SPECIES

Chilean Mockingbird Mimus thenca Scientific name definitions

Natacha González, Vicente Pantoja, Maria Jesus S. Mallea, Matías Garrido, Antoine Touret, Angélica Almónacid, Heraldo V. Norambuena, and Fernando Medrano
Version: 2.0 — Published March 3, 2023

Breeding

Introduction

Chilean Mockingbird breeds in the austral summer. For many years it was considered a breeding endemic of Chile, but a small breeding population inhabits Neuquen, Argentina (43). The species is presumably socially monogamous and can breed twice in a season (6).

Phenology

Phenology in Chile

Phenology in Chile has been historically described to prolong from October to late January (67, 68, 6). However, eBird observations and photos confirm that it may start nest building as early as late July (Piñones, eBird S71692908) and August (Hostens, eBird S72885724, Escobar, eBird S59373691) and extend until February, feeding fledglings (González, eBird S44481746, Arcaya, eBird S102015892, Jara, eBird S16888443). In Fray Jorge National Park, phenology is strongly linked to the presence and fruiting of quintral (Tristerix aphyllus) (1). Egg-laying and incubation potentially take place during August to January, with a peak in October (6). Considering Chilean Mockingbird can breed twice in the season, birds can be seen nest building and transporting nesting material in different months during the season, extending this task from late July until January (Lemoine, eBird S101092750, Gherardi, eBird S51608158, 6). Fledglings have been reported as early as 1 September (Maldonado, eBird S73081056) and seen until late February (González, eBird S44481746, 32).

Phenology in Argentina

Breeding information on the population in Neuquen, Argentina is scarce and may not reflect the real phenology of the species. Matarasso (43) reported two nests during the summer of 2007, with no precise date. After this, only a few records of breeding activity have been reported. An adult transporting food was reported in November (Roesler, eBird S41096666) and nest building during early January (Areta, eBird S26679633). The species could potentially breed twice in Argentina as in Chile, but further studies are required.

Nest Site

Selection Process

Information needed.

Site Characteristics

Nests are constructed in hidden places in trees or bushes with thick, leafy foliage, at a mean height of 1.64 meters from the ground where it is well protected (6). In Marin (6), out of 38 nests examined, 33 (86.8%) were found in thorny and leafy trees/shrubs and five (13.2%) in very leafy non-thorny shrubs.

Tree and bush species are predominantly tall and leafy, such as trevo (Trevoa trinervis), espino (Acacia caven), maiten (Maytenus boaria), chagual (Puya chilensis), blackberry (Rubus ulmifolius), palqui (Cestrum parqui) and tamarugo (Prosopis chilensis), or cacti such as quisco (Trichocereus chiloensis [= Echinopsis chiloensis]) (38, 6).

Nest

Construction Process

Nest building takes place from late July to January (eBird 2022), with a peak in October (6). A Chilean Mockingbird pair took 8 days to complete the construction of their nest (6). For reference, a similar species such as Chalk-browed Mockingbird (Mimus saturninus) takes 10–15 days to complete construction of its nest (69).

Structure and Composition

Cup-shaped nest with a coarse outer surface of thorny twigs, dry grass leaves, and stems, while the interior is covered with soft vegetable fibers as cardo (Cynara sp.), other materials such as rabbit hair, horse hair and to lesser extent feathers (67, 68, 46, 6, Carrion, iNaturalist 35390361).

Dimensions

Mean internal diameter of 107 mm (range 88–125) (n = 16) and a depth of 58 mm (range 55–60mm) (n = 5) (6).

Eggs

Shape

Marin (6) found that of the shape of the eggs, 47% were subelliptic, 21% long subelliptic, 17% oval, 11% long oval, 2% short oval, and 2% short subelliptic (n = 104).

Size

Mean size 28.98 mm x 19.97 mm (n = 104) (6).

Mass

Mean mass of 6.37 g (n = 71). The mean mass of one egg is 8.2% of the total mean mass of an adult Chilean Mockingbird (6).

Color and Surface Texture

Bluish-green in color with reddish-brown spots and a great variation in the type of markings that the eggs present. Eggs may be crowned, hooded, speckled, with overlapping spots, speckled, or spotted, with color markings varying from reddish-brown to dark olive (6).

Clutch Size

Clutch size varies from two to four eggs, being three most frequent (38,70, 46, 6).

Egg Laying

Egg-laying and incubation potentially take place from August to January, with a peak in October (6, eBird 2022). The period between the end of the nest construction and the laying of the first egg is between 3 to 8 days (n = 5) (6).

Incubation

Incubation Patches

Well-developed incubation patches were reported during September in female individuals in Fray Jorge National Park, Chile (1).

Incubation Period

The incubation period varies between 14–16 days, on average 14.8 days (n = 5) (6). As a reference, Northern Mockingbird (Mimus polyglottos) has an incubation period of 12 days (71) while Chalk-browed Mockingbird (Mimus saturninus) takes 14–15 days (69).

Parental Behavior

In Marin's study (6), it could not be ascertained if both adults were incubating since there is no marked sexual dimorphism. However, every time an adult was incubating, the other was close and vigilant near the nest (6).

