (Branta canadensis), Mallard (Anas platyrhynchos), Killdeer (Charadrius vociferus),
Caspian Tern (Sterna caspia), Forster’s Tern (Sterna forsteri), White Pelican (Pelecanus
erythrorhyncos), Bank and Rough-winged Swallows (Hirundinidae), Purple Martin (Progne
subis), American Golden-Plover (Pluvialis dominica), Least Sandpiper (Calidris minutilla),
and other, unidentified sandpipers (Scolopacidae).
Fingerling size fish of Gizzard Shad (Dorosoma cepedianum), and Threadfin Shad
(Dorosoma petenense) were found at multiple colonies and are presumed to have been dropped
by foraging adults. Previous surveys usually included more varieties of fish species, but most
colonies during this year’s survey were without chicks and there was less opportunity for
foraging adults to drop fish while feeding chicks. However, during a return visit to Densford Bar,
RM 754.5 (Appendix A-II-42), when more chicks had hatched, a fingerling White Bass (Morone
chrysops) and an unknown minnow species were also observed.
The amount of human disturbance in and around the colony sites was somewhat less
when compared to last year’s survey. As before, however, recreational use was mostly restricted
to areas close to the waterline. In 12 of 68 active colonies (18%), recreation or signs of recent
human activities were noted. These signs included camping debris, campfire remnants, human
tracks along with their pets’, and ATV tracks. Nine of these colonies were located within 5
miles of a public boat ramp or harbor and probably are subject to more heavy recreational use.
Eight of these 12 colonies had chicks and/or eggs present, while 4 had chicks indicating the
potential for successful reproduction. Though subject to heavy recreational use, Densford Bar,
RM 754.5, continues to be one of the more successful and densely populated colonies with this
year’s count yielding 700 birds. At the time of the survey, eggs were found to have been
displaced by human intruders, with 2 scrapes observed containing 13 eggs each. Further egg
displacements were noted one week later.
Besides Densford Bar, two other colonies also had impressive counts this year. As
mentioned earlier, Island #84 Dikes, RM 533, with 729 birds and Nebraska Point Dikes, RM
807, with 600. Taken together, these 3 colonies account for 25% of the entire 2003 Interior
Least Tern population along the lower Mississippi River.
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It is perhaps worth noting that even though the number of colony sites identified this
year, 68, did not change from last year’s 2002 survey, which also identified 68 colony sites, 20
of their locations did change. Because the 2003 river stages were anywhere from 3 to 9 feet
higher than the 2002 stages at the time of the surveys, it is likely that river stages affect the
selection of colony sites by Least Terns. It appears that when one bar becomes submerged due to
rising water, another bar, attached at lower river stages, becomes unattached at higher stages and
becomes more inviting as a potential Least Tern colony site.
Compared to last year’s survey, the total number of colony sites remained the same (68),
but the average number of birds per colony was significantly greater (112 vs. 80), with 7581
Least Terns counted at 68 nest colonies. The total 2003 count (which includes birds counted on
resting bars, feeding over water, or in transit) was 8,082. This exceeds the highest previous
survey (6,971) which occurred in 1995 (Table 4), by 1,111 birds. It exceeds last year’s survey
(5,802) by 2,280 birds for a 39% net increase in Interior Least Terns in one year.
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Population Survey of the Interior Least Tern
On the Mississippi River from
Cape Girardeau, Missouri to Vicksburg, Mississippi
2003
APPENDIX A
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APPENDIX A – I
LEAST TERN SURVEY - 2003
Mississippi River
Cape Girardeau, MO to Vicksburg, MS
NEST COLONY LOCATIONS
Memphis District, Corps of Engineers
Maps
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Appendix A – II
LEAST TERN SURVEY - 2003
Mississippi River
Cape Girardeau, MO to Vicksburg, MS
SMALL BOAT SURVEY July 13 - 17, 2003
Memphis District, Corps of Engineers
Data Sheets
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