In this post, I will provide a synopsis of several scientific papers that recently appeared in scientific journals. All of the papers deal with birds that occur regularly in Maine and most of the research was performed in our state.
Michael Schummer, Brad Allen and Giuming Wang recently published a paper in the Northeastern Naturalist where they analyze changes in reproductive success of Maine waterfowl. They used brood survey data collected between 1955 and 2007. The authors analyzed six species: Mallard, American Black Duck, Wood Duck, Common Goldeneye, Ring-necked Duck, Hooded Merganser.
Brood sizes declined for all species except Mallards and Hooded Mergansers over the survey period. For those four species, clutch size declined between 0.9 ducklings per brood to 1.7 ducklings/brood.
The authors do not know the reasons for the declines but the results should arouse concern. The four species are quite different in nesting behavior and in diet suggesting that a general loss of habitat quality may be to blame.
Bald Eagles are adaptable predators that sometimes kill live prey and at other times scavenge. The diet of Bald Eagles varies seasonally and geographically but commonly includes invertebrates, fish, reptiles, birds and mammals.
Medium-sized mammals in their diets include rabbits, sea otters, arctic foxes and raccoons. Eagles can kill any of these mammals. We do have evidence that Bald Eagles will feed on hoofed mammals (deer, elk, caribou and domesticated cattle and sheep. For these larger mammals, biologists presumed they were scavenged by the Bald Eagles.
A recent study by Jared Duquette and colleagues published in the Northeastern Naturalist provides evidence that a Bald Eagle killed a white-tailed deer fawn and flew it to its nest for its two eaglets. The fawn had been radio-collared. It weighed six pounds. Thanks to the radiocollar, the workers could locate the carcass (including two legs and most of the hide) to the eagle’s nest.
In an article in the Wilson Journal of Ornithology, Mason Cline and Joanna Hatt describe the potential of lobster traps in causing deaths of landbirds. The two authors participated on the Bath-Brunswick Christmas Bird Count in January 2010. At Merepoint Neck in Brunswick, they noticed 80 metal lobster traps stored above the tideline. Three live Blue Jays were in three of the traps. Furthermore, the Blue Jays were ripping the flesh from nine other Blue Jays that had perished within the traps. The jays were apparently enticed to enter the traps to feed on the baitfish remnants inside and then could not exit. This observation established idle lobster traps as potentially lethal traps and demonstrated cannibalism for the first time in Blue Jays.
More than three million lobster trap tags have been issued to the Maine lobster industry. The number of traps is certainly higher than that with all the spare or unused traps not in current use. The authors rightly point out that if only a fraction of those traps are stored after the lobster season in such a way that birds can enter, the traps represent a potentially significant source of mortality. Fortunately, storage of the traps to obscure the openings and removal of the baitfish can reduce the threat.
In the journal Frontiers in Ecology and the Environment, Jory Brinkerhoff and three co-authors explore the impact of birds in increasing the risk of Lyme disease. The black-legged tick is the primary vector of the Lyme disease bacterium (Borellia burgdorferi). Borellia parasitizes a wide variety of vertebrate species, including at least 71 species of North American birds.
The black-legged ticks in their early (larval) stages are mostly found on white-footed mice. The immature (nymphs) occur on a variety of birds, mammals and even reptiles. The adult ticks are primarily dependent on white-tailed deer. However, the blood of white-tailed deer causes the Borellia bacteria to die. So, deer can certainly disperse black-legged ticks but not the Lyme disease bacterium.
Anytime, an infected black-legged tick bites a host it can transfer Borellia to the host or, if the host is infected, the tick may become infected. It’s a two-way street.
Although some dispute the role of birds in transmitting Borellia, the Brinkerhoff team showed that 58% of the bird species evaluated may harbor Borellia and may infect black-legged ticks with the Lyme disease bacterium. When an infected tick bites a human, the Borellia may be transferred and Lyme disease follows.
As global warming continues, the geographic range of black-legged ticks and the Lyme disease bacterium will shift northward. Currently, there are only seven established populations of black-legged ticks in Canada. Six are in southern Ontario and one in Nova Scotia. The authors believe that these tick introductions were made by birds.
[First published on May 29, 2011]