The blunt pole is not a source of more salient recognition cues than the sharp pole for the rejection of model eggs by American robins (Turdus migratorius)

Abstract

Hosts of obligate avian brood parasites can reduce the costs of raising parasitic offspring by rejecting foreign eggs from their nests. Rejecter hosts use various visual and tactile cues to discriminate between own and foreign eggs. The blunt pole hypothesis specifically states that avian-perceivable visual information at and around the broader pole of the eggshell contains more salient recognition cues than does the sharp pole of the same egg. The directional prediction is, therefore, that eggs painted non-mimetically on their blunt pole should more likely be rejected relative to those similarly painted on their sharp pole. This hypothesis had been experimentally tested and its predictions supported solely in mimetic avian host-parasite systems, with hosts producing denser and more variable eggshell maculation patterns at the blunt pole, and in one species with immaculate eggs but still with distinctly discernible blunt-pole specific colouration. Here we aimed to expand upon these previous works and assessed whether the blunt pole of model eggs contains more salient egg rejection cues, relative to the sharp pole, for the American robin (Turdus migratorius), a robust rejecter of non-mimetic brown-headed cowbird (Molothrus ater) eggs. In this system host eggs are uniformly immaculate whereas the brood parasitic shell is maculated. We painted model cowbird-sized eggs on either the blunt or the sharp half to mimic the immaculate robin egg colours and the other half to resemble non-mimetic egg colours and patterns. There was no statistical support for the predicted outcomes of the blunt pole hypothesis in our trials as rejection rates were similar regardless of whether eggs were painted with non-mimetic colours on the blunt or sharp poles. Future work should test the role of asymmetrical signalling content for anti-parasitic rejection of eggs in additional host species, especially those with both immaculate own and mimetic parasitic eggs.

The limits of egg recognition: testing acceptance thresholds of American robins in response to decreasingly egg-shaped objects in the nest

“…[51,52]), but also whether (e.g. [40]) and how much they shift, and in which direction, with changing stimulus salience and multiple recognition cue components [53,54]. Here, the use of more extended shape series (relative to [26]) allowed us to estimate the curvature of the egg-rejection logistic function along with our gradients of shape variation, making these new results comparable with the recent series of egg-rejection experiments using continuous model (egg) trait variation(s) as predictor(s) of egg-rejection response patterns (sensu [31]).…”

A review of the cues used for rejecting foreign eggs from the nest by the Eurasian blackbird (Turdus merula)

“…Color discrimination may be more heavily weighted to the egg's blunt pole (Polačiková et al, 2007). Evidence for greater salience of the cues in this region appears in a variety of species (Polačiková & Grim, 2010), including the song thrush (Polačiková & Grim, 2010) (though not the American robin: see Hauber et al, 2021). As with other thrushes, maculation is denser at the blunt pole of blackbird eggs, and painting this pole immaculate blue led to significantly higher rejection rates than applying the same paint to the sharp pole (Polačiková & Grim, 2010).…”

Patterns of stress response to foreign eggs by a rejecter host of an obligate avian brood parasite

“…Importantly, the only other study we are aware of in Turdus thrushes investigating the effect of experimental parasitism on plasma corticosterone detected elevated corticosterone 24+ hours after the addition of the model egg (Ruiz‐Raya et al, 2018 ). However, sampling robins at 24 h following the experimental parasitism without tethering the model eggs would have meant that nearly all the females would have rejected the non‐mimetic parasitic eggs (e.g., Hauber, Hoover, et al, 2020 ) and that they would therefore have been unstimulated for several hours before capture. The 2‐h time window thus allowed us to catch females during or closely following stimulation by model eggs, and, according to our estimate, allowed sufficient time for any changes in corticosterone in most females reach or remain near peak levels.…”

“…If natural glucocorticoid release in response to parasitic eggs stimulates egg rejection, then minimal but long-term upregulation of the HPA axis in response to non-mimetic parasitic egg stimuli is consistent with the timeframe of rejection of parasitic eggs by American robins. For example, >90% of robins reject non-mimetic beige cowbird-sized eggs within 5 days (Luro et al, 2018), 2 days (Hauber et al, 2019), and even 1 day (Hauber, Hoover, et al, 2020), but only 33% (seven out of 21) birds rejected the same non-mimetic egg type within 2 h in this study (also see Scharf et al, 2019). However, individual variation in glucocorticoid levels in this study did not predict egg rejection: circulating corticosterone at our single time-point of sampling did not differ between females who accepted or rejected the non-mimetic beige eggs (Figure 3).…”