Breeding season length
Nests can be found anytime of the year. Roper (1996) found nests between late December and early September. CET found nests between January and September. Peak breeding is from April-July. How much of the population breeds from January-March depends on the year (CET, pers. obs.). During one study over five years, in 2 out of 5 years most pairs bred in January-February, while in the other 3 years, most pairs did not begin until April (CET, unpubl. data). A minority of pairs breed after July.
Nests and nest building
Nests of Western Slaty-Antshrikes in Panama sometimes can be confused with other species if one doesn’t have the correct search image. In the younger forest on Barro Colorado Island and the Pacific slope of Panama (and other drier sites), Western Slaty-Antshrikes rarely use moss on their nests and use lighter lining (Oniki 1975, CET, pers. obs.). In these areas, their nests can be confused with nests of Spotted Antbird (Hylophylax naevioides). Nevertheless, Western Slaty-Antshrikes nest higher on average and they do not build nests with sticks or leaves. Nests (with moss) can also be confused with nests of Spot-crowned Antvireo (Dysithamnus puncticeps). Antvireo nests are usually higher in the canopy and are slightly smaller.
Nests are located 2.0-2.5 meters high (range=0.25-7.0 meters; Oniki 1975, Roper 2005, CET, unpubl. data). Both sexes build open cup nests in the forks of trees. The majority of nest building is complete in three days, with both parents’ first adding moss and then rhizomorphs. After nest completion, parents may add rhizomorphs to strengthen the nest, even after eggs are laid. Nests are thin and are not re-used. The lining of nests is usually made of black rhizomorphs and the outside is moss (with rhizomorphs woven into the moss) (Skutch 1934, Oniki 1975). They also use spider webs and rootlets in their nests (Oniki 1975). The cup-shaped nest is tied to two or three small twigs of a horizontal fork. The twigs of the fork are 0.2 to 0.6 cm in diameter (Oniki 1975). For 32 nests the external diameter averaged 9.0 x 6.9 cm, the internal diameter 6.5 x 5.3 cm, the external height 7.7 cm, and the internal depth 4.9 cm (Oniki 1975).
Clutch size and eggs
They have a modal clutch size of two (1.89 ± 0.3 [sd], range: 1-2 eggs, n=474 nests; Tarwater and Brawn 2010b). Eggs are oblong, whitish with reddish-brown spots (Oniki 1975). The average dimensions were 23.5mm x 17.0mm and weigh 3.26 g (Oniki 1975).
Incubation and nestling period
The two eggs are laid two days apart. Although some incubation starts prior to the second egg being laid, full incubation does not start until the clutch is complete (Oniki 1975, CET, pers. obs.). Both parents incubate eggs. Incubation bouts are long, sometimes lasting 3-4 hours (Skutch 1934, Oniki 1975). Eggs hatch synchronously, usually within a few hours of each other. Nestlings hatch naked and weigh on average 2.7g (n=12, CET, unpubl. data). The incubation period is approximately 16 days (from lay date of first egg to hatching, 14 days from lay date of second egg) and the nestling period is 10 days (Johnson 1953, Oniki 1975, Robinson et al. 2000 [n=14, 7], CET, unpubl. data [n=130 nests followed from building]). Nestlings fledge most often 10 days after hatching, except when eggs hatch late in the day, then offspring fledge the following day. Nestlings fledge on the same day, within a few hours of each other. The male typically cares for the first to fledge (generally the larger offspring) and the female feeds the second to fledge (smaller individual) (Tarwater and Brawn 2008). See 'Parental care' below for more information.
Renesting, multi-brooding, and intervals between nests
The average interval after a failed nest is 11.9 days, the average interval after offspring are raised to independence is 56.1 days, and the longer parents care for the first nest, the longer the interval length (Tarwater and Brawn 2010b). Experienced birds have shorter renesting intervals after a failed attempt than inexperienced birds (Roper 2005). Western Slaty-Antshrikes often attempt second nests after the first successful brood, and in some cases, successfully raise two broods in one year (Tarwater and Brawn 2008). They renest up to 9 times in one breeding season if they are continually unsuccessful in producing offspring (CET, unpubl. data). They typically nest 4 times in one year because usually a pair produces one successful brood/year (CET, unpubl. data). Average annual productivity was 0.91 fledged young/female (± 0.06, n=4 years) and varied from 0.73-1.0 fledged young/ female (Tarwater et al. 2011).
Parents divide broods, with the male typically feeding the larger offspring and the female feeding the smaller offspring (Tarwater and Brawn 2008). Brood division begins in the nest and continues throughout the post-fledging period. The duration of parental care to fledglings is on average 55.3 ± 1.96 days (mean ± SE; n = 38 fledglings, Tarwater and Brawn 2010b). When comparing broods (n=9) that were observed during both periods, feeding rates in the nest were on average 1.2 ± 0.07 feeding visits/hr/offspring and after fledging, 3.79 ± 0.27 feeding visits/hr/offspring with no differences in food load between the periods (Tarwater and Brawn 2010b). Provisioning rates increase throughout the nestling period and post-fledging period until offspring are approximately 41 days out of the nest (Tarwater and Brawn 2010b). Male and female parents do not differ in provisioning rates (Tarwater and Brawn 2008). The prey times that parents most frequently bring to nestlings are: katydids, caterpillars, spiders, and beetles, with the rest of the prey items encompassing less than 5% of prey items brought (Tarwater et al. 2009). Even after parents stop feeding offspring, parents typically allow offspring to remain on the natal territory after reaching independence (See 'Dispersal' below).
After fledging, juveniles are fairly immobile. They remain low to the ground for the first two days, hiding in dense liana tangles and shrubs (Oniki 1975, Tarwater and Brawn 2008). Then juveniles move to 15-20m up and stay hidden and relatively immobile for the next two weeks. Not only are broods divided during this time, but the two offspring are segregated spatially (in different areas of the territory) (Tarwater and Brawn 2008). After two weeks, the juveniles begin to travel with parents more frequently, and the two offspring are brought back together (although each parent continues to feed its focal offspring). Juveniles start out pecking at inedible prey items, and the minimum age a successful prey capture was observed was 15 days after fledging (CET, unpubl. data). Foraging success and the number of maneuvers used slowly increase with age until offspring reach independence around 40 days after fledging (based on when parents stop feeding young) (CET, unpubl. data). See Vocalizations and Molts for other descriptions of offspring.
Fledglings have been observed helping build nests (Tarwater 2006). This appears to be more beneficial for offspring in terms of learning rather than helping parents. The fledglings collected nest material and in some cases they put the nesting material on the nest (but did not actually weave the material into the nest) and in other cases they dropped the nesting material while trying to fly with it. Older fledglings, which have developed the ability to sing, will help defend the natal territory with their parents (Tarwater 2006).