Breeding biology of Pale-breasted Thrush Turdus leucomelas (Turdidae) in the north of Atlantic Forest, Brazil

Reproduction is a key process in the life of organisms and represents high-energy cost to the parents, and often a tradeoff between survival and reproductive success. The breeding biology is an important aspect to be studied, which has inspired theories about bird life history evolution, such as the latitudinal gradient in the clutch size related to different survival rates between the temperate and tropical regions. To contribute with the knowledge of breeding ecology of tropical birds we monitored two reproductive periods of the Pale-breasted Thrush Turdus leucomelas in the northeastern Brazil. The breeding period in the studied area occurred from December through April. We found the nests mainly on Facheiro cactus (Cactaceae) at 1.43 ± 0.35 m above the ground. They were composed by roots, mosses, fungus mycelium, leaves, twigs, and a mix of fragmented vegetal material with soil and sand at the base. We found clutch sizes of three (n = 9), two (n = 3) and one (n = 1) eggs. The eggs (n = 21) had mass of 5.1 ± 0.9 g and measured 26.6 ± 1.3 mm by 19.5 ± 0.5 mm. We observed 12 days of incubation and 14 days of nestling period. The Mayfield nest success in 2013 was 7.3% and in 2014 it was 4.5%, lower during the nestling period than during the incubation in both years. Our results show that T. leucomelas breeds in the area in the beginning of rainy season, when the frequency of occurrence of the migrant Turdus amaurochalinus is low. In addition, the low nest success observed may be a consequence of the localization of the studied area in the periphery of the species range, where limited resources probably result in reduced fitness. KEY-WORDS: breeding success, egg, Mayfield, nest, northeastern Brazil, predation. al. 2000). Basically, breeding depends on environmental and ecological circumstances throughout the annual cycle, necessitating the integration of these components to understand it (Sherry et al. 2015). Density-dependent feedback is not restricted to the same population, sometimes the increase of competitor presence can be an important driver of nesting. Interspecific competition can reduce breeding opportunities for subordinate species, resulting in lower fledgling rates and breeding density (Brazill-Boast et al. 2011, Edworthy 2016). Competition-mediated habitat selection is widely believed to change the range of habitats or resources exploited by different species (Sherry & Holmes 1988, Dhondt 2012). The dominance is a primary factor in determining the realized niche among species and the community structure of an area (Dhondt 2012, Thornton et al. 2015). Thus, the seasonal variation in the frequency of occurrence of possible competitors may influence the breeding biology of a resident species. Breeding biology of Pale-breasted Thrush Turdus leucomelas Ruiz et al. 111 Revista Brasileira de Ornitologia 25(2): 2017 Revista Brasileira de Ornitologia 25(2): 2017 Additionally, different predator assemblages can change in distinct ways the reproductive success of the species. Predators are widely accepted as one of the main cause of breeding loss in tropical bird populations (Lack 1954, Nice 1957, Ricklefs 1969, Oniki 1979, Skutch 1985, Martin 1993). Nest predation influences reproductive strategies and nest-site selection (Martin 1995, Fontaine & Martin 2006). Consequently, nests are not randomly spread across the environment, they are generally hidden in the habitat or in places difficult to access (Klopfer 1963, Cink 1976, Ricklefs 1984, Sonerud 1985, Martin & Roper 1988). Nest concealment is known to improve nest survival for a variety of open-cup avian species (Berl et al. 2014). The mechanism for this is linked to the effect of vegetative cover on predator foraging efficiency (Li & Martin 1991, Segura et al. 2012). In consequence, birds in general have applied a plethora of behavioral techniques to avoid predation (Martin 1998, Clark & Shutler 1999, Rauter et al. 2002, Davis 2005). Therefore, predators are a powerful ecological force shaping many aspects of breeding biology and life histories of birds (Clark & Wilson 1981). Another important environmental aspect for bird nesting is the precipitation, considered the main weather condition perceived by birds at tropical areas (Boag & Grant 1984, Lloyd 1999, Hau et al. 2008). The onset of rains is associated with greater food availability, either fruits or arthropods (Wolda 1978, Grant & Boag 1980, Leigh-Jr. et al. 1996, Ahumada 2001, Dantas et al. 2002). Presumably, this peak must also match with the greater breeding period energy demand for egg production (Lack 1968, Ewald & Rohwer 1982, Martin 1987), parental care (Lack 1954) or juveniles' dispersal (Morton 1971). Additionally, the breeding season may be adjusted by molting process that also occurs most often in the rainy season, when there is plenty of food supply (Poulin et al. 1992). The Pale-breasted Thrush (Turdus leucomelas) is a common