Nest, eggs and nestling description of the Silvery-flanked A ntwren Myrmot h eru l a l uctuosa (Pelzeln, 1 8 6 8 ) in the A tlantic Forest of northeast Brazil

d on 10 De c em b er 2019. A BSTR A CT: In t h e present stu d y, we d es c ri b e t h e nest, eggs, an d nest l ings of t h e Si l very-flanke d Antwren, Myrmotheru l a l uctuosa . One nest was foun d on 08 May 2017 on t h e e d ge of one of t h e forest fragments of t h e Serra d e Ita b aiana Nationa l Park, in t h e state of Sergipe, nort h east Brazi l , an d was monitore d unti l 21 May 2017. The nest, presente d t h e fo ll owing d imensions: externa l h eig h t: 7.6 c m; outsi d e d iameter: 21 × 19 c m; interna l d iameter: 13.3 × 11 c m; d ept h of t h e oo l ogi c ch am b er: 4.7 c m; an d h eig h t in re l ation to t h e groun d : 84 c m. The two eggs foun d in t h e nest were w h ite wit h re dd is h - b rown d ots, pat ch es an d stripes an d weig h e d 1.7 an d 1.5 g. In c u b ation was performe d b y b ot h t h e ma l e an d fema l e, as we ll as fee d ing of t h e nest l ings. A sing l e egg h at ch e d , giving rise to a 3.9 c m, 3.7 g nest l ing. A cc or d ing to our estimates, t h e young fle d ge d wit h a b out 11 d ays. This is t h e first stu d y to present information a b out t h e repro d u c tive b io l ogy of t h e Si l very-flanke d Antwren.


INTRODUCTION
Most species of the Thamnophilidae family are considered socially monogamous and form permanent breeding pairs who defend their territories with considerable vigor throughout the year (Zimmer & Isler 2003). Within this family, the genus Myrmotherula is the most numerous. Chaparro-Herrero & Ruiz-Ovalle (2014) pointed out that information on the reproductive biology of this genus is limited to nest descriptions and parental care in a few species. Of the 20 species within this genus in Brazil, there is enough information about reproduction only for seven species (M. surinamensis, M. pacifica, M. multostriata, M. axillaris, M. longipennis, M. unicolor and M. menetriesii); for other six species there is no information about reproduction or, only anecdotal information is available (Zimmer et al. 2019).
Nests are described as a ball of dead leaves with a mean diameter of 8 cm, suspended in forks varying from 2 to 10 m above the ground; normal clutch size of 2 white eggs with reddish spots; incubation varying from 16 to 19 days and parental care provided by both parents (Zimmer et al. 2019).
The Silvery-flanked Antwren (Myrmotherula lucutosa) inhabits the understory of primary and secondary Atlantic Forest areas from Brazil (Sick 1997, del Hoyo et al. 2019. It presents a marked sexual dimorphism, with males paler than females, presenting a blackish belly and a pale grey flank; females are more olive-tinged grey with an ochraceous belly (Grantsau 2010). It is insectivorous and little is known about its reproduction, only mentioned that its nest is similar in shape, size and materials to that of the White-flanked Antwren M. axillaris (Zimmer & Isler 2003, del Hoyo et al. 2019. In this way, this study aimed to provide information on the reproductive biology of the Silvery-flanked Antwren, presenting for the first time information about nest, eggs, and parental care.

METHODS
The study was carried out in May 2017 in the Serra de Itabaiana National Park (PARNASI), Sergipe state, northeastern Brazil (10 o 40'S; 37 o 25'W). PARNASI is formed by a complex of three mountain ranges: Cajueiro, Comprida and Itabaiana. The three mountain ranges together comprise an area of 7966 ha, and the highest point is in the Itabaiana mountain range, 660 m a.s.l. (Carvalho & Vilar 2005). The park area is in a transition between the Atlantic Forest and Caatinga Biomes. The climate of the region is tropical with dry summer and moderate wet winter, with temperatures varying between 17 and 23 o C (Vicente 1999).
Nest monitoring started at the time of its discovery; it was visited at intervals of 3 to 4 days. The nest, eggs and the nestling were described, weighed and measured using a caliper, accurate to 0.01 mm, and a spring scale, accurate to 0.1 g. During critical phases, such as hatching and fledging, the nest was monitored at shorter intervals of 1 to 2 days, always trying to minimize the impacts caused by human presence. The measurements of the nest were obtained on the day of its discovery, to avoid natural deformations due to its use. Nest characteristics measured were: height of the external part of the nest, external and internal diameter, depth of the oological chamber (edge of the nest until the deepest area of its center) and height of the nest in relation to the ground (the latter with the aid of a measuring tape). In the case of the nestling, its developmental stage was described based on its mass (g), size (cm), shape and color of feathers, beak and tarsi, and on the presence of closed or open eyes.

