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The only living mammals that lay eggs are echidnas and platypuses. In the latter, the eggs develop in utero for about 28 days, with only about 10 days of external incubation (in contrast to a chicken egg, which spends about one day in tract and 21 days externally). [11] After laying her eggs, the female curls around them.
The shiny cowbird's diet consists mainly of insects, other arthropods [4] and seeds, and they have been recorded foraging for grains in cattle troughs. [2] Like most other cowbirds, it is an obligate brood parasite, laying its eggs in the nests of many other bird species such as the rufous-collared sparrow. [5]
The eggs are incubated for 12 to 13 days and the chicks are altricial at hatching. [9] Chicks fledge 10 to 12 days after hatching. Most pairs raise two broods per year, and the male may feed newly fledged young while the females incubate the next clutch of eggs. [27] The brown-headed cowbird may parasitize this species. [8]
The screaming cowbird frequently parasitizes its main host, the baywing, during the pre-laying period. [20] Screaming cowbirds lay 31% of their eggs before the first baywing egg but most of the eggs laid are ejected, and often within 24 hours. [29] By ejecting parasitic eggs with their feet, baywings can reduce the parasitic egg load by 75%. [30]
The female cowbird may continue to observe this nest after laying eggs. Some bird species have evolved the ability to detect such parasitic eggs, and may reject them by pushing them out of their nests, but the female cowbird has been observed to attack and destroy the remaining eggs of such birds as a consequence, dissuading further