Inter-Individual Variation in Anti-Parasitic Egg Rejection Behavior: A Test of the Maternal Investment Hypothesis

Abstract

Synopsis Hosts of avian brood parasites may reduce or forego the costs of caring for foreign young by rejecting parasitic eggs from the nest. Yet, many host species accept parasitic eggs and, even among rejecter species, some individuals go on to incubate and hatch them. The factors explaining the variation in egg rejection between species have received much theoretical and empirical attention, but the causes of intraspecific variation in different individuals’ propensity for accepting parasitic eggs are less well understood. Here we tested the maternal investment hypothesis, which predicts that hosts with costlier clutches will be more likely to reject parasitic eggs from their nest. We studied variation in the egg rejection responses of American robins (Turdus migratorius), a robust egg-rejecter host of the brood parasitic brown-headed cowbird (Molothrus ater), to 3D-printed cowbird-sized eggs which were painted dark blue, a color known to induce variable and repeatable egg rejection responses in individual robins. Costlier clutch investment was estimated by earlier laying date, larger clutch size, heavier unincubated yolk mass, and variable yolk steroid hormone concentrations. There was no statistical support for most of our predictions. However, we detected more concentrated and greater overall amount of deoxycorticosterone deposited in egg yolks of rejecters relative to acceptors, although this accounted for no more than 14% of variance in the data. Future work should test experimentally the potential physiological linkage between maternal egg yolk steroid investment and egg rejection propensity in this and other host species of avian brood parasites.

Self‐referent phenotype matching is a poor predictor of egg rejection by American Robins

“…1) was not experimental, but rather a correlative test of our focal hypothesis and, even with the statistically significant result, our ability to explain our result remains severely limited (i.e., PC3 only explained 4% of variance in receptor reflectance, and its model had R 2 = 12%; see Results). Therefore, beyond self‐referencing, other aspects of the ecological, life history, and physiological (including endocrine) traits of female robins must also contribute to their rejection patterns in this particular study paradigm (e.g., Abolins‐Abols and Hauber 2020a,b, Hauber et al 2020).…”

“…We assessed egg‐rejection behaviors as a function of the egg traits of individual robins throughout Champaign County, IL, USA, during the 2019 breeding season (May–July). Details of the study area and general methods are provided in Hauber et al (2020). Briefly, we searched for robin nests by inspecting planted trees in private orchards and tree farms.…”

“…1) and did not have an experimental comparison or control. Therefore, our approach must be classified as a correlational study (e.g., Abolins‐Abols and Hauber 2020a, Hauber et al 2020), where we quantified egg rejection as a function of the impact of variation in the natural robin‐egg coloration and size or shape between individuals. Moreover, we did not capture and band robins.…”

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

“…[ 32 ] for the egg-rejection rates of robin-blue mimetically painted control eggs (one rejection out of n = 15 trials). To deploy the two new series of model eggs, during May–June, 2020, we located active nests of free-living American robins, with permission, throughout private tree farms, and gardens, near Urbana, IL, USA (for details of the study area and search methods, see [ 34 ]. A nest was considered active if (i) the clutch size grew on consecutive days, (ii) a robin was flushed from the nest, and/or (iii) robin eggs were warm to human touch.…”

Natural and artificial scents do not increase egg rejection rates of model brood parasitic eggs by American robins (Turdus migratorius)

“…For the scent-application experiments, I conducted a new set of trials. During May-June, 2020, I located active nests of American Robins, with permission, throughout private orchards and tree farms near Urbana, IL, USA (for details of the study site and search methods, see Hauber et al 2020a). Nests were deemed active (i.e.…”