The intricacies of food webs

We tend to see “food chains” as a simple succession, from herbivores eating plants, to carnivores eating herbivores but in reality, nature rarely fits neatly into little boxes. The notion of chains is more accurately replaced by “food webs” and these webs are as complex, intricate, and tangled as is the web each of us has become very accustomed to: the World Wide Web. Natural food webs are worldwide as well, with connections and ramifications reaching far beyond simple or straightforward relationships: think, for example, of a Nashville Warbler in the boreal forest gorging on insects all summer long. Once it migrates to its wintering grounds in Central America, its diet shifts dramatically. The Nashville Warbler prefers to then drink nectar from flowers it pierces with its sharp beak!

Animals can change their diets during the annual cycle or throughout their lives, with young eating different food types than do adults (many young of sparrows, for example, are fed a protein-rich diet of insects while in the nest, even if their parents prefer seeds and plant matter). A food web in a graphic form would resemble a bowl of pasta with connections in all directions.

At Cabot Head, a few years ago, I watched – with delight – an adult Bald Eagle eating a big fish on a boulder on the shore of Wingfield Basin. A few crows were nearby, waiting for their chance to snatch a morsel. Apparently satiated, the eagle let go of the barely-eaten fish, which slipped back into the water. Before the crows managed a rescue, however, an otter swimming by grabbed the fish, swam further away and hopped out onto the land and into the forest to enjoy its happenstance meal, away from the pesky crows.

This fall, on October 18, a family of three otters were feasting on fish on the other side of Wingfield Basin. Their meal was hidden behind big boulders but they seemed to be enjoying themselves tremendously. Later, we watched the otters swimming across Wingfield Basin and then hauling ashore before disappearing into the forest. Later, in the afternoon on that same day, we looked back to where the otters had been, only to witness three Bald Eagles (one adult and two immature) and two Common Ravens finishing the otters’ leftover meal! It was a reversal of the relationship observed before and a small, but telling, illustration of the intricacies of food webs.

There is now only a week left of migration monitoring at Cabot Head Research Station. Fall temperatures have finally arrived, delivered on the backs of strong North winds blowing from Georgian Bay. Over the last four days, nets have stayed closed on three of them due to high winds. Only on Sunday, October 23, were we able to run the nets. And to our surprise, we caught 90 birds, 82 of them Black-capped Chickadees! We were kept quite busy with these feisty little balls of feathers.

In the last week, another surprise was the capture of a Northern Parula on October 17, the latest date ever for this species at Cabot Head. This individual was recaptured a few days later, on October 20, raising doubts as to the chances of this bird completing its migration. Northern Parula is a species not commonly detected in the fall: it was seen in eight years between 2002 and 2015, mostly in September, and in very small numbers.

Posted in STATION NOTES / BLOG

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