Abstract
Brood parasitized and/or colonial birds use egg features as visual identity signals, which allow parents to recognize their own eggs and avoid paying fitness costs of misdirecting their care to others’ offspring. However, the mechanisms of egg recognition and discrimination are poorly understood. Most studies have put their focus on individual abilities to carry out these behavioural tasks, while less attention has been paid to the egg and how its signals may evolve to enhance its identification. We used 92 clutches (460 eggs) of the Eurasian coot Fulica atra to test whether eggs could be correctly classified into their corresponding clutches based only on their external appearance. Using SpotEgg, we characterized the eggs in 27 variables of colour, spottiness, shape and size from calibrated digital images. Then, we used these variables in a supervised machine learning algorithm for multi-class egg classification, where each egg was classified to the best matched clutch out of 92 studied clutches. The best model with all 27 explanatory variables assigned correctly 53.3% (CI = 42.6–63.7%) of eggs of the test-set, greatly exceeding the probability to classify the eggs by chance (1/92, 1.1%). This finding supports the hypothesis that eggs have visual identity signals in their phenotypes. Simplified models with fewer explanatory variables (10 or 15) showed lesser classification ability than full models, suggesting that birds may use multiple traits for egg recognition. Therefore, egg phenotypes should be assessed in their full complexity, including colour, patterning, shape and size. Most important variables for classification were those with the highest intraclutch correlation, demonstrating that individual recognition traits are repeatable. Algorithm classification performance improved by each extra training egg added to the model. Thus, repetition of egg design within a clutch would reinforce signals and would help females to create an internal template for true recognition of their own eggs. In conclusion, our novel approach based on machine learning provided important insights on how signallers broadcast their specific signature cues to enhance their recognisability.
Highlights
The ability to identify other individuals is an essential cognitive skill of animals
The best classification was achieved when the algorithm used the 27 variables calculated by SpotEgg and was trained with 4 eggs per clutch (Table 2)
This is a good achievement taking into account that the algorithm faced a multi-class classification problem where probability of classification success by chance was only ca. 1%. This achievement appears good due to the extraordinary resemblance of eggs among clutches of the studied Eurasian coots, at least, from a human eye perspective. This accuracy level was similar to the discriminatory ability found in several bird species towards parasitic eggs [23, 25, 42, 43]
Summary
The ability to identify other individuals is an essential cognitive skill of animals. There is substantial evidence on the ability of birds to discriminate voices at individual level [1,2,3,4, 15,16,17,18,19], while the mechanisms underlying individual visual recognition are still poorly understood [1, 20,21,22]. Due to the fitness costs paid by parasitized hosts, their ability to identify their own progeny and discriminate it from the parasite confers an obvious adaptive advantage [24,25,26,27]
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