Article: Drew, G. S.; Piatt, J.F.; Hill, D.F. Effects of currents and tides on fine-scale use of marine bird habitats in a Southeast Alaska hotspot. 2013. Mar Ecol Prog Ser. DOI: 10.3354/meps10304
Have you ever watched a bird on the water hunt for food? The ease with which the animal dips under the water’s surface, remains unseen for a period of time, and bobs back up to gulp down their prey seems quite simple to an ordinary observer. But what happens beneath the water has remained a mystery… until now.
Birds are often the most visible part of a complex marine ecosystem; their abundance and diversity reminds us that these predators cannot be sustained without a rich ecosystem of benthic invertebrates, zooplankton, and fish. It has long been known that large-scale ocean movement can alter the distribution of many marine populations, including birds. However, only recently did scientists start investigating the interactions between organisms and their marine habitats on a smaller scale. Birds have preferred niches, thereby using specialized strategies to find food. Depending on the movement of the water, it can make it more or less energetically costly to forage for food.
Advances in technology have enabled research scientists to model high-resolution environmental data to look at water movement at a fine spatial and temporal scale and to find answers to new questions, such as the one Dr. Drew and his research team addressed in Glacier Bay. Located in southeast Alaska and deemed a marine bird hotspot by the North Pelagic Seabird Database, Glacier Bay is rich in bottom topography. The presence of sills, basins, islands, headlands, and channels, as well as tides ranging between three and five meters, create numerous habitat types that differ in current speed and direction, two tidal forces thought to influence bird habitat use. Drew’s team set out to discover how fine-scale tidal features vary across the various marine habitats in Glacier Bay and how the resulting niches influence bird distribution.
Sample Collection and Analysis
To tackle this question, Dr. Drew and his research crew conducted a marine bird survey which covered the entire Glacier Bay coastline during the summers of 2000 through 2003 and classified fifteen bird species into three groups: surface feeders, mid-water feeders and bottom feeders based on foraging strategies. In addition, they used a computer-modeling program to model the tidal conditions of the bay, which was achieved by estimating the water surface elevation (depth) and tidal velocity (current speed) every 30 minutes at every location where a bird was observed. The model produced an instantaneous reading of the depth and current values associated with each bird.
Drew’s team found that differences in depth and current speed existed for each foraging group. Bottom foragers used habitats that were shallower and had slow currents speeds, surface forgers used habitats that were deeper and had fast current speeds, and mid-water foragers fell somewhere in between.
Bottom-water foragers are known to feed largely on sessile, filter-feeding organisms near the bottom and so the observation that this group was associated with shallow habitats was expected. This is because deeper water and higher current speeds expose birds to conditions that put a greater constraint on feeding efforts due to increased energetic costs. One bottom-water forager, white-winged scoters, was found to favor low current speeds. Current speeds are closely associated with tides and the observation suggests that the species may be feeding at particular times of the day when current speeds are lower to minimize the energetic costs required for foraging. Just as humans eventually adopted certain times of the day for eating (i.e. breakfast, lunch, and dinner-time), white-winged scoters might be participating in a similar “mealtime” moment during particular times of the day when current and tidal speeds are optimal!
The dive range of mid-water foragers appears to be influenced by the location of their prey in the water column, where trade-offs exist. For example, increased current speeds can lead a school of fish closer to the surface, but a current speed that is too high can be disruptive and increase the energetic costs of swimming. A member of the mid-water foragers is the pigeon guillemot, which feeds on pelagic schooling fish and epi-benthic fishes. Cormorants, also a mid-water forager, share a similar diet, but the two species revealed different foraging strategies. The tidal-features associated with pigeon guillemots most closely resembled that of diving ducks, indicating that pigeon guillemots take a benthic-feeding approach to fishing, whereas the cormorants take a mid-water forager approach. The cormorant’s long neck likely provides the species with an advantage at high current speeds.
Within the surface foraging group, arctic terns and mew gulls favored shallower habitats with higher than expected current speeds whereas other species used deeper habitats. High current speeds bring nekton to the surface and the fast currents that enter the narrow entrance of Adams Inlet –located on the northwestern arm of the bay- might be the reason that the terns selected the location as a colony site.
The complexity of Glacier Bay’s bottom topography creates varies niches. By observing water movement on a fine scale at numerous locations, Drew’s team successfully linked bird observations to instantaneous descriptions of small-scale water movement to help explain the distribution of bird populations in Alaska. The team hopes to improve the understanding of coastal habitat use and to increase the appreciating for marine protected areas that are rich with biodiversity.