Breeding

Known to breed in Argentina in November, December, and January (de la Peña 2007). A male with partially enlarged testes was collected in southern Brazil in February (Belton 1984).

Nest height is 0.5-2 m above the ground (Pereyra 1927, de la Peña 2007). Three nests described in Esperanza, Argentina were between 1-2 m above ground and contained 2-5 eggs. Nests were exposed and held in the branches of shrubs and bushes (Pereyra 1927, de la Peña 2007).). A nest was also found 40 cm above the ground (Pereyra 1927).

The nest is a domed, globular, structure composed of sticks, with an elongated tunnel entrance; the general appearance of the nest is like a long, dense heap of spiny sticks. The eggs are laid in an enclosed inner chamber which is lined with small leaves, moss, hair, and soft plant materials (Sick 1993, de la Peña 2007). The nest almost always includes the shed skins of snakes and lizards. The entrance is on the side and leads directly to the inner chamber. Above the inner chamber, there is an accumulation of heavy pieces of bark and branches that can form a high, conical protuberance from the top, which "apparently provides protection against rain" (Sick 1993). Sick (1993) reported "an assemblage of 3 nests" that were constructed primarily of wire (including barbed wire), gathered from a nearby factory; the weight of the entire structure was estimated 15 kg.

Nest measurements (n = 3; (de la Peña 2007): total length, 30-40 cm; length of tunnel, 20-25 cm; internal diameter of tunnel, 3-5 cm; external diameter of tunnel, 10-12 cm; height of external chamber, 25 cm; external width of chamber, 16 cm; internal diameter of chamber, 12 cm. Another nest was 28 cm long with a diameter of 31 cm diameter, and an entrance diameter of 3 cm (Pereyra 1927).

The eggs are ovoid and pale green or greenish white (de la Peña 2007). Mean dimensions of the eggs as 20-21 mm x 15-16 mm (Serie and Smith 1923.)

Spix's Spinetail regularly is parasitized by the brood parasite Striped Cuckoo (Tapera naevia) (Wetmore 1926, de la Peña 2007). Striped Cuckoo is reported to tear a hole in the side of S. spixi's nest to access the nest cavity, since it is too large to enter through the entrance (Wetmore 1926). Spix's Spinetail also is a host to the brood parasite Shiny Cowbird (Molothrus bonariensis), although it is unclear how the cowbird had reached the inner chamber, given that the entrance would be too small for it (Pereyra 1927). Friedmann (1931) reported that cowbirds arely enter the domed nests of the spinetail, although cowbirds often hop around curiously outside.