TROUBLED WATERS: 
What is Happening
to Marsh Bird Populations?

The BBS is rather good at detecting population trends of many species of landbirds.  Because it is a roadside based survey, however, it is not particularly suited for monitoring waterbird populations, especially shy, secretive species like rails, bitterns and grebes. They require special survey techniques and which may also have a tendency to avoid roadside marshes.  Despite its limitations, the BBS nevertheless points to some rather worrisome trends. 

Since the survey began in 1966, many marsh dependent birds have declined across North America.  Among them are Pied-billed Grebe, American Bittern, Least Bittern, Northern Harrier, Virginia Rail, Sora, American Coot, Common Snipe, Black Tern, Common Yellowthroat, and even the ubiquitous Red-winged Blackbird.  Most of these declines are statistically significant.  Only a few species show increases (Common Moorhen, Marsh Wren and Swamp Sparrow), none of which are statistically significant.  Marsh birds appear to be declining more seriously than other groups of birds.  Why? 

Despite the progress that is being made in Remedial Action Plans (RAPs) and other areas, marshes are still under considerable pressure.  The rate of wetland loss has slowed, but marshes are still disappearing incrementally.  Highly invasive, non-native species (purple loosestrife, European phragmites, carp, zebra mussels, and even Mute Swans) are degrading many marshes.  The water pollution problem is still with us.  And we are only now just beginning to think about the possible damaging effects that a diminished ozone layer might have on wetland ecological processes.  We're not out of the woods yet . . . 

Bad News for Black Terns  

The Black Tern is a little unusual in several respects: 
 

•    Unlike most other kinds of terns that nest on tiny islands and beaches, the Black Tern is an obligate marsh-nesting species. 
•    Whereas most terns and gulls are strongly colonial, densely packing as many nests as possible into the available habitat, the Black Tern is only loosely colonial, and generally prefers to space its nests out.  Black Tern colonies seldom exceed 50 pairs; most colonies consist of about 10 pairs. 
•    We normally think of terns as being expert fishers.  Although the Black Tern does eat plenty of minnow-sized fish, its diet is dominated by insects, which are often "hawked" out of the air. 

The Black Tern doesn't nest in just any old marsh.  The marsh must have the right ratio of open water:emergent vegetation (generally about 50:50) as is evidenced by the Marsh Monitoring Program data gathered on Black Terns over the last two years. 

Black Terns nest in various kinds of marsh vegetation, but cattail marshes are generally favoured.  Indeed, the vast majority of the MMP routes on which Black Terns have been recorded are dominated by cattail.  Since their nests are often built on floating mats of dead cattails, rootballs and patches of floating mud, Black Terns need sheltered pools of open water for nesting.  Stands of cattail are very good at buffering wind and wave action, helping to protect the vulnerable nests. 

Black Terns generally prefer deep water marshes (about 1 m).  Deep water conditions tend to create the interspersed stands of vegetation and open pools of water that the terns need.  If the water is too shallow, the vegetation is too dense.  If the water is too deep, then the vegetation is too interspersed. 

Black Terns like relatively large marshes.  Just as with landbirds, several species of marsh birds are "area sensitive," requiring large tracts of habitat in order to successfully reproduce.  The Black Tern is moderately area-sensitive.  According to the MMP data, it generally requires permanent marshes that are at least 50 ha.  Black Terns have disappeared from many marshes that have been reduced much below this threshold. 

Though still locally common in some areas, the Black Tern has seriously declined throughout its range.  It was recorded on only 65 of the 273 Marsh Monitoring Program routes surveyed in 1995 and/or 1996. 

A recent analysis of Breeding Bird Survey data showed that the population has been declining by an average rate of 4.7% per year since 1966.  Over the span of just 30 years, this translates to an overall loss of about 75% of the population!  This species is clearly in serious trouble. 

We don't really know what is causing the decline.  In coastal marshes along the Great Lakes, sustained high water levels may be effectively washing out key habitat.  Elsewhere, the decline might be more directly connected to habitat loss due to development pressures, coupled with increased recreational use of marshes by power boats that can wash out tern nests.  Pollution may also play a role, especially on the Black Tern's Latin American wintering grounds, where persistent toxins like DDT are still in use. 

The future of the Black Tern is quite uncertain.  It is already considered endangered in New York, Pennsylvania and Ohio, threatened in Ontario, and a species of special concern in Michigan. 

Across the Great Lakes basin, data collected by Marsh Monitoring Program volunteers from hundreds of marshes will continue to throw light on the Black Tern's habitat requirements and monitor regional population trends.  Together, this crucial information can be used to identify the geographic extent and seriousness of the problem, help pin-point the source(s) of the trouble, and ultimately aid in the design, application and evaluation of recovery efforts.  In the meantime, this species would likely benefit from large-scale marsh creation projects, eliminating powerboat access to nesting colonies, and installing floating nesting platforms in degraded sites. 

— Jon McCracken

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