Testing assumptions of coevolution in an egg-rejecting brood parasite host: Uncovering sensory, cognitive, and evolutionary drivers of responses to parasitism in American robins (Turdus migratorius)

Hosts of brood parasitic birds face fitness costs associated with rearing unrelated offspring. In response, the recognition and rejection of parasitic eggs is a common host defense. Brown-headed cowbirds (Molothrus ater) challenge coevolutionary theory, because although they exploit over 200 host species, they lay non-mimetic eggs, and most hosts do not combat cowbird parasitism with egg rejection. American robins (Turdus migratorius) are one of a handful of cowbird hosts known to recognize and remove cowbird eggs from the nest. I addressed the mechanistic and evolutionary drivers of egg rejection in this host species, by disentangling the roles of spectral tuning and visual physiology on the behavioral outcome of egg rejection, by estimating the costs of parasitism which may drive egg rejection behavior, and by addressing the reciprocal effects parasitism on host egg color variation and its role in mediating rejection decisions. I also test assumptions underlying the evolvability of host egg rejection responses in this system. In Chapter 1, I lay out an overview of brood parasitism as a reproductive strategy and brood parasite-host ecology, and highlight evolutionary mechanisms and consequences of coevolution in these systems. In Chapter 2, I test the hypothesis that foreign egg rejection is driven proximately by perceivable differences in ground color between host and parasitic eggs across the entire avian spectral sensitivity range. I show that the rejection of artificially dyed eggs is mediated by input from all four avian single-cone photoreceptors, and that more divergent model `parasitic\u27 eggs are indeed rejected at higher rates. However, the cowbird egg does not conform to this prediction, because both model and real cowbird eggs are rejected in 100% of experimental trials despite their lower overall discriminability from robin eggs. This may indicate a cowbird-egg specific rejection response in robins. In Chapter 3, I test a critical assumption underlying the evolution of cowbird-specific egg rejection responses in robins, by assessing the hypothesis that cowbird parasitism imposes recoverable costs on robin hosts. My results indicate that cowbird chicks fare poorly when reared alongside robin chicks, but parasitism per se still reduces nesting success for robins; thus, rejection of cowbird eggs serves a function to eliminate the cost of parasitism. In Chapter 4, I examine a critical assumption underlying all of host-parasite coevolutionary theory, namely that host defenses can evolve genetically in response to parasitism. I address the hypothesis that egg rejection is repeatable in our study population, as repeatability is prerequisite to the evolution and spread of a behavioral trait, including a predictor of the trait\u27s genetic heritability. As predicted, egg rejection behavior in American robins was found to be highly repeatable for intermediately-rejected model egg colors within the same nesting attempt, irrespective of potentially confounding ecological and temporal factors. Finally, in Chapter 5, I test predictions stemming from alternate hypotheses that egg rejection evolved in response to cowbird (non-mimetic) versus conspecific (mimetic) parasitism, by investigating the degree of color variation within robins\u27 own clutches, and the effect of experimentally manipulating intraclutch color variation. I used both observational and experimental data, and found that egg color varies more between clutches than among egg within a single clutch, yet experimental manipulated intraclutch color variation did not affect rejection rates. These results support the scenario of historical parasitism by non-mimetic parasites. Variation among the findings of similar studies pertaining to hosts of mimetic parasites may be explained by hosts\u27 use of different cognitive mechanisms in the decision to reject foreign eggs, However, for hosts of non-mimetic parasites, investigating egg color variation and its effect on egg rejection is not informative about different cognitive decision-making rules, as predictions under each mechanism are similar - that there will be no effect of a history of parasitism on intraclutch color variation (observational patterns) or rejection rate (experimental data). This body of research presents compelling evidence in support of egg rejection by robins as a specific response to historical cowbird parasitism, and has highlighted important components of the sensory, cognitive, functional and evolutionary processes underlying egg rejection in this paradoxical brood parasite-host system

Which egg features predict egg rejection responses in American robins? : replicating Rothstein's (1982) study

Rothstein (Behavioral Ecology and Sociobiology, 11, 1982, 229) was one of the first comprehensive studies to examine how different egg features influence egg rejection behaviors of avian brood parasite-hosts. The methods and conclusions of Rothstein (1982) laid the foundation for subsequent experimental brood parasitism studies over the past thirty years, but its results have never been evaluated with replication. Here, we partially replicated Rothstein's (1982) experiments using parallel artificial model egg treatments to simulate cowbird (Molothrus ater) parasitism in American robin (Turdus migratorius) nests. We compared our data with those of Rothstein (1982) and confirmed most of its original findings: (1) robins reject model eggs that differ from the appearance of a natural robin egg toward that of a natural cowbird egg in background color, size, and maculation; (2) rejection responses were best predicted by model egg background color; and (3) model eggs differing by two or more features from natural robin eggs were more likely to be rejected than model eggs differing by one feature alone. In contrast with Rothstein's (1982) conclusion that American robin egg recognition is not specifically tuned toward rejection of brown-headed cowbird eggs, we argue that our results and those of other recent studies of robin egg rejection suggest a discrimination bias toward rejection of cowbird eggs. Future work on egg recognition will benefit from utilizing a range of model eggs varying continuously in background color, maculation patterning, and size in combination with avian visual modeling, rather than using model eggs which vary only discretely

Which egg features predict egg rejection responses in American robins? Replicating Rothstein's (1982) study

