A first year study of breeding success of marsh terns on St. Clair Flats, MI
Report to Detroit Audubon
Caleb G. Putnam, Michigan IBA Coordinator, Audubon
Introduction
Black Tern (Chlidonias niger) is ranked Special Concern and Forster’s Tern (Sterna forsteri) Threatened by
the Michigan Department of Natural Resources (MDNR). St. Clair Flats and Harsen’s Island is one of the
most significant Important Bird Areas (IBAs) in Michigan, representing the world’s largest freshwater
delta and Michigan’s largest remaining marsh tern colony. Black Tern (Fig. 1) and Forster’s Tern nest on
floating mats of dead vegetation and other debris in protected areas of larger emergent marshes,
particularly those dominated by bulrush, cattail, and pond lily. Marsh tern populations have experienced
drastic range-wide declines in recent decades, including in Michigan (Michigan Breeding Bird Atlas II)
between the 1980s and 2006. Both species nest in the temperate latitudes and winter in the tropical
latitudes, typically at sea, and the cause of these declines is not well understood. In 2012 National
Audubon Society (NAS) approached Detroit Audubon with a proposal to study the nesting success of
these species on St. Clair Flats in order to begin to understand whether either species is in need of
conservation work on their breeding colonies in Michigan, and to identify threat abatement strategies if
necessary. The project would also aid in understanding whether the causes of observed declines are
operative on the breeding grounds or winter grounds. The goals for this year were:
1) To document locations of nesting behavior during 2013;
2) To estimate the breeding success (fecundity) of each species during 2013;
3) To test whether artificial platforms are used by either species, and if so, whether they increase
nest success;
4) To identify the causes of any nest failures and chick mortality, and to determine if anything can
be done to abate such threats;
5) To recommend a course of action for 2014 and beyond.
Fig. 1. Adult Black Tern. Photo: Caleb G. Putnam.
Methodology
Creation of the marsh tern group
Members of the marsh tern group brought together by National Audubon and Detroit Audubon to serve
as an advisory group and develop a monitoring plan include Caleb G. Putnam (National Audubon),
Richard Quick (Detroit Audubon), John Darling (Michigan Department of Natural Resources [hereafter
MDNR]), Terry McFadden (MDNR), Joe Robison (MDNR), and Tom Schneider (Detroit Zoo). Each brought
management expertise and/or regulatory authority to the team.
Monitoring Methodology
The marsh tern group met on 14 January 2013 and again on 25 March 2013 to devise a conservation
plan. The group selected Randy Kling of St. Clair, Michigan as lead volunteer monitor. Randy made
approximately weekly visits to the colony using a MDNR boat. During each visit he searched for new
breeding activity(they are often visible flying above the nesting areas), visited the nest platforms,
documented new nests (both on artificial platforms and natural substrates), weighed and measured
eggs and chicks, banded chicks, and returned to check on all known active nests until chicks or eggs
could no longer be located. He sought to follow each nest and chick as far to fledging as possible.
Artificial nesting platforms
Artificial platforms were placed in two areas selected by Putnam, Quick, and Kling as looking appropriate
for these species. Both sites were well-protected from boat activity by shallow water, and characterized
by small openings amongst large areas of emergent bulrush and cattail. Both species nest in a heavily
clumped pattern; we deemed each such area a colony. The clumped distribution of nests allows all
adults in the colony to swarm and drive away any predators that enter the area, to the benefit of all of
the pairs in that colony.
Visits to the flats are as summarized in Table 1.
Table 1. Visitation schedule (CP= Caleb Putnam, RK=Randall Kling, JD=John Darling, RQ=Richard Quick,
TM=Terry McFadden, B=Blake (last name unknown), HH=Harold Huber
Date
Observers
Notes
May 3, 2013
CP, RK, JD, RQ
installed platforms
May 9, 2013
CP, RK
put carpet on several platforms
May 22, 2013 RK
first BLTE and FOTE nests
May 29, 2013 RK
many new BLTE and FOTE nests
June 4, 2013 RK
June 14, 2013 RK
June 20, 2013 RK
June 27, 2013 RK, CP, RQ, JD, DS, B, HH
1st chick banding, adult trapping
June 28, 2013 RK, CP, DS, B
continued chick banding and trapping
July 3, 2013
RK, TM
banded 5 chicks
July 8, 2013
RK
July 23, 2013 RK
August 1, 2013 RK, CP
removed platforms
22 used, floating platforms received from David Shealer were installed at two locations on 3 May 2013.
