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Mockingbirds tend to reject pure white, unspotted, cowbird eggs (i), while they accept other cowbird colour morphs: blue–green (ii) and both heavily and lightly spotted white eggs (iii and iv, respectively). The chalk-browed mockingbird has a ( b) blue–green spotted egg and is parasitized by the shiny cowbird, which has polymorphic eggs that vary in coloration and spotting (eggs i–iv). Thus, dotted areas represent acceptance errors, the crosshatched area represents rejection errors, and the linear hatched area represents correct rejection decisions. last laid eggs that appear different) and stimulus set 2 represents eggs that should be rejected (e.g. Here we illustrate two sets of stimuli, such that stimulus set 1 represent eggs that should not be rejected (e.g. When the difference between the template and the focal stimulus is greater than the acceptance threshold (solid vertical line), the theory predicts rejection by the host, but hosts should accept the stimulus when the difference is less than the threshold. Hosts of avian brood parasites can compare their own eggs (or an internal template of their eggs) with other eggs in their nests. in making decision between potential non-self and self (modified from ). The optimal acceptance threshold hypothesis is a theoretical model used to understand the outcomes of discrimination decisions between ( a) stimuli that vary relative to a template, e.g. an acceptable choice in one situation would not necessarily be an adaptive decision in another ( figure 1 a).įigure 1.
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In each case, stimuli vary and the optimal choice is context dependent, i.e. This ‘optimal acceptance threshold hypothesis' was proposed almost 30 years ago and has been used to predict decisions across a range of contexts: pollination, same-sex partnership, social insect nest-mate recognition and foreign egg rejection. One approach for making decisions between stimuli is to set a (flexible) threshold based on the similarity of the traits of each stimulus, such that differences greater than a threshold elicit a response. Thus, the ability to produce flexible responses to novel situations is necessary for adaptive decisions. This article is part of the theme issue ‘The coevolutionary biology of brood parasitism: from mechanism to pattern’.ĭecision-making is critical for survival and reproduction, but animals often face challenging decisions involving discriminating between novel stimuli in a variety of contexts. These results uncover how a single component of a multicomponent cue can shift a host’s discrimination threshold and illustrate how the optimal acceptance threshold hypothesis can be used as a framework to quantify the direction and amount of the shift (in avian perceptual units) of the response curve across relevant phenotypic ranges. However, as predicted, hosts tolerated spotted eggs more than unspotted eggs, irrespective of colour. Although tests of this hypothesis typically assume that decisions are based on perceived colour dissimilarity between own and foreign eggs, we found that decisions were biased toward rejecting browner eggs. To do this, we recorded responses to eggs that varied with respect to two components: colour, ranging from bluer to browner than the hosts' own eggs, and spotting, either spotted like their own or unspotted. We tested how individual components of a relevant recognition cue (experimental eggs) contributed to behavioural responses of chalk-browed mockingbirds, Mimus saturninus, a frequent host of the parasitic shiny cowbird, Molothrus bonariensis. Decision-makers should respond to a stimulus when the perceived difference between that stimulus and a comparison template surpasses an acceptance threshold. The optimal acceptance threshold hypothesis provides a general predictive framework for testing behavioural responses to discrimination challenges.