Rothstein (Behavioral Ecology and Sociobiology, 11, 1982, 229) was one of the first comprehensive studies to examine how different egg features influence egg rejection behaviors of avian brood parasite-hosts. The methods and conclusions of Rothstein (1982) laid the foundation for subsequent experimental brood parasitism studies over the past thirty years, but its results have never been evaluated with replication. Here, we partially replicated Rothstein's (1982) experiments using parallel artificial model egg treatments to simulate cowbird (Molothrus ater) parasitism in American robin (Turdus migratorius) nests. We compared our data with those of Rothstein (1982) and confirmed most of its original findings: (1) robins reject model eggs that differ from the appearance of a natural robin egg toward that of a natural cowbird egg in background color, size, and maculation; (2) rejection responses were best predicted by model egg background color; and (3) model eggs differing by two or more features from natural robin eggs were more likely to be rejected than model eggs differing by one feature alone. In contrast with Rothstein's (1982) conclusion that American robin egg recognition is not specifically tuned toward rejection of brown-headed cowbird eggs, we argue that our results and those of other recent studies of robin egg rejection suggest a discrimination bias toward rejection of cowbird eggs. Future work on egg recognition will benefit from utilizing a range of model eggs varying continuously in background color, maculation patterning, and size in combination with avian visual modeling, rather than using model eggs which vary only discretely.we thank the Human Frontiers Science Program (to MEH) and the Harley Jones van Cleave Professorship in Host- Parasite Interactions at the University of Illinois, Urbana- Champaig

Experimental Shifts in Intraclutch Egg Color Variation Do Not Affect Egg Rejection in a Host of a Non-Egg-Mimetic Avian Brood Parasite

<div><p>Avian brood parasites lay their eggs in the nests of other birds, and impose the costs associated with rearing parasitic young onto these hosts. Many hosts of brood parasites defend against parasitism by removing foreign eggs from the nest. In systems where parasitic eggs mimic host eggs in coloration and patterning, extensive intraclutch variation in egg appearances may impair the host’s ability to recognize and reject parasitic eggs, but experimental investigation of this effect has produced conflicting results. The cognitive mechanism by which hosts recognize parasitic eggs may vary across brood parasite hosts, and this may explain variation in experimental outcome across studies investigating egg rejection in hosts of egg-mimicking brood parasites. In contrast, for hosts of non-egg-mimetic parasites, intraclutch egg color variation is not predicted to co-vary with foreign egg rejection, irrespective of cognitive mechanism. Here we tested for effects of intraclutch egg color variation in a host of nonmimetic brood parasite by manipulating egg color in American robins (<i>Turdus migratorius</i>), hosts of brown-headed cowbirds (<i>Molothrus ater</i>). We recorded robins’ behavioral responses to simulated cowbird parasitism in nests where color variation was artificially enhanced or reduced. We also quantified egg color variation within and between unmanipulated robin clutches as perceived by robins themselves using spectrophotometric measures and avian visual modeling. In unmanipulated nests, egg color varied more between than within robin clutches. As predicted, however, manipulation of color variation did not affect rejection rates. Overall, our results best support the scenario wherein egg rejection is the outcome of selective pressure by a nonmimetic brood parasite, because robins are efficient rejecters of foreign eggs, irrespective of the color variation within their own clutch.</p></div

Summary of results of experimental parasitism following the manipulation of clutch contents.

<p>Bars represent the rejection rates for parasitic eggs in each experimental group (<u>+ binomial</u> SE estimates). Sample sizes are indicated inside bars.</p

Summary of published studies on egg rejection responses (relative to controls) to experimental brood parasitism, where the methodology included manipulations to increase intraclutch egg appearance variation.^{^a}

<p>^a “Parasite Mim./Nonmim.” indicates whether natural parasitic eggs mimic those of hosts. “Exp. Mim./Non.” indicates whether eggs used in artificial parasitism mimicked those of hosts. “Effect” indicates the induced change in the rate of rejection of experimental eggs.</p><p>^b To our knowledge, this is the only previous study to experimentally both increase and decrease intraclutch color variation.</p><p>Summary of published studies on egg rejection responses (relative to controls) to experimental brood parasitism, where the methodology included manipulations to increase intraclutch egg appearance variation.^{^{<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0121213#t002fn001" target="_blank">a</a>}}</p

Univariate ANOVA outputs.

<p>Summary of ANOVA results describing differences in the proportional photoreceptor catches between eggs within versus between unmanipulated host nests. For each photoreceptor type, ‘Mean (SE)’ represents the proportionate receptor catch per egg, and standard error. JNDs values indicate discriminable chromatic difference between two eggs, as perceived by avian visual physiology (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0121213#sec002" target="_blank">Methods</a>). For JNDs, mean JND values are shown both for within (W) and between (B) nest comparisons. Significant p values for JNDs indicate that mean discriminability was greater between nests than would be expected based on variation within nests. For all measures, there is significantly more variation between nests than within clutches.</p><p>Univariate ANOVA outputs.</p

Summary of binomial GLMM outputs.

<p>Summary of GLMM outputs describing the effects of experimental manipulation, nesting stage (laying versus incubation), and Julian date on the likelihood of the rejection of ‘parasitic’ eggs.</p><p>Summary of binomial GLMM outputs.</p

Representative egg color spectra, with experimentally manipulated nest (inset).

<p>Representative spectra showing each of the three colors used in the egg rejection experiment, in addition to natural American robin egg spectrum. Pale-mimetic and vivid-mimetic paints were used to manipulate the color of real robin eggs. Blue paint was used to color plaster-of-Paris model parasitic eggs. The unmanipulated spectrum represents the average spectrum of real robin eggs. Inset shows a representative nest with experimentally increased variation in egg color, showing two natural robin eggs painted with vivid-mimetic paint, one painted with pale-mimetic paint, and one blue model egg.</p