Bluebill ducks have been having a rough time. Despite population increases in other waterfowl species, bluebill (also known as the lesser scaup) numbers have been declining since the 1970s. Numbers also fluctuate from year to year. While it is still one of the most numerous and widespread duck species in North America, wildlife officials would like to see overall population growth. Adding to the problem, in the past week another mass mortality event occurred in Minnesota.
One possible factor for the general decrease is declining spring conditions along the Mississippi, which forms the basis of the ducks’ migration path. Mild winters may allow them to spend more time further north, as water and food sources persist for longer. Additionally, regional changes in agricultural practices have affected duck habitat, by providing less cover and poorer quality food sources. Researchers have also looked at parasite loads. While today’s ducks have similar parasite levels compared to their 1970s counterparts, there may be changes in some features, such as the infectious species responsible.
Over the last five years along the upper Mississippi River, several waterfowl species including bluebills and coots have suffered mortality due to parasitic trematodes, particularly during the spring migration. Infected birds appear unwell, getting lethargic and experiencing difficulty with diving (feeding) and flying. The resultant damage to their internal organs and circulatory system is fatal.
Parasitic trematodes require two intermediate hosts in their life cycles, including aquatic snails and insect larvae. Several snail species may be implicated in transmitting harmful trematodes. The exotic faucet snail, originally native to Europe, can act as both intermediate hosts. Additionally, three trematode parasites, Leyogonimus polyoon, Cyathocotyle bushiensis and Sphaeridiotrema globulus, are associated with the faucet snail and were first documented in the US in 1998 and 2002. These events marked the first observation of faucet snails in the upper Mississippi region. Then, as with the current bluebill die-off, some of these parasites were responsible for waterfowl mortality.
The concern is with growing impacts on wildfowl. Not only can various duck species be affected, but other snails may act as vectors, and the parasites may spread south via the Mississippi’s many waterways. Once introduced, these alien species have the potential for devastating repercussions.