Eleven were installed at Baltimore colony (within 100m of 42.61946N, -82.66348W) and the other
eleven were installed at Doty colony (within 100m of 42.60525N, -82.68276W). Platforms were spaced
at least 15-20 feet apart in openings within the emergent vegetation, with each platform located no
more than 10 feet from a patch of living vegetation (Fig. 2). Platforms measure 12” X 18” and are
constructed of plywood with Styrofoam core and carpet mat on the upper surface. They are individually
numbered and held in place by a 10’ length PVC pipe painted green and jammed into the substrate in
water between knee and hip deep, by hand. The platform freely slides up and down the PVC pipe as
wave action demands, via a metal U-bracket.
Fig. 2. Black Tern platforms at Baltimore colony, St. Clair Flats, 3 May 2013.
All new tern nests discovered were given unique nest cards, and were visited subsequently until no eggs
or chicks could be located.
Results
Nest locations and number of pairs
Fifty Black Tern nests and fifteen Forster’s Tern nests were located and monitored on St. Clair Flats
during summer 2013. Five colonies were discovered (Fig 3). Nests and platforms were monitored using a
GPS unit (Fig. 3).
Fig. 3. Four of five colonies of nesting marsh terns at St. Clair Flats, MI, during summer 2013. Baltimore
(upper right), Doty (two center left clumps), FisherSW (center right), and Strawberry (bottom left). A
fifth subcolony, not shown, is located well to the SE at Little Muscamoot Bay. Yellow pins represent
artificial platforms, red pins Black Tern nests, and blue pins Forster’s Tern nests.
In both species, because tern pairs which fail prematurely will re-nest, it is not possible to say how many
pairs were represented by these fifty nests. The actual number of pairs on the flats is difficult to
estimate. At each colony we halved the number of birds simultaneously seen in flight during predator
mobbing, and then summed these totals. However this method is not exact, and the number of birds
absent from the colony during any particular visit is unknown.
Based on repeated visits, we believe a conservative minimum number of Black Terns pairs present on
the flats to have been at least 200 pairs during summer 2013, possibly as high as 300 or 400 pairs.
Previous estimates from the Great Lakes Colonial Waterbird Survey (Cuthbert et al. 2008) put the
number of Black Tern pairs at around 40-50 pairs, based on single visit flyovers. The extent of year-toyear variation in numbers is not known currently. Forster’s Terns were very common at the flats, but
were more loosely distributed throughout the flats than Black Terns, and we were unable to estimate
the number of pairs actually present. Forster’s Tern nests were less common than Black Tern nests at
each colony. Cuthbert et al. (Great Lakes Colonial Waterbird Survey) estimated 588 pairs of Forster’s
Terns in 1999 and 515 pairs of Forster’s Terns in 2009. This number seems possible, although our
estimate would have been lower than this during summer 2013.
Number of nests at each colony
Baltimore: A total of 19 Black Tern nests and 7 Forster’s Terns nests were discovered at Baltimore
colony. (Fig. 4)
Fig. 4. Baltimore colony, one of the two largest colonies of Black Terns on St. Clair Flats. Yellow pins
represent artificial platforms, red pins Black Tern nests, and blue pins Forster’s Tern nests.
Doty: A total of 25 Black Tern nests and 3 Forster’s Tern nests were found at Doty colony. (Fig. 5)
FisherSW: Four Black Tern nests were found at FisherSW colony. (Fig. 5)
Fig. 5. Doty (left and center) and FisherSW (right) subcolonies. Yellow pins represent artificial
platforms, red pins Black Tern nests, and blue pins Forster’s Tern nests.
Strawberry: One Black Tern nest and 6 Forster’s Terns nests were discovered at Strawberry colony.
Little Muscamoot: Two Black Tern nests were found at Little Muscamoot colony.
Use of artificial platforms
Only two Black Tern nests appeared on our platforms. The first had one egg which did not hatch (Doty),
and the second (Baltimore) had three eggs which were not present on subsequent visits. Dozens of
active nests were placed on clumps of natural floating bulrush from the previous season in the
immediate vicinity of the artificial platforms. Adult Black Terns and Forster’s Terns regularly perched
upon the PVC poles, as these were the only available exposed perches in the colony, but showed an
obvious preference for nesting on natural substrates.
Hatching success
Ideally, we wanted to know the fate of each egg and chick in the colony, but this is rarely possible (D.
