New York Natural Heritage Program
Black Tern
Chlidonias niger (Linnaeus, 1758)

Threats [-]
Current State and Federal regulations (i.e., the Migratory Bird Treaty Act) appear to offer adequate protections to the birds themselves, though habitat protections such as the New York State Wetlands Law, and Section 404 of the Clean Water Act are not adequate to prevent all wetland habitat losses (Holst 2005). Loss, degradation, isolation, and fragmentation of wetlands via drainage for agriculture and/or urbanization are often considered to be the the primary factors for the decline of this species and other freshwater marshbirds. For example, in Canada nearly half of the wetlands along the St. Lawrence River were destroyed between 1945-75, and a similar pattern occurred on the U.S. side (Novak 1992). This has left many localized marshes that were too small themselves, or were not part of larger wetland complexes, unsuitable because Black Terns are an area- dependent species (Brown and Dinsmore 1984). Water level management on Lake Ontario and the St. Lawrence Seaway has undoubtedly negatively affected lakeshore marsh habitat by allowing succession to proceed, leading to the dominance of wetlands by single species monocultures such as Typha and/or Phragmites. Since the mid-1980s, virtually all of the Black Tern colonies in lakeshore marshes in Monroe/Wayne and Oswego/Jefferson/St. Lawrence Counties have either disappeared entirely or dramatically declined so that today virtually all Black Terns nest on inland marshes on public lands which have the ability to manipulate water levels. Lake Ontario/ St. Lawrence Seaway water level stabilization, implemented primarily for shipping interests, has curtailed dramatic annual water level fluctuations on these commercial waterways leading to compositional changes in marsh vegetation, diminishing their quality as tern nesting habitat (McGowan and Corwin In press). Because the Black Tern is sensitive to water level fluctuations it has been adopted as a performance indicator species by the International Joint Commission Lake Ontario - St. Lawrence River Study ( At inland marshes, siltation and run-off from intensive agricultural practices and urbanization leads to the eutrophication and chemical contamination of wetlands, affecting habitat quality and diminishing the invertebrate prey base. One study in the northern Great Plains found that wetland invertebrate and seedling emergence was greatly reduced by agricultural sediment loads, hampering successional changes and severely impacting nearly all key ecological functions of wetlands (Gleason et al. 2003). The highly intensive agricultural practices (i.e., large monocultures needing heavy inputs of synthetic chemicals and use of heavy machinery) adopted especially since the 1980s have been implicated not only in the severe decline of Black Terns in Europe (Bientema 1997), but also in the general decline of a whole suite of agricultural and wetland birds in the St. Lawrence Valley (Jobin et al. 1996), as well in the entire central and eastern U.S. (Murphy 2003). In certain other cases, lack of stochastic flushing events (i.e., flooding) promotes the establishment of large monotypic stands of emergent vegetation (cattails) as well as exotic invasive plants like purple loosestrife and phragmites which can become too dense and crowd out native emergent plants which provide more favorable nesting cover for nesting Black Terns and other marshbirds (Holst 2005). In addition to diminishing the insect and fish prey base and causing dietary problems for Black Terns (Bientema 1997), chemical contaminants, including organochlorines (PCBs, DDT, DDE, Dieldrin) have been detected in Black Tern eggs in Canada (Weseloh et al. 1996) and the U.S. Severe contaminant concentrations, and very poor reproductive success at a site in Monroe County was reported by Firstencel (1987). The levels of contaminants she found were about two times higher than in Black Tern eggs in Ontario and Quebec (Weseloh et al. 1996), likely causing decreases in eggshell thickness and nest failure. Because contaminant concentrations in sediments were low, Firstencel suggested that the high levels of contaminants were coming from fish (greater bioaccumulation than insects) which the terns feed to their young. Because many banned pesticides are still used in South America where the birds spend most of the year feeding on fish, exposure on the wintering grounds must also be be considered to be high. Nevertheless, strong circumstantial evidence suggests that the high levels of contamination reported by Firstencel (1987) in western New York may be at least partially responsible for the complete elimination of Black Terns from this (more industrialized) region of the State. The loss of the pre-migratory staging area at the mouth of the Niagara River, another highly polluted waterway, also lends some support to the deleterious role that chemical contamination has played in the severe decline of the Black Tern in New York. Nisbet (1997) reported that population declines in Europe were more pronounced in more highly degraded habitats than in more natural ones. Various types of human disturbance have often been cited as potential threats to nesting Black Terns. Excessive boat traffic as well as waves caused by boat wakes can swamp nests, but boat traffic did not appear to have overtly visible effects on nesting terns at four different colonies in New York (Novak 1992). Canoes moving through areas where terns are nesting produces mobbing behavior by the adults and may increase the likelihood of nest abandonment, predation or exposure to inclement weather (Novak 1992). Shealer et al. (2000) demonstrated that Black Terns have a high tolerance for intensive disturbance (repeated nest visits for trapping and banding) but that these nests suffered high (47%) mortality rates mostly due to predation. Likewise Heath and Servello (2008) documented the importance of nest predation to chick survival, but were unable to identify the predator species. Many different birds and mammals have been implicated as Black Tern nest predators (Heath and Servello 2008), but extreme caution should be taken when high nest mortality rates are provided by researchers visiting nests because both bird and mammal predators can follow human scent trails looking for prey.

