Chalk-browed Mockingbird Mimus Saturninus Research Articles

Parasitic egg rejection decisions of chalk-browed mockingbirds Mimus saturninus are independent of clutch composition.

Obligate avian brood parasites lay their eggs in nests of other host species, which assume all the costs of parental care for the foreign eggs and chicks. The most common defensive response to parasitism is the rejection of foreign eggs by hosts. Different cognitive mechanisms and decision-making rules may guide both egg recognition and rejection behaviors. Classical optimization models generally assume that decisions are based on the absolute properties of the options (i.e., absolute valuation). Increasing evidence shows instead that hosts' rejection decisions also depend on the context in which options are presented (i.e., context-dependent valuation). Here we study whether the chalk-browed mockingbird's (Mimus saturninus) rejection of parasitic shiny cowbird (Molothrus bonariensis) eggs is a fixed behavior or varies with the context of the clutch. We tested three possible context-dependent mechanisms: (1) range effect, (2) habituation to variation, and (3) sensitization to variation. We found that mockingbird rejection of parasitic eggs does not change according to the characteristics of the other eggs in the nest. Thus, rejection decisions may exclusively depend on the objective characteristics of the eggs, meaning that the threshold of acceptance or rejection of a foreign egg is context-independent in this system.

Breeding biology of Chalk-browed Mockingbird Mimus saturninus in a natural savanna of central Brazil

The Chalk-browed Mockingbird Mimus saturninus (Mimidae) is a common bird in savannas, grasslands, and farmlands, also occurring in urban areas. We studied its breeding biology in a Cerrado savanna inside a reserve at central Brazil, from 2003 to 2013. We monitored 74 nests, which were usually found in the more open phytophysiognomies of the Cerrado. The breeding season lasted from late August to late December. The nest is a cup built with twigs and generally placed on shrubs and small trees. Mean nest height above the ground was 1.3 ± 0.5 m (n = 73). Mean clutch size was of 2.9 ± 0.9 eggs (n = 63), ranging from one (possibly an incomplete clutch) to six eggs (possibly a communal nest). Incubation was asynchronous, the incubation period lasted 14.2 ± 0.9 days (n = 17 nests), and the nestling period lasted 14.1 ± 1.2 days (n = 21 nests). Nestling infestation by botfly larvae was common (60% of nests), but did not cause any nest loss. Its breeding season was similar to most birds of the Cerrado region, but its clutch size was larger. The Chalk-browed Mockingbird breeding biology in central Brazil is similar to that reported from the southern part of its distribution. The species is capable of multiple broods, has cooperative breeding and shows evidence of communal breeding, a rare behavior reported so far for Mimidae. The main difference between our study and those conducted with the Chalk-browed Mockingbird in Argentina, is the very low prevalence of brood parasites, demonstrating the importance of reserves to allow breeding with low brood parasitism rates.

Molecular tracking of individual host use in the Shiny Cowbird - a generalist brood parasite.

Generalist parasites exploit multiple host species at the population level, but the individual parasite's strategy may be either itself a generalist or a specialist pattern of host species use. Here, we studied the relationship between host availability and host use in the individual parasitism patterns of the Shiny Cowbird Molothrus bonariensis, a generalist avian obligate brood parasite that parasitizes an extreme range of hosts. Using five microsatellite markers and an 1120‐bp fragment of the mtDNA control region, we reconstructed full‐sibling groups from 359 cowbird eggs and chicks found in nests of the two most frequent hosts in our study area, the Chalk‐browed Mockingbird Mimus saturninus and the House Wren Troglodytes aedon. We were able to infer the laying behavior of 17 different females a posteriori and found that they were mostly faithful to a particular laying area and host species along the entire reproductive season and did not avoid using previously parasitized nests (multiple parasitism) even when other nests were available for parasitism. Moreover, we found females using the same host nest more than once (repeated parasitism), which had not been previously reported for this species. We also found few females parasitizing more than one host species. The use of an alternative host was not related to the main hosts' nest availability. Overall, female shiny cowbirds use a spatially structured and host species specific approach for parasitism, but they do so nonexclusively, resulting in both detectable levels of multiple parasitism and generalism at the level of individual parasites.

Partial host fidelity in nest selection by the shiny cowbird (Molothrus bonariensis), a highly generalist avian brood parasite

Obligate avian brood parasites can be host specialists or host generalists. In turn, individual females within generalist brood parasites may themselves be host specialists or generalists. The shiny cowbird Molothrus bonariensis is an extreme generalist, but little is known about individual female host fidelity. We examined variation in mitochondrial control region sequences from cowbird chicks found in nests of four common Argentinean hosts. Haplotype frequency distributions differed among cowbird chicks from nests of these hosts, primarily because eggs laid in nests of house wrens Troglodytes aedon differed genetically from those laid in nests of the other three hosts (chalk-browed mockingbird Mimus saturninus, brown-and-yellow marshbird Pseudoleistes virescens, and rufous-collared sparrow Zonotrichia capensis). These differences in a maternally inherited marker indicate the presence of a nonrandom laying behaviour in the females of this otherwise generalist brood parasite, which may be guided by choice for nest type, as house wrens nest in cavities whereas the other three species are open cup nesters.