Shealer, pers. comm.). Incubation ranges from 20-22 days for Black Terns and 23-25 days for Forster’s
Terns, and fledging last 3-4 weeks for Black Terns and approximately the same time period for Forster’s
Terns. The problem in both species is that hatched young will leave the nest and swim or run to safety as
early as four days after hatching (D. Shealer, pers. comm.). And in both species, the young can fly as
early as 3 weeks, while they are still being tended by adults. It is thus rarely possible to know the fate of
chicks that are absent at the nest site during subsequent visits. This uncertainty usually made it
impossible for us to determine whether eggs we located hatched, and whether chicks we observed or
banded fledged.
As a result of these limitations, we were only able to document the number of eggs and chicks that we
observed (a metric of hatching success), without knowing their eventual fate (fledging success). We
documented 85 Black Tern eggs, at least 46 of which hatched. We were unable to follow any of these to
fledging, but believe it is highly likely that many of these chicks did fledge. Some juvenile Black Terns
were observed flying with adults in the colonies during late July and early August, but in much smaller
numbers than eggs and chicks observed. We hypothesize that fledged chicks may have moved out of our
study area to the west side of Lake St. Clair shortly after fledging, where T. McFadden observed large
number of juveniles in late July (pers. comm.).
25 of 49 (51.0%) Black Tern nests were documented to hatch at least one chick. However, of the other
24 nests, 19 had unknown fates and 5 definitely failed. So the actual hatching success of this species was
somewhere between 51% (25 of 49 nests) and 89.8% (44 of 49 nests). It is not possible to increase the
precision of this estimate without additional data. 5 of 16 (31.3%) Forster’s Terns nests were
documented to hatch at least one chick. However, the fate of the other 11 nests was unknown, so it is
possible that the actual hatching success was as high as 100% (16 of 16 nests) or as low as 31.3% (if all
11 failed).
We documented 31 Forster’s Tern eggs, at least 12 of which hatched. We were unable to follow any of
these 12 to fledging. These juveniles were highly mobile and it is possible that many of the birds we
documented made it to fledging age, but we are unable to say anything conclusive in this regard without
more data. There were large numbers of juveniles present on the west side of Lake St. Clair in
midsummer, but whether these birds were the result of successful fledging at St. Clair Flats is unknown.
Clutch Size Distribution
In Black Terns, 0 of 32 nests had 4 or 5 eggs (0%), 28 of 32 (78.5%) nests had three eggs, 1 had 2 eggs
(3.1%), and 3 had one egg (9.4%). The nest with 2 eggs, and all 3 nests with one egg, may have been
nests discovered during the laying process which had not yet realized their full clutch size. In Forster’s
Terns 0 of 14 nests had 4 or 5 eggs (0%), 8 had 3 eggs (50.0%), 4 had 2 eggs (25.0%), and 2 had 1 egg
(6.3%). The same caveat for Black Tern nests also applies to Forster’s Terns nests with only one or two
eggs. In summary, both species had clutch sizes of three most or all of the time.
Banding
32 Black Tern chicks were banded, and none were recaptured. 9 adult Black Terns were trapped on the
nest and banded, with no returns. 12 Forster’s Tern chicks were banded, and none were recaptured. No
adult Forster’s Terns were captured or banded. The ability of chicks of all ages to swim and hide and
move away from the nests soon after hatching complicated our efforts greatly.
Discussion and Future Recommendations
Marsh terns nested in large numbers on St. Clair Flats during summer 2013. This IBA is even more
important than previously thought, as it is by far the largest remaining colony in Michigan for either
species. The total number of pairs is much higher than previous estimates suggested. Hatching success,
the best possible surrogate of breeding success given our data set, is at least 51% in Black Tern and at
least 31.3% in Forster’s Terns. Fledging success is not possible to estimate without enclosing the nests
such that chicks cannot hide and will automatically be relocated during subsequent visits by the
monitor. This is highly recommended during future years of this study.
Most marsh terns did not use artificial platforms in this location, as there is an abundant supply of dead
floating vegetation upon which to nest. But marsh terns may prefer to nest near platforms, perhaps
because platforms allow parents to perch above the vegetation and watch for predators. If so, future
projects could install PVC pipes without platforms in order to increase the attractiveness of colonies. It is
unclear whether the presence of artificial platforms influenced site selection at Baltimore and Doty (we
placed platforms in areas that looked good to us) or whether birds may have been attracted to the area
by our artificial platforms. We lack colony location data from any previous years. If it is determined that
birds are attracted to initiate colonies at sites with artificial platforms, we could attempt to attract birds
away from sites rich with predation pressures to nest in safer areas.
