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Johnny Rooks digging for kelp maggots on the Falkland Islands.

By Keith L. Bildstein, Ph.D., Sarkis Acopian Director of Conservation Science
and  Katie Harrington, Hawk Mountain Research Associate
Hawk Mountain Sanctuary

29 August 2016

Bear with me on this; it is really quite exciting…

Kelp maggots are the larvae of coastal dipterid flies that feed on rotting seaweeds, including kelp that drifts up along sandy shorelines.  In the northern hemisphere, these larvae (aka maggots) are themselves fed upon by shorebirds and passerines, including crows, which excavate them from kelp wracks that accumulate along the shorelines of sandy beaches.  Although studied little south of the equator, kelp maggots also inhabit the rotting kelp that drifts up along the shorelines of the Falkland Islands, including those on Saunders Island, a 49-square mile landmass in the northwestern part of the archipelago.

Neck Maggot Feeding Summer

Johnny Rooks digging for kelp maggots

For more than four years, we and our colleagues have watched the Striated Caracaras (aka Johnny Rooks) that we have been studying on Saunders Island, digging and probing beached kelp adjacent to penguin colonies there, and have wondered if these largely scavenging birds were receiving ecologically significant amounts of nutrition from the buried invertebrates.  In February of this year, we began to study this feeding behavior in detail using a protocol that allows us to quantify the rate at which individual caracaras secure maggots.  Our observations involve counting the numbers of maggots individual birds catch and consume during 30-sec feeding bouts.  We continued to collect data using this protocol on our most recent trip this August to assess the extent of seasonal differences in feeding rates.  We also collected maggots and weighed them to determine their individual mass, allowing us to determine their nutritional value. Our results, albeit preliminary, suggest that although birds capture maggots at higher rates in austral summer than in austral winter, in both seasons they manage to do so at rates of capture that are high enough to provide substantial nutrition for the birds engaging in this behavior for several hours or more daily.

The dogged determination and methodic nature with which the rooks dig is impressive, with many birds digging alternately with their left and right feet five inches or more into the rotting kelp while securing dozens of maggots over brief periods.  Clearly, more study is needed, but our initial observations suggest that this nutritional resource plays a significant role in the life of Striated Caracaras year round.

Summer maggots

Exposed kelp maggots

Intriguingly, on the most recent trip we also saw groups of rooks digging in upland pastures where they were feeding on what appeared to be small earthworms and grass grubs, with about the same rate of capture as when they were catching kelp maggots.  On our next trip in February 2017, we plan to expand our observations considerably.  In the interim, we will be presenting preliminary results of our work at the annual meeting of the Raptor Research Foundation in Cape May, New Jersey, in October.

Of course, insect eating is not unknown in raptors.  Kites, American Kestrels, and many other falcons routinely do so, as do larger birds including Steppe Eagles overwintering in Africa.  However, digging in the ground for insects is relatively uncommon among birds of prey.  Honey buzzards reportedly do so, and kites and Common Buzzards dig for earthworms in recently plowed fields in Europe.  That said, at least some rooks appear to do it routinely as well, and not only on Saunders Island.  In August 2013, during a short trip to Steeple Jason, a tiny island in the Falklands more than 50 kms from Saunders Island, one of us saw large numbers of caracaras digging for earthworms in peaty soil at the base of the island’s steep escarpment.

Our work with caracaras indicates that they are severely food stressed in winter on the Falklands.  Digging in the ground for invertebrates at first may not seem “raptorly,” but beggars cannot be choosers, and the taste of a live invertebrate may beat that of a rotting vertebrate anyway.

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