species with breeding biology poorly studied (Collar 2005, Davanço et al. 2013). Its large occurrence in South America makes it a good model species to investigate the variations of reproductive traits among regions, habitats and climate conditions. The present study contributes to this knowledge describing the nesting biology of the species in the extreme of its distribution. In addition, our objectives were to correlate some aspects of the reproduction of the species with habitat characteristics, precipitation and molt occurrence. We also discussed the effects of the frequency of occurrence of the migrant congener Creamy-bellied Thrush (Turdus amaurochalinus), a possible competitor in the area. METHODS Study area and species We monitored T. leucomelas breeding biology in a plot of 550 × 550 m (30.25 ha) formed by an array of eleven rows and columns (50 m apart) located at Centro de Lançamento Barreira do Inferno CLBI (Barreira do Inferno Launch Center, Brazilian Air Force) city of Parnamirim, Rio Grande do Norte state, Brazil (05o54'S; 35o10'W, 1800 ha). The area has tropical coastal vegetation of Atlantic Forest sandbank (Scarano 2002) and classified as semi deciduous forest of lowlands (Cestaro 2002). According to the Köppen (1936) classification system, the climate of the region is AS (tropical and humid) with dry summer and rainy winter (IBAMA 2003). The species T. leucomelas is widespread in central, east and north of South America (Sick 1997, Collar 2005), where it inhabits pristine and disturbed humid forests, drier deciduous woodland, savannas, gallery woodland, and anthropogenic environments (Collar & Garcia 2016). This common, non-threatened, species feeds mainly on fruits, arthropods, worms and small lizards (Collar & Garcia 2016). Data collecting and analysis From 2010–2012 we determined the breeding period of the species through records of the brood patches occurrence resulting from a monthly demographic monitoring captures at the same study area. In 2013– 2014, we started the search for active nests one month before the breeding season and extended it to one month after to avoid loss of any reproduction attempt. During this time interval, we searched the whole study area for nests at least once a week in the early hours of the morning (05:00–10:00 h), using previous established parallel paths at 50 m each (1050 h-observer in total). We applied ordinary nest-searching methods through the area, looking for visual contacts and behavioral clues of adults in breeding activity (vocalizations, territory defense, carrying of nest materials or food for nestlings) (Lopes & Marini 2005). We photographed, georeferenced and described the active nests found according its stage (construction, incubation, nestling) and we monitored them and the parental behavior at intervals between 2 to 4 days until it became inactive. We manipulated the contents once in the incubation stage (to measure and to weigh the eggs with a 20 g scale and 0.05 mm caliper) and another time in the end of the nestling stage for ringing. The description of the egg shape was based on Baicich & Harrison (1997) and the colors on Smithe (1975). The nestlings were marked with aluminum bands provided by the Centro Nacional de Pesquisa e Conservação Breeding biology of Pale-breasted Thrush Turdus leucomelas Ruiz et al. 112 Revista Brasileira de Ornitologia 25(2): 2017 Revista Brasileira de Ornitologia 25(2): 2017 de Aves Silvestres (CEMAVE/ICMBio), and with colored bands to follow them during the post-nestling period. We also monitored the nests after each breeding attempt (successfully or not) in order to check reuse. We considered as incubation period the time between the laying and hatching of the last egg, and the nestling period between the hatching of the last egg and when the last nestling leaving the nest. When we could not register the exact day of an event, we assumed the day at half of the period from the last research visit to establish these periods. We considered successful nests those that produced at least one nestling, and failure when no eggs hatched (during incubation) or no offspring was produced (during nestling period). We estimated the breeding success as a simple ratio of successful nests to total nests found (apparent success, see Jehle et al. 2004) and by using Mayfield's method, which estimates the mortality rate as a ratio between failures and observation period (Mayfield 1961, 1975). Since the nests were not monitored daily, we assumed as the exact date of loss or success the middle day between the last two consecutive visits (following Mayfield 1975). Based on our results, we considered 12 days the incubation period and 14 days the nestling period to obtain the survival rates. For each nest, we identified the plant species where it was built, the perpendicular distance of the superior ridge of the nest to the ground (height from the ground) and its position in the plant support (branches or main axis). We also measured the largest and smallest internal and external diam