Conservation Strategies and Management Practices [-]
A review of management actions at the Iroquois/Tonawanda/Oak Orchard wetland complex in western New York showed a consistent pattern of response by nesting Black Terns (Hickey and Malecki 1997). After the drawdown of water levels in an impoundment, the terns recolonized the wetland the year following re -flooding, reaching peak numbers in the second and third years after the drawdown. In the first year post-inundation, vegetation responded, the muskrat population grew and Black Tern nesting was likely limited by the lack of suitable nest substrates. In the second and third years, muskrat (Ondatra zibethicus) feeding and house-building activities removed vegetation from the marsh, improving the interspersion of vegetation to water (50:50 ideal) and providing nesting substrates. Because muskrat structures are heavily used as nesting substrates, especially in marshes dominated by less persistent emergents such as bur-reed, their population dynamics (including the effects of trapping), suggests that muskrat ecology is an important feature to Black Tern nesting success (Hickey and Malecki 1997). Thus, a 4-6 year cycle of drawdown should be used, with re-flooding during years 2-5. Water levels should be maintained higher than normal in the first year following re-flooding in order to allow muskrat populations to recover. Removal of vegetation by muskrat herbivory benefits Black Terns by improving the interspersion of vegetative cover and open water and by increasing the availability of nesting substrates (Zimmerman et al. 2002). Because nest substrates are often limiting, the placing of artificial nest platforms, especially in the first year after drawdown, may enhance productivity (Hickey 1997).

This type of management strategy requires at least three nearby marsh impoundments to be managed on a staggered 3 year management cycle, with one of the impoundments being drained, disced, and re-flooded each year. Such a strategy is time and labor intensive, costly, and necessitates a large wetland mosaic amenable to intensive management with heavy machinery to disc up marsh vegetation (Shambaugh 1996). Since many managed areas will not have the resources and area available to implement such an intensive strategy (which also is beneficial to other marshbirds and waterfowl) other, less costly strategies have been attempted in the northern Great Plains (Linz et al. 1994; Linz and Blixt 1997). Because these management strategies involved the application of potentially dangerous herbicides to reduce cattail growth, the authors themselves recommended that manual methods were preferred, and that herbicides be used only as a last resort (Linz and Blixt 1997). Water level control measures, discing, and healthy muskrat populations could potentially all be used in concert to control dense monotypic stands of vegetation and promote the hemi-marsh stage (a 50:50 ratio of well interspersed vegetation and open water) (Zimmerman et al. 2002).

Black Terns use different wetlands or different locations within a wetland mosaic from year to year because suitability varies with yearly fluctuations of water levels and resulting vegetational changes. The presence of Black Terns is related to the total area of wetlands in the surrounding landscape; thus wetland complexes must be maintained because they are more likely to have at least some wetland components with water and plant regimes favorable for nesting. Areas of suitable habitat >10 ha that have equal proportions of well-interspersed emergent vegetation and open water, with stable water levels (> 30 cm depth) throughout the breeding season (May-June). Maintaining stable water levels during the nesting season decreases the probability of nest destruction and decreases the chances of nest predation (Zimmerman et al. 2002).

Research Needs [-]
Nisbet (1997) summarized important research needs for Black Terns: 1) improve ecological understanding on the wintering grounds; 2) monitoring population trends in the main range in the prairie states and provinces; 3) measure factors limiting productivity; 4) study foraging, diet, and nutrition in relation to habitat quality, water quality, and prey populations; 5) study behavioral ecology, including nocturnal incubation, mate fidelity, spacing behavior, coloniality, dispersal, and post-fledging parental care; 6) study the demography of metapopulation dynamics; and 7) implement comparative studies across regions.

In New York specifically, determining the cause of the decline is paramount so that steps can be taken to mitigate limiting factors. Two hypotheses have generally emerged: 1) productivity problems related to contaminant effects on the birds themselves and/or their prey base, or 2) habitat alterations associated with wetland loss and modifications. Declines of this species in Europe (Bientema 1997) have essentially tied these factors together such that land use changes associated with intensive agriculture and urbanization have led to the eutrophication and contamination of wetlands, which in turn collapses the prey base and lowers productivity below that which can sustain the population. This hypothesis has also been invoked to explain ongoing declines in other agricultural avifauna in both Europe and North America (Murphy 2003).