Factors Influencing Mortality of Eared Grebes (Podiceps nigricollis) during a Mass Downing

Abstract

During the fall, hundreds of thousands of Eared Grebes (Podiceps nigricollis) stage on the Great Salt Lake, Utah. Mortalities resulting from birds being forced to the ground (downings) during migration occur every few years. A large (>7,000 birds) die-off of migrating Eared Grebes occurred 12 December 2011 in and around Cedar City, Utah. We examined body condition, age, sex, and heavy metal concentrations among downed birds and those that still occupied the Great Salt Lake both pre- and post-downing. Eared Grebes that we collected pre-downing were heavier (523 g) than deceased birds (433 g). Body weight (g) and subcutaneous fat thickness (mm) were lower in downed birds than pre-downing birds, likely the result of the 400-km flight from the staging area to Cedar City. Liver, heart, and intestine weights were all significantly greater in both pre- and post-downing birds than downed birds. This may be because of tissue catabolism during flight or incomplete physiological changes before migration. Adult biased age ratios were observed in all groups but pre-downing (34 adults: 5 juveniles) and post-downing (39: 2) groups were less biased than the downed group (100∶ 1). Mercury and selenium wet weight concentrations in all groups were above the level observed to impact some birds' reproduction and locomotion. Mercury concentrations in the liver ranged from 4.4–25.8 ppm wet weight and mercury concentrations measured in birds collected pre- and post-downing were lower than downed birds. Despite high levels of mercury and selenium, no adverse effects of heavy metal contamination have been noted in studies of Eared Grebes on the Great Salt Lake. Weather was likely the proximate cause of the downing, but impacts of heavy metal toxicity should be examined further to determine their effect on migrating Eared Grebes.