Size and material of model parasitic eggs affect the rejection response of Western Bonelli's Warbler Phylloscopus bonelli

Abstract

Given the high costs of brood parasitism, avian hosts have adopted different defences to counteract parasites by ejecting the foreign egg or by deserting the parasitized nest. These responses depend mainly on the relative size of the host compared with the parasitic egg. Small hosts must deal with an egg considerably larger than their own, so nest desertion becomes the only possible method of egg rejection in these cases. The use of artificial model eggs made of hard material in egg-recognition experiments has been criticized because hard eggs underestimate the frequency of egg ejection. However, no available studies have investigated the effect of softer material. Here, we test the potential effect of size of dummy parasitic eggs in relation to egg-rejection behaviour (egg ejection and nest desertion rates) in Western Bonelli's Warbler Phylloscopus bonelli, a small host, using plasticine non-mimetic eggs of three different sizes. In addition, we tested the potential effect of material, comparing ejection and desertion responses between real and plasticine eggs. As predicted, small eggs were always ejected, whereas nest desertion occurred more frequently with large eggs, thus suggesting that nest desertion occurs because of the constraints imposed by the large eggs. We found that plasticine may misrepresent the responses to experimental parasitism, at least in small host species, because this material facilitates egg ejection, provoking a decrease in nest desertion rate. Thus, particular caution is needed in the interpretation of the results in egg-rejection experiments performed using dummy eggs made of soft materials.

Egg rejection and egg recognition mechanisms in Oriental Reed Warblers

“…In this study, Oriental Reed Warblers, as one of the most common hosts of cuckoos, have a nearly 100% rejection rate for non-mimetic eggs, similar to the population in northeastern China, which rejected both blue (n = 15) and white (n = 24) model eggs at a rate of 100% (Wang et al 2021) and the Japanese population (94%, n = 33; Lotem et al 1995), suggesting that the Oriental Reed Warbler possesses an extremely strong egg recognition ability. However, egg rejection of another population of Oriental Reed Warblers (Li et al 2016(Li et al , 2020 was slightly lower than that of this population, possibly because the material and size of experimental eggs used may influence on the host's completion of egg rejection (Roncalli et al 2017;Li et al 2020).…”

Contrasting effects of egg size and appearance on egg recognition and rejection response by Oriental reed warblers

“…Additionally, hosts may accept a parasitic egg sometimes, even if it has been recognized (i.e., evidence of egg‐pecking behavior from video sampling) (Guigueno & Sealy, 2012; Soler, Fernández‐Morante, Espinosa, & Martín‐Vivaldi, 2012). This phenomenon has been recorded in several small‐bodied hosts (about 10 g; Roncalli et al., 2017), especially in experiments using artificial model eggs made from a hard material (Martín‐Vivaldi, Soler, & Møller, 2002; Šulc et al., 2016). However, this effect is thought to be less constrained in large or medium‐sized hosts that grasp or puncture foreign eggs in their nests (Soler, Ruiz‐Raya, Roncalli, & Ibáñez‐Álamo, 2017).…”

“…A large number of studies have confirmed that hosts use cues from egg coloration or maculation mimicry to reject eggs (Honza & Cherry, 2017; Šulc, Procházka, Capek, & Honza, 2016), but evidence for using egg coloration as an egg rejection cue is rare (Šulc et al., 2019). Egg size can significantly affect the mode of egg rejection by forcing hosts to desert nests instead of ejecting eggs if they are constricted by their limited bill size or the weight of a heavy parasitic egg (Stokke et al., 2010; Roncalli, Ibáñez‐Álamo, & Soler, 2017; but see Soler, Ruiz‐Raya, Roncalli, & Ibáñez‐Álamo, 2015). Additionally, hosts may accept a parasitic egg sometimes, even if it has been recognized (i.e., evidence of egg‐pecking behavior from video sampling) (Guigueno & Sealy, 2012; Soler, Fernández‐Morante, Espinosa, & Martín‐Vivaldi, 2012).…”

A Meta-Analysis of Avian Egg Traits Cueing Egg-Rejection Defenses Against Brood Parasitism

“…This conclusion is still important from a methodological point of view, because various materials are used to manufacture the artificial egg models. However, what is still missing from the experimental repertoire is a model egg stimulus that can be pierced by hosts whose beaks are too small for grasp rejection (e.g., Roncalli et al, 2017). Finally, we compared the eggshell traits' impact on egg rejection by hosts parasitized by Old World cuckoos vs. cowbirds and, contrary to expectations, found no statistical differences between these diverse set of hosts (Figure 4).…”