Exploration speed in captivity predicts foraging tactics and diet in free-living red knots

Variation in foraging tactics and diet are usually attributed to differences in morphology, experience, and prey availability. Recently, consistent individual differences in behaviour (personality) have been shown to be associated with foraging strategies. Bolder or more exploratory individuals are predicted to have a faster pace-of-life and offset the costs of moving more or in risky areas, with higher energetic gains by encountering profitable foraging opportunities and prey. However, the relationship between personality, foraging, and diet is poorly understood.

We investigated how exploratory behaviour in red knots (Calidris canutus) is associated with foraging tactics and diet by combining laboratory experiments, field observations, and stable isotope analysis. First, we developed a mobile experimental arena to measure exploration speed in controlled settings. We validated the method by repeated testing of individuals over time and contexts. This setup allowed us to measure exploratory personality at the field site, eliminating the need to bring birds into captivity for long periods of time. After releasing birds within days of their capture, we asked whether exploration speed was associated with differences in foraging tactics and diet in the wild.

We found that tactile foraging red knots mainly caught hard-shelled prey that are buried in the sediment, whereas visual foraging knots only captured soft preys located close to or on the surface. We also found that faster explorers showed a higher percentage of visual foraging than slower explorers. By contrast, morphology (bill length and gizzard size) had no significant effect on foraging tactics. Diet analysis based on δ15N and δ13C stable isotope values of plasma and red blood cells confirmed our field observations with slower explorers mainly consumed hard-shelled prey while faster explorers consumed more soft than hard-shelled prey.

Our results show that foraging tactics and diet are associated with a personality trait, independent of morphological differences. We discuss how consistent behaviour might develop early in life through positive feedbacks between foraging tactics, prey type, and foraging efficiency.

Methods
Isotope data: Red blood cell and plasma d13C and d15N isotopes collected from the blood samples of red knots.

Exploration speed: Movement tracjectories were collected from the recordings of the top camera during experiments in the mobile arena. We used the distance between estimated positions to calculate speed. Errors in the positioning algorithm were filtered by excluding speeds higher than 200 cm/s. An individual’s exploration speed was calculated as the average speed during each 20 min trial. The dataset includes four repeats of exploration speed.

Proportion of explorative behaviour: Behaviours were collected from the recording of the side camera during experiments in indoor arena and mobile arena. Behavioural budget is calculated of the time that the focal bird spend exploring (walking and probing) the arena. 

Foraging behaviour: Foraging behaviour was collected from the observations in the field. Focal birds were filmed for 20 min or up to the moment they flew away. Behavioural budget is calculated for two types of foraging tactics: ‘tactile foraging’ when a bird is probing continuously with the bill into the substrate, and ‘visual foraging’ when a bird is scanning the area in front of it and pecking at items seen on the substrate surface. 

Prey-foraging behaviour: In cases that we could identify the ingested prey in the field, categorized as hard-shelled prey (e.g., cockles or Baltic tellins) or soft prey (e.g., brown shrimp or polychaete worms) to investigate the relationship between foraging tactics and diet.