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Biology

Diving birds find an abundant food source in a surprising place

 

Paper: Sato, N. N., Kokubun, N., Yamamoto, T., Watanuki, Y., Kitaysky, A. S. and Takahashi, A. (2015). The jellyfish buffet: jellyfish enhance seabird foraging opportunities by concentrating prey. Biol. Lett. 11, 20150358.

Background
Jellyfish blooms (large aggregations of jellyfish) can have a substantial impact on the food web of an ecosystem. Often, these blooms are considered to be negative events because the jellyfish can eat a large proportion (up to 50%, in some cases) of the zooplankton population in a given area. That has trickle down effects on the food web – when the jellies eat all that food, there is not as much food left to support fish and other invertebrate populations, and then in turn, there are not as many fish or other invertebrates available to higher level predators like tuna. However, this team of researchers working in Alaska had another idea: what if these jellyfish blooms were actually helping certain animals feed better?

Small fish are drawn to jellyfish tentacles for two main reasons: they find protection from predators there, and they can feed on the jelly’s sloppy seconds (whatever zooplankton that do not end up in the jelly’s mouth). Because these smaller fish aggregate among the tentacles, it makes the jellyfish a target for fish-eating animals, like diving sea birds. The idea is that these jellies represent a one-stop shop for the diving birds – they can eat more in the same location while also expending less effort to get the same amount of food.

Methods
To see if diving birds were actually taking advantage of these mini schools of fish in the Bering Sea of Alaska, the researchers used a new type of technology: an animal-borne video logger. They were able to attach these mini recorders to thick billed murres (Figure 1) to get video data of what those birds were seeing in the water as well as environmental data about the water itself – kind of like a go pro with a thermometer and a salinity meter. Separate sensors on the camera also recorded the bird’s speed, acceleration, and wingbeat frequency. They were able to analyze the footage with the environmental and speed data to get a more complete picture of what these birds were doing as they dove underwater to feed.

Figure 1 – Thick billed murre diving

Figure 1 – Thick billed murre diving

 

 

 

 

 

 

Results
The researchers analyzed footage from 200 feeding events and found that 179 of those events included encounters with jellyfish. Out of those encounters, about 28% of the jellyfish tentacles had small fish hiding inside. Out of all the fish feeding events the researchers saw in the tape, 20% involved feeding on those small fish hiding in the tentacles. That may seem like a small percentage, but the behavior is widespread – every individual the researchers followed fed on those fish (Figure 2). Additionally, the probability that the bird would attack the fish underneath the jelly increased as the number of fish surrounding the jelly increased.

Figure 2 – Video of the murre feeding on the fish underneath the jelly

The other interesting thing that the researchers found was that the birds are feeding on the ascending portion of their dives. Previously, scientists thought that the birds only fed on the bottom of the dive. Figure 3 shows how the bird’s diving behavior changes when it sees a jelly with fish around it – the bird will increase its wingbeat frequency and speed in order to stop and eat the fish at the jellyfish.

Figure 3 – Changes in the murre’s diving behavior with and without a jellyfish. The feeding event is marked with the black triangle – notice that the acceleration (red line) and velocity (green line) both decrease as the bird pauses to feed.

Figure 3 – Changes in the murre’s diving behavior with and without a jellyfish. The feeding event is marked with the black triangle – notice that the acceleration (red line) and velocity (green line) both decrease as the bird pauses to feed.

Discussion
This paper was important because it revealed two new things about the behavior of the diving murre: 1) the bird uses fish underneath jellyfish as feeding opportunities and 2) the birds are feeding on the ascent of the dive instead of at the bottom. Neither discovery would have been possible without the new animal-borne video logger to record the birds’ behavior as they dive!

The findings here suggest that the increase of jellyfish might actually be a good thing for these birds – more jellies means more tiny fish underneath them for sea birds to eat. The blooms are altering food webs from the bottom (jellyfish eating zooplankton), but they’re also creating new foraging opportunities for top predators like these diving birds. For once, news about the blooms isn’t all doom and gloom!

Engage
What shorebirds are there in your area and what do they feed on? If you don’t know, get out there with some binoculars and a bird book from your public library and find out!

Discussion

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  1. […] to observe behaviors of animals that previously eluded in-depth (or at-depth!) study (ex. diving seabirds). The researchers attached these cameras on adult green turtles using temporary glue. After 24-72 […]

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