Throughout its broad geographic range, Slate-throated Redstart builds a well-disguised domed nest placed on the ground on steep slopes in undisturbed forests or in banks along roads and trails in areas of human activity; occasional nests are built among epiphytes on a fallen log or in the root mass of an overturned tree, and some nests may lack a roof when the overlying vegetation provides a natural roof (Skutch 1954, Stiles and Skutch 1989, Collins and Ryan 1994, Mumme 2010, Ruggera and Martin 2010). Skutch (1954) reported one nest in Costa Rica at which both members of the pair appeared to participate in nest construction, but this observation is likely erroneous or exceptional; in more extensive studies in both Costa Rica (Mumme 2010) and Venezuela (Ruggera and Martin 2010), only females engaged in nest building.
Extensive data on breeding biology are available from two populations at 10º N latitude: Monteverde, Costa Rica (subspecies comptus; Mumme 2010) and Yacambú National Park, Venezuela (subspecies ballux: Ruggera and Martin 2010). At both locations, Slate-throated Redstart is a strongly seasonal breeder that initiates nests from late March through late June, with the peak of nesting occurring in late April and early May. Limited data from equatorial regions suggest that breeding seasons are more extended at low latitudes. Nesting occurs from late December through July near Cali, Colombia (3.5º N; Miller 1963, Hilty and Brown 1986, Fjeldså and Krabbe 1990). In eastern Ecuador (1º S) the congeneric Spectacled Redstart (Myioborus melanocephalus) nests May-December (Greeney et al. 2008), suggesting that Ecuadorian populations of Slate-throated Redstarts may have a similarly extended nesting season. In a high latitude Bolivian population at 18º S, nest building has been observed in mid-October, and two nests with nestlings were found in early November and mid-December (S. K. Herzog, unpublished data).
Eggs are dull white with dark red or brown spots (Ruggera and Martin 2010). Egg size in Costa Rica averages 16.7 x 13.4 mm, and neither length nor width is significantly related to laying order (Mumme 2010). In Venezuela, mean egg mass is 1.6 g (range 1.2-1.9 g; Ruggera and Martin 2010). Clutch size varies geographically. It averages 2.9 (range 1-4) in Costa Rica (Mumme 2010), but is substantially smaller in Venezuela, averaging 2.1 in Yacambú National Park (Ruggera and Martin 2010) and 2.4 in Henri Pittier National Park (Collins and Ryan 1994). Slate-throated Redstart lays the eggs early in the morning, usually within the first few hours after dawn. Eggs typically are laid on consecutive days, and incubation does not begin until clutch completion; the incubation period in Costa Rica is 14-15 days (mean 14.2 days) but somewhat longer and more variable in Venezuela (14-18 days, mean 15.3 days; Mumme 2010, Ruggera and Martin 2010). No information is available on clutch size or incubation period from populations at either the northern (Mexico) or southern (Bolivia) limits of the breeding range.
Incubation is performed only by females. However, some males occasionally feed females on the nest; at 52 nests observed 6-8 hours during incubation in Venezuela, 21 males (40%) fed their mates, averaging 0.1 feeding trips/hour. Daily nest attendance by incubating females averages 59% in Venezuela (Ruggera and Martin 2010).
Eggs hatch synchronously, and the nestling period is typically 11-12 days in both Costa Rica and Venezuela (Mumme 2010, Ruggera and Martin 2010). The female alone broods the chicks, and in Venezuela brooding attentiveness averaged 63% during the first three days after hatching but dropped to 11% after feather emergence. In Costa Rica females cease brooding 5-6 days after hatching. Nestlings are provisioned by both males and females. In Venezuela the average rate of provisioning was 3.8 feeding visits/hour during the first three days after hatching, but the rate increased to 9.4 feeding visits/hour after feather emergence (Ruggera and Martin 2010). In Costa Rica provisioning rates appear to be considerably higher, averaging 20.3 feeding visits/hour when nestlings were 5-9 days old (Mumme 2010). Nestling growth curves in Costa Rica and Venezuela appear to be very similar (Collins and Ryan 1994, Mumme 2010, Ruggera and Martin 2010). Body mass averaged 10.8 g for Venezuela nestlings immediately prior to fledging (age 10 and 11 days), and nestlings reach mean adult body mass (9.5 g) 7-8 days after hatching at both locations (Mumme 2010, Ruggera and Martin 2010). Parents continue to feed the young after fledging, and may do so at least until the young are 40 days old (28-29 days after fledging); juveniles, however, begin to forage on their own when 30 days old (Mumme 2010).
Reported rates of nest success differ dramatically between Venezuela and Costa Rica; only 15% of nests in Venezuela were successful, compared to 40% in Costa Rica. Overall daily nest mortality rates were 0.065 in Venezuela and 0.032 in Costa Rica. Nestling predation rates in Venezuela increased with nestling age, and the overall daily nest predation rate was 0.053; in Costa Rica predation accounted for 83% of all nest failures (Ruggera and Martin 2010, Mumme 2010).