Reproductive Strategies Change With Time in a Newly Founded Colony of Spotless Starlings (Sturnus unicolor)

Variation in avian reproductive strategies is often studied from a comparative perspective, since even closely-related taxa differ greatly in the degree of polygyny, extra-pair paternity (EPP) or intra-specific brood-parasitism. However, substantial variation at the species level suggests that ecological factors are important in shaping these patterns. In this study, we examined the temporal plasticity of these strategies, following a population from the year of colony formation to 2 years after this. Parentage data from these years shows that polygyny decreased with time, likely as a consequence of increased competition for nesting sites and mates by new recruits, and immigrants of higher quality arriving to the colony as time passed. In parallel to this temporal change, we found an increase in intra-specific brood-parasitism and quasi-parasitism (QP). We interpret these patterns as a consequence of an increase of floaters with time; these birds pursue a mixture of alternative mating strategies to succeed in the population. We also found evidence of conspecific brood parasitism (CBP), by nesting females that laid part of the clutch in another nest or that after losing a partially laid clutch resorted to lay the last eggs in another nest. Analyses of the distance between the main nest and nests containing the secondary polygynous brood or extra-pair or parasitic young showed an avoidance of contiguous nests for conducting these alternative reproductive tactics. At the same time, these secondary nests were closer to the main nest than random distances within the colony, suggesting that access to public information was restricted to a narrow area around the main nest. Our study emphasizes how behavioral patterns are plastic traits that vary not only with individual circumstances, but also with time, tracking changes in density and social structure.

Nesting of the Spotless Starling, Sturnus unicolor, on the island of Favignana (Aegadian Islands, Sicily)

The presence of the Spotless Starling in Sicily was reported for the first time at the beginning of the XIX century, although the first nests for the province of Trapani are reported almost at the end of the XX century. As a breeder, the Spotless Starling remained absent from all the circumsicilian islands until the first report for the island of Pantelleria, in 2012. In 2020, the nest of Spotless Starling was observed on the island of Favignana, within a hole on an antenna. The couple fed the chicks with Ligia italica crustacean isopod of the family Ligiidae. The nesting on the island of Favignana confirms the continuous expansion of the species in Sicily.

Beak coloration of starling (Sturnus unicolor) males depends on the length of their throat feathers

Within the context of complex sexual signaling, most research has focused on exploring the associations between several signals and/or their relationships with different proxies of individual quality. However, very few studies have focused on checking whether the expression of one signal is conditioned by the expression of the others. Here, by experimentally shortening the throat feathers of male spotless starlings (Sturnus unicolor), we evaluated the influence of this trait on the color expression of the beak base. In addition, we tested the relationship between these two sexually dimorphic characters with traits indicating individual qualities, such as body condition and color reflectance at the wavelength related to carotenes in the tip of the beak. Our results show that the coloration of the beak base in males, but not in females, is positively related to body condition and to the length of ornamental throat feathers. Moreover, the experimental shortening of throat feathers in males had a negative effect on the blue chroma intensity of their beak base 1 year after manipulation. These results support for the first time a causal link between the expression of two sexually dimorphic characters, which is essential to understand their functionality in a multiple signaling framework.