It appears that stochastic processes, primarily weather-related, are to blame for most losses of eggs and
chicks on St. Clair Flats. Predation appears to be a minor cause of losses. This is the norm for marsh
terns, and does not represent an unusual liability of St. Clair Flats (D. Shealer, pers. comm.). A large
storm, which occurred on or around 25-26 May 2013, is believed to have destroyed nearly all of the
existing nests on Baltimore colony. Another in early July appears to have caused similar damage, and
2013 witnessed an unusually large number of strong storms locally (R. Kling, pers. comm.) Floating mats
of vegetation were relocated by up to several hundred feet during these events, and presumably wave
action simply overtook the eggs and chicks when this occurred. However, adults of these two species are
long-lived, buffering them from the effects of occasional years of poor nesting success.
There is no obvious evidence of artificially high nest depredations due to avian or mammalian predators
on St. Clair Flats. Gulls are nearly absent from the site during May and June, but more abundant after
nesting ends. Black-crowned Night-Herons were not observed, but should be watched for (and
appropriate removal of willow thickets on islands on the flats is recommended should they appear). We
believe Great Blue Herons represent the highest potential avian threat on the flats, but this species
cannot forage in the water depths preferred by the terns, and so they appear to represent a negligible
threat, at least when water levels are not exceptionally low. Northern Harriers are present in small
numbers throughout the nesting period, and represent a possible threat. However, terns were highly
successful at mobbing these hawks and chasing them out of the colony. We observed indirect evidence
of muskrat predation of Black Tern eggs at Doty colony on 26 June 2013. No minks or raccoons or any
other mammalian predators were observed, as would be expected given the very long swimming
distances required to reach the colonies (well over a mile). This is no doubt a large part of why this site is
so attractive to nesting marsh terns.
In summary, marsh terns nest at St. Clair Flats in larger numbers than previously recognized. The
colonies are located in at least 6 distinct units, with breeding success apparently quite high. Marsh terns
prefer strongly to nest on natural substrates, especially floating dead bulrush mats, often near artificial
platforms with their raised PVC pipe perches. This site appears to be very important to the maintenance
of marsh tern populations in Michigan, and merits additional work to help us fully grasp the breeding
biology and conservation status of this species in our state. We recommend continuing work on the flats
for several more years, until a fuller grasp of the breeding success of these two species is in hand. We
recommend the following modifications for the 2014 season plan:
1) Do not use artificial platforms again unless there is an insufficient amount of floating debris
available, based on the judgment of Randy, Caleb, and the local MDNR staff.
2) Monitor the locations of colonies between years to see if they remain static.
3) Continue monitoring hatching and fledging rates, visiting as often as possible during the 8-10
week period when the most hatching is taking place (~May 20-July 20). This will allow us to
more accurately assess hatching and fledging success.
4) If feasible, seek a graduate student to conduct a thesis on this colony, who can then visit the site
daily or nearly daily throughout the entire summer, allowing the most complete possible
estimation of fledging success.
5) If possible, build hardware cloth enclosures around each nest to prevent chicks from leaving the
immediate area of the nest, allowing for an accurate assessment of fledging success. Although
this method may increase the mortality level of some chicks, it is the only way to accurately
estimate the fledging success of Black and Forster’s Terns chicks (D. Shealer, pers. comm.), and it
does allow the researcher to limit his/her time at each nest during subsequent visits compared
to the alternative method. This method has been successfully employed at other field sites in
Wisconsin, without obvious detriment to the colony.
6) Weigh and measure all eggs, which allows for an accurate estimation of hatching date, and a
much clearer picture of the fate of each egg/chick in estimating overall hatching and fledging
success.
7) Promote Phragmites control throughout the flats, as this plant seriously depletes or even
removes habitat for nesting marsh terns;
8) Continue to monitor for any increase in nest depredation rates, and to quickly identify and
remove problem predators such as Black-crowned Night-Herons that have the potential to wipe
out an entire colony with one individual predator.
9) Retain the marsh tern group and schedule two meetings in 2014, coordinated by NAS to revise
the conservation plan.
10) Retain Randy Kling to do the monitoring work in 2014.
11) To support continued NAS staff involvement, request an additional $4,000 from Detroit
Audubon. Funds will support oversight, coordination, planning, and reporting for a second year
of breeding monitoring of marsh terns on St. Clair Flats.