RESULTS
On 08 May 2017, a nest of M. luctuosa was found on the edge of one of the fragments of Atlantic Forest located in the PARNASI. The nest was found hidden beneath a leaf of Heliconia sp. (Heliconiaceae) and was strongly attached to a horizontal fork of a sapling, 84 cm above the ground. At that time, the female was incubating two eggs. The nest was a deep cup made up mainly of fragments of dry leaves of different plant species, and the incubatory chamber was lined with a large amount of light brown flexible rootlets. The whole structure was contained within a flimsy basket of black rootlets filaments that also provided attachment to the horizontal fork (Fig. 1). Nest measurements were: height of the external part of the nest (7.6 cm), external diameter (21 × 19 cm), internal diameter (13.3 × 11 cm), depth of the oological chamber (4.7 cm), and height of the nest in relation to the ground (84 cm).
Eggs were white with reddish-brown dots and stripes on the terminal portion of the rhombus pole (Fig. 1A). On 12 May 2017, it was observed that only one chick had hatched. The mass, diameter and length of the eggs are presented in Table 1.
The nestling registered on 12 May 2017 measured 3.9 cm and weighed 3.7 g. It had no feathers, had closed eyes, pale skin and tarsi, and yellow-orange beak, labial commissure, tongue and mouth lining (Fig. 1B)  presence of black feather cannons, mainly on the wings, back and head (Fig. 1C). The presence of the first feathers was recorded on 18 May 2017. On that day, the nestling measured 7.2 cm, weighed 7.2 g, had greyish feet and tarsi and black beak, contrasting with the yellow labial commissure and mouth lining (Fig. 1D).
On 21 May 2017, the nestling was seen outside the nest with very short wings and tail, moving between the medium forest extract through short and weak flights. A plumage of color similar to that of the adult female was already observed. longipennis and M. menestrierii build nests high above the ground (more than 300 cm high) (Zimmer et al. 2019).

DISCUSSION
In general, the species of the Thamnophilidae present a clutch of two eggs, with rare cases of three eggs (Skutch 1996, Sick 1997, Robinson et al. 2000, Zimmer & Isler 2003, Pinho et al. 2006, Perrella et al. 2015. Thamnophilidae species present eggs different in colors, and in the genus Myrmotherula important differences can be observed in the color patterns of their eggs (Zimmer & Isler 2003, Chaparro-Herrera & Ruiz-Ovalle 2014. The M. luctuosa eggs observed in this study are similar to the color pattern found in M. axillaris eggs (Zimmer & Isler 2003, Chaparro-Herrera & Ruiz-Ovalle 2014. The young fledged with 11 days, but it could have occurred early due to the manipulation to obtain its measurements. Other species of Thamnophilidae showed similar nestling periods, such as C. melanaria (10 days; Pinho et al. 2006), H. maculicauda (11 days; Evangelista 2008), Rhopias gularis (11 days; Perrella et al. 2017) and Taraba major (13 days; Lara et al. 2012).
During the reproductive activity, it was observed that both the female and the male participated in the incubation of the eggs (Fig. 2), and fed the nestling, like observed in other Thamnophilidae, such as C. melanaria    Pinho et al. 2006), H. maculicauda (Pinho et al. 2006), T. pelzelni (Silva & Carmo 2015) and R. gularis (Perrella et al. 2017). This is the first study to present information about the reproductive biology of the Silvery-flank Antwren M. luctuosa. The description of its nest, eggs and parental care add valuable information about the reproductive biology of the genus Myrmotherula, thus contributing to the understanding of the life history pattern of this group of Neotropical passerines.