Experimental shifts in egg–nest contrasts do not alter egg rejection responses in an avian host–brood parasite system

Abstract

Obligate brood parasitic birds exploit their hosts to provide care for unrelated young in the nest. Potential hosts can reduce the cost of parasitism by rejecting foreign eggs from the nest. Observational, comparative, and experimental studies have concluded that most hosts use the coloration and patterning of eggshells to discriminate between own and foreign eggs in the nest. However, an alternative hypothesis is that birds use the colour contrasts between eggshells and the nest lining to identify parasitic eggs (egg-nest contrast hypothesis). In support of this hypothesis, we found that the avian perceivable chromatic contrasts between dyed eggs and unmanipulated nest linings significantly and negatively covaried with the rejection rates of different dyed eggs of the great reed warbler Acrocephalus arundinaceus, a frequently parasitized host of the common cuckoo Cuculus canorus. To experimentally test whether egg-nest contrasts influence rejection, we reciprocally dyed both eggs and the nest lining of this host species with one of two colours: orange and green. Contrary to the egg-nest contrast hypothesis, host rejection patterns in response to dyed eggs were not altered by dyeing nests, relative to unmanipulated control eggs and nests. In turn, experimental egg colour was the only significant predictor of egg rejection rate. Our results demonstrate that egg-nest contrast is a collateral, not a causal factor in egg rejection, and confirm the conclusions of previous studies that hosts can rely on the parasitic egg's appearance itself to recognize the foreign egg in the nest.

The Perceptual and Cognitive Processes That Govern Egg Rejection in Hosts of Avian Brood Parasites

“…Consistent with this observation, hosts found to disproportionately reject brown egg models laid (at least moderately) blue-green eggs (Honza et al, 2007;Cassey et al, 2008;Soler et al, 2012;Hanley et al, 2017Hanley et al, , 2019bAbolins-Abols et al, 2019;Manna et al, 2020). Although past experiments found that nest contrast did not influence host egg recognition (Aidala et al, 2015;Hauber et al, 2015a), a blue nest lining did increase performance on an egg discrimination task by 25% in the blue-green egg laying American robin Turdus migratorius (Aidala et al, 2015). We feel this is still a promising line of research, particularly if future studies employ an experimental manipulation of nest background (Aidala et al, 2015) alongside sufficient variation in the color of egg stimuli to determine threshold values (sensu Lind, 2016).…”

Increased egg–nest visual contrast does not induce egg ejection in the eastern phoebe (Sayornis phoebe), an accepter host of the brood parasitic brown-headed cowbird (Molothrus ater).

“…increasing the visual contrast between artificial parasitic eggs and the nest background did not induce egg-rejection behavior, suggesting that egg-nest contrast is not a salient visual cue in making egg-rejection decisions in this parasite-host system. Because phoebes do not respond to cowbird parasitism at the egg stage, our experimental results do not allow us to conclude a general noneffect of egg-nest visual contrast across all parasite-host systems, though recent work suggests that is likely the case (Aidala et al, 2015;Hauber et al, 2015). We caution that phoebes' lack of behavioral response to natural and artificial parasitism is a limitation of this study system because it impedes our ability to determine which, if any, variables influence parasitic egg rejection.…”

“…In our study, phoebes used neither a discordancy nor template-based rule to reject parasitic eggs (as no rejection behavior occurred). Future studies should address the cognitive bases of egg-rejection behavior (such that may exist) in nonrejecting hosts along an egg color continuum that encompasses and surpasses the natural variation of parasitic and host eggs (Aidala et al, 2015; Croston & Hauber, 2015; Hanley et al, 2017; Hauber et al, 2015). Here, we only used discrete egg colors that matched the background and maculation of phoebes’ cowbird parasites in our artificial parasitism experiments because we were specifically testing whether egg–nest visual contrast could facilitate detection of cowbird parasitism (see Method).…”

“…And yet, relatively few studies have examined whether eggs’ proximate environmental milieu contributes to egg detection/ejection (Siefferman, 2006), with those that have done so garnering equivocal results (Aidala, Croston, Schwartz, Tong, & Hauber, 2015; Avilés, Martín-Gálvez, De Neve, Soler, & Soler, 2015; Gloag et al, 2014; Hauber, Aidala, Igic, Shawkey, & Moskát, 2015; Honza, Procházka, Morongová, Čapek, & Jelínek, 2011; Honza, Šulc, & Cherry, 2014). Increasing evidence suggests that parasitic eggs have evolved a cryptic function in some host–parasite systems, particularly among Australasian cuckoo–host systems (Gloag et al, 2014; Langmore et al, 2005; Langmore, Stevens, Maurer, & Kilner, 2009; Thorogood, Kilner, & Rasmussen, 2017).…”

Revealing the roles of egg darkness and nest similarity for a cryptic parasite egg versus host's cognition: an alternate coevolutionary trajectory

“…Furthermore, we suggest that the cryptic egg hypothesis is more suitable for explaining host nests that are enclosed or in dense vegetation cover with poor light conditions rather than open host nests, because dim-coloured eggs with an egg darkness or nest similarity effect are only presumed to have an effect under poor light conditions [17]. This may be one of the major reasons why some studies supported the egg darkness or nest similarity component of the cryptic egg hypothesis [13,21,24,25] while others did not [23,26–28]. Although poor light conditions may favour cuckoos evolving dark eggs, egg colour mimicry is still an alternate strategy in such conditions.…”