Our results showed that although Brown-breasted Bulbuls have strong nest sanitation abilities, their nest sanitation behavior did not directly affect their recognition of non-mimetic eggs. The egg rejection rates were similar for the nests with and without the addition of peanut shells. In addition, the Brown-breasted Bulbuls did not treat the non-mimetic eggs as foreign objects, such as peanut shells. This was shown in the nests with both peanut shells and non-mimetic eggs, in which all of the birds removed the peanut shells, but only 52.6% rejected the non-mimetic eggs. Therefore, this indicated the influence of the shape of the object on egg discrimination ability, similar to the results of previous studies (Ortega and Cruz 1988; Guigueno and Sealy 2009, 2012).
Nest sanitation behavior is considered a pre-adaptation of host bird egg discrimination ability, as the two behaviors share a common aspect of the removal of objects from the nest, although some nest sanitation behavior is accomplished by burying or swallowing (Guigueno and Sealy 2012). However, the two behaviors differ with respect to the shape and color of the foreign object and the similarity to the host egg. There is also no clear link between nest sanitation in cowbird hosts (Rothstein 1975; Peer and Sealy 2004). In addition, some hosts do not practice nest sanitation, but can reject some eggs (Peer 2017). Underwood and Sealy (2006) suggested that the ability of American Robins and Gray Catbirds (Dumetella carolinensis) to recognize non-egg objects was significantly higher than that of other species without egg discrimination ability.
Moskát et al. (2003) examined the relationship between nest sanitation and egg rejection in the Great Reed Warbler (Acrocephalus arundinaceus) and showed that the rejection rates of the non-egg models were significantly higher than those of cuckoo model eggs, either during hatching or in the early stage of incubation. Guigueno and Sealy (2009) also showed that in American Yellow Warblers (Setophaga petechia) the burial probability of non-egg objects was significantly greater than that of the simulated cowbird-egg-shaped models during the pre-hatching and hatching stages. These results strongly indicated that host egg discrimination ability is likely to be an extension of nest sanitation behavior.
Yang et al. (2015a) studied the nest sanitation and egg rejection of Barn Swallows, which are hosts of the Common Cuckoo (Cuculus canorus). In nests with only non-mimetic eggs, the probability of swallows rejecting the model eggs was 27%, whereas in nests with model eggs and peanut shells, all of the individuals were able to remove the peanut shells and the probability of rejecting the model eggs increased to 56%, demonstrating the significant influence of host bird nest sanitation behavior on egg discrimination. Similar to Barn Swallows, Brown-breasted Bulbuls also showed moderate egg discrimination ability against non-mimetic eggs (64.1%, Su et al. 2016; 53.8%, this study), which were similar to American Robins (65%; Luro and Hauber 2017) but stronger than the discrimination ability of Red-winged Blackbirds (0%; Peer 2017). However, our results showed no effect of nest sanitation behavior on egg discrimination, but were consistent with those by Luro and Hauber (2017) and Peer (2017), who found that although the tested hosts showed nest sanitation behavior, the addition of foreign objects to the nest did not influence host egg rejection. Rothstein (1975) also found that Red-winged Blackbirds show a very low level of response to cowbird eggs, and foreign objects removal did not influence their egg rejection.
Previous studies have shown that when a host bird sees an adult cuckoo around its nest, it is stimulated to increase the probability of rejecting the parasitic eggs in its nest (Moksnes et al. 1993; Davies 2000; Hosoi and Rothstein 2000; Guigueno and Sealy 2011). Yang et al. (2015a) further showed that this type of stimulation was also produced by foreign objects, which increased egg rejection in Barn Swallows. Such type of stimulation was effective in Barn Swallow populations, but not in American Robins, Red-winged Blackbirds or Brown-breasted Bulbuls. We suggested the possible reason for this discrepancy could be that the nesting habitat of Barn Swallows is different from the latter three, as Barn Swallows are a communally nesting species and their nests are usually built under the eaves of human habitation (Liang et al. 2013; Yang et al. 2015b). In such a nesting habitat, there are relatively fewer pieces of debris that fall into swallow nests during the incubation period. In addition, since the swallow nests are next to the eaves and are relatively dark, small pieces of debris may be difficult for the birds to see. As a result, swallows do not frequently clean their nests during incubation. Thus, when peanut shells suddenly appeared as apparent foreign objects in their nests, nest sanitation behavior was activated and their egg discrimination abilities were also stimulated. By contrast, in the natural habitat of the Brown-breasted Bulbuls, American Robins and Red-winged Blackbirds, many pieces of debris often fall into their nests during the incubation period. In addition, as their nests are cup-shaped and open, the fallen small-sized debris is easily seen. Thus, they often clean their nests throughout the incubation period, and nest sanitation behavior is a normal part of their behavior during incubation. When the peanut shells were added to their nests, the additional stimulation effect was not sufficient to be detected or to induce them to reject the non-mimetic eggs. Clearly, large-scale and multiple-population studies, and more in-depth explanation of these results, e.g., the costs of nest sanitation versus egg rejection and the role of host age structure in nest sanitation, need further investigation in the future.