Hatching

Hatching of the eggs can extend from several hours to a day. In one nest the first two eggs hatched fairly synchronously while the third or fourth asynchronous with a day or more apart (6).

Young Birds

Condition at Hatching

On hatching, chicks are typically altricial with closed eyes and poor mobility. They have dark gray down feathers, mainly on the back and nape. The body, the tarsus, and the legs are orange, the nails are ivory, and the bill is orange with a yellowish tip. The oral ridges are pale yellow, while the palate and tongue are bright orange (6).

Growth and Development

Three days after hatching, the culmen turns grayish, and at 10 days, it possesses a blackish tone, except for the cutting edges (tomia), which are of a yellowish gray color, and the oral margins of a pale yellowish-white tone. Tarsus and feet turn grayish at 6–7 days and completely gray at 9–10 days. The eye line is visible after two days, and chicks begin to open their eyes on day three, which after seven days are fully open (6).

The mean body mass at hatch is 5.7 g (range 4.4–5.8 g, n = 15) (7.37% of the adult size). Chicks increase their mass daily in an almost linear way, and the body mass acquired by chicks at 13 days is 64.7 g (83.6% of the adult size) (6).

At hatching the average wing length of chicks is 8.6 ± 0.54 mm, 7.2% of the adult size. The wing pin feathers begin to emerge at 3 to 4 days and open their sheaths at 7 to 8 days. Measurements at day 12 are 72 mm, reaching 60% of the adult size. Tail pin feathers begin to emerge at 5–6 days and open their sheaths at 8–9 days. Measurements of the tail are 31 mm at 12 days, (26.6% of the adult size). At hatching, the average tarsus is 9.1 mm (range 8.3–9.9), (23.7% of the adult size), and it acquires adult size at 10–11 days. Tarsus measurement is the only parameter that acquires the size of the adult before leaving the nest. The average culmen at hatching is 5.6 mm, (30.8% of the adult size), and 16.8 mm at 12 days, (92.6% of the adult size). (6). The first body feathers appear as subcutaneous black dots at 2–3 days, emerging as pins at 3–4 days and opening their sheaths at 6-7 days. Body feathers have light brown tips (6).

Parental Care

Brooding

Information needed.

Feeding

Adults feed their chicks larvae of beetles (Coleoptera), caterpillers (Lepidoptera, including Noctuidae), and fruits while in the nest (6).

Nest Sanitation

Information needed.

Cooperative Breeding

Not reported.

Brood Parasitism by Other Species

Identity of the Parasitic Species

Nests are frequently parasitized by Shiny Cowbird (Molothrus bonariensis) both in Chile and Argentina (33, 6). The interaction between Shiny Cowbird and Chilean Mockingbird in Chile seems to be something relatively recent, for example, Goodall et al. (72) and Johnson (70 do not mention Shiny Cowbird chicks fed by Chilean Mockingbirds in their publications (6). The first work that documents a Chilean Mockingbird feeding Shiny Cowbird chicks was elaborated in 1970 (73).

Frequency of Occurrence

In a study in central Chile, Shiny Cowbird parasitism reached 53% of study nests, where 34 of 63 clutches contained 1 to 3 Shiny Cowbird eggs. In terms of clutch size, it is quite possible that most clutches of less than three eggs are a reduction of the original clutch affected by Shiny Cowbirds. Out of 46 clutches studied in detail, in 15 of them (33%) when a reduction of the original clutch was observed, Shiny Cowbird eggs were found instead (6). Despite the high level of parasitism by Shiny Cowbird, in general, it is rarely observed or reported (73).

Response to Parasitic Female, Eggs, or Nestlings

Chilean Mockingbird is very susceptible to changes in the nest, particularly with their eggs during early incubation. If it finds many Shiny Cowbird eggs in the nest, it generally abandons the nest. In Marin (6), of the parasitized nests with more than one Shiny Cowbird egg, 16 were abandoned eventually, which ultimately ended up predated (6).

Success of Parasite with this Host

Information needed. Fraga (74) found that in Chalk-browed Mockingbird (Mimus saturninus), there is a high incidence of parasitism by Shiny Cowbird, but only about 6% of cowbird eggs were successfully reared as young.

Fledgling Stage

Departure from the Nest

Chicks take approximately 15 days to completely leave the nest, more frequently fledging at 11–12 days and as early as 9 days in some cases (6).

Association with Parents or Other Young

After leaving the nest, the chicks are still fed by the adults. Chicks follow their parents and perch alongside them in bushes and trees if necessary. Both original chicks and parasitic Shiny Cowbird chicks (if present) can be seen together being attended by adults (6). Further studies are required to describe the length of this interaction and diet items.

Immature Stage

Information needed.

Recommended Citation

González, N., V. Pantoja, M. J. S. Mallea, M. Garrido, A. Touret, A. Almónacid, H. V. Norambuena, and F. Medrano (2023). Chilean Mockingbird (Mimus thenca), version 2.0. In Birds of the World (N. D. Sly, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.chimoc1.02