LISTENING IN ON DIVING MURRES

Thick-billed murres (Uria lomvia). Credit: Art Sowls, USFWS, via Wikimedia Commons.

There's an exciting new paper in Marine Ecology Progress Series (MEPS) that lets us listen in on diving murres to learn more about their feeding ecology. Specifically:

1. what prey they catch by day
2. what prey they catch by night (really exciting!)
3. and how they might find these prey that exist in patchy distributions

The lead author is Kelly Benoit-Bird, at Oregon State University, whose acoustics work I profiled in The BP Cover-Up, my cover article in Mother Jones last year. You can read about her MacArthur Foundation fellowship here.

Red arrow marks the Pribilof Islands. Credit: Aleut International Association.

  
Research for the MEPS paper was conducted in Bering Sea off the Pribilof Islands, home to some 200 million breeding seabirds, one of the largest concentrations in the North Pacific.

The birds nest here because of the phenomenal seasonal abundance that cranks up in these waters each spring with the return of sunlight.

  

Phytoplankton bloom around the Pribilof Islands, Bering Sea. Credit: NASA image by Jeff Schmaltz, MODIS Rapid Response Team, Goddard Space Flight Center, via the Earth Observatory.

Here's how multiple streams of data were gathered simultaneously as the researchers' boat motored along predetermined 10-kilometer (6.2-mile) long transect lines through the waters around the Pribilofs:

• An observer with binoculars stood on the bow of the boat, sighting murres flying through the air and/or foraging at the surface
• A 4-frequency echosounder system was used to provide nearly continuous information from underwater on the presence of zooplankton and fish (identified by their acoustic "signatures")
• To verify the truth of those acoustic signatures, net tows were made at the the beginning of each transect to capture fish and zooplankton
• At the start and finish of each transect, a CTD (conductivity, temperature, depth) profile was conducted to a depth of 100 m (328 feet) or to within 5 m (16 ft) of the seafloor in shallower waters; the CTD also gathered data on dissolved oxygen and, through a transmissometer and a fluorometer, different data on the presence of fish and/or zooplankton

Common murres (Uria aalge). Credit: Michael Haferkamp via Wikimedia Commons.

  
This system allowed the team to observe thousands of bubble trails made by diving murres and correlate them with schools of krill, juvenile pollock, and squid—their primary prey.

Some of their findings:

• Individual diving murres were more likely to be found alone in shallow water (<100 m) during the day
• Aggregations of birds were more likely to be found in the outer shelf and slope zones (>100m) at night

Why? The authors suggest:

[T]hat murres use different tactics when foraging on prey in different habitats and likely reflects the differences in diet noted during the same time period, with parents feeding their chicks small fish that could be caught near the colony while consuming krill and squid that are primarily accessible in deep waters, far from the colonies at night. There was, however, no significant difference in the rate of bird detections during the day and night, suggesting that both periods are important for foraging despite these different tactics. These results indicate that using only daytime observations of murres as an index of habitat use and foraging activity can significantly skew any results drawn from those data, underestimating the importance of deepwater habitats and making the calculation of accurate energy budgets impossible. 

  

Murres on nests, Pribilof Islands, Alaska. Credit: Allen Shimada NOAA/NMFS/OST/AMD via Flickr.

   
As to how the birds find and track their prey—in deep water, often in the dark—the authors note:

The ability of murres to successfully track the abundance, density, and accessibility of prey that occurred at depths between 10 and 100 m (32 and 328 feet) raises questions about how they gather information to exploit these resources effectively, particularly at night. Birds could be directly detecting the prey using vision or chemosensory mechanisms. However, both would likely be inefficient for deep prey, and vision would not be effective at night when foraging activity is equal to daylight foraging activity.

  

Murre. Credit: christopher.woo via Flickr.

   
Possibilities include: 

• Birds cue to surface ocean color, which indicates the presence or absence of phytoplankton [see 3rd photo in this post, above]: no correlation was found
• Birds cue to a physical characteristic, such as temperature: a correlation was found between murres and warmer surface temperature 

Further possibilities revolve around birds cuing to other birds—something that seems obvious if you observe birds at sea... though obvious isn't always correct: 

While the hypothesis that colonies function as ‘information centers’, where group members can learn the location of food by following successful foragers, has largely been discounted... there have been a number of studies showing that the detection of other predators that are already exploiting a prey patch, a process known as ‘local enhancement’, can be an effective foraging strategy... Local enhancement occurs in another species of murre, but its importance is dependent on the predictability of prey... Substantial experimental and at-sea data suggest that local enhancement works whenever feeding groups are more conspicuous than food patches... which is likely to be the case for the deep prey observed here. The local enhancement hypothesis is supported by the prevalence of aggregation among acoustically observed murres... Whatever the reason for aggregating, murres in groups were spaced in a highly regular pattern with an average of 50 m between individuals, suggesting that, while grouped, birds may be attempting to avoid direct competition, a common cause of uniform spacing between individuals.

 

Patchiness of prey and distribution of birds. Click for larger image. Credit: Benoit-Bird, et al. MEPS. DOI:10.3354/meps09408.

 

The authors conclude:

Using this [fisheries acoustic] technique, we were able to observe the diving depths of individual birds, assess these dives in relation to [prey] patch size, density, depth, and type of prey both day and night, and show that birds on the surface were a good proxy for foraging effort taking place in a given area. The concomitance of these data is made possible by acoustic approaches and provides new insights into the predator−prey relationship. During the breeding season, diving murres in the southeastern Bering Sea show strong selection for prey patches with specific characteristics, and the high degree of overlap between murres and their prey at a range of scales shows effective information gathering about prey by these birds.  

 

Thick-billed murre. Credit: susanvg Susan van Gelder via Flickr.

  
The paper (♥ open access): 

• Benoit-Bird KJ, Kuletz K, Heppell S, Jones N, Hoover B (2011) Active acoustic examination of the diving behavior of murres foraging on patchy prey. Mar Ecol Prog Ser 443:217-235 DOI:10.3354/meps09408