Selin Ersoy is watching red knots and trying to find one with a tag. Photo credit: Dieke de Boer

Red knots' preferred prey

Red knots are molluscivores, meaning the main prey type in their diet is shellfish. To digest these shellfish, they have a specialized muscular stomach, a ''gizzard'', which is capable of crushing the shells. Knots find these shellfish, like cockles and baltic tellins, by probing in the mud with their bill. To forage more efficiently, they have a sensory organ located in the tip of their bill, which uses differences in pressure to detect shellfish in the mud. Once found, the shellfish are handled and ingested whole

Video 1: Knot probing and ingesting shellfish (clip 25 sec)
''In this clip a red knot is probing with its bill in the mud, searching for shellfish. If you pay close attention, you can also see a red knot running by with a crab that it just caught"

But there are exceptions...

Although shellfish are the main prey item on their menu, red knots also forage on other prey items. For example, red knots regularly attempt to catch very agile shrimps, which requires a different foraging strategy than shellfish. In the clip below, you will see that a red knot, trying to catch a shrimp, will constantly look around to find it, and must be very quick when trying to catch it. They often fail, and in many cases, the shrimp is stolen by an alert gull in the neighborhood that noticed the knot had a shrimp.

Video 2: Knot catching and handling a shrimp (clip 57 sec)
-> ''Here you see a red knot going through the trouble of catching a shrimp, while also having to run from a hungry gull. Handling a shrimp also takes significantly longer than handling shellfish.''

Apart from shrimp, we have also witnessed red knots eating worms. Worms and shrimp have an advantage over shellfish, as they are rich in nutrition and cost little energy to digest. Whereas when eating shellfish, the red knots must use energy to crush the shells in their gizzard to get to the flesh. In only a few of the more than 100 videos of foraging red knots, they were seen eating worms, even though this seems like a very attractive prey type.

Video 3: Knot eating a worm (clip 35 sec)
-> ''This red knot displays a rare foraging strategy to catch worms, yet it seems to be quite successful.''

Back at NIOZ, we analyzed the field videos of the knots to determine how knots spend their time during low tide. This can be tricky, as the interpretation of the behaviours must be objective and precise. To do that, we started the analysis with making an ethogram in which all visible knot behaviours are listed and described in detail. We scored all knot behaviours as in ethogram using specialized software for behavioural coding, then we extracted duration and frequency of these behaviors to look at the percentage of time knots spend on foraging, what kind of prey they are eating, and other behaviours like sleeping, or vigilance.

Understanding the variety in foraging strategies, prey choice and individual differences as described in the previous blog post can give insight in how red knots deal with differences in environmental conditions and prey availability, especially in times of climate change and possible anthropogenic threats to their natural habitat.

This blog is written by Maartje van Deventer together with Selin Ersoy.  Analysis of the field videos are part of Maartje’s internship project with Selin and Allert Bijleveld.

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