Feeding preference and foraging impact of wintering coots on submerged aquatic vegetation

Abstract

While it has been well established that waterbirds can consume substantial amounts of submerged aquatic vegetation (SAV) on their wintering grounds, relatively little is known about their effects on SAV in the northern Gulf of Mexico (nGOM). We measured the impact of wintering American coot (Fulica Americana) foraging on native wild celery (Vallisneria americana) and exotic Eurasian water milfoil (Myriophyllum spicatum) using caging experiments at two locations in upper Mobile Bay during winter 2013–2014. We also determined feeding preferences using tethering experiments, and monitored the location of coots and the feeding behavior of individual birds. Coots were significantly more abundant over Eurasian watermilfoil than native wild celery. Caging experiments usually showed higher SAV biomasses in exclusion cages, and suggested a larger impact of coot foraging on milfoil than wild celery. Video recordings confirmed that coots were responsible for the SAV losses detected with both caging and tethering experiments, and dietary analyses supported experimental results and highlighted the role of milfoil in the coot's diet (86.9 ± 8.9% of stomach contents). Tethering results showed a preference for wild celery over milfoil, which is likely explained by the higher nutritional quality of wild celery (19.26 ± 1.21 C:N ratio) compared to Eurasian milfoil (25.01 ± 2.45 C:N ratio). Overall, our results are similar to those of several prior seagrass herbivory studies in showing that herbivores do not always feed on their preferred food, presumably because other factors, such as proximity of refuges from predators or competition for food resources, are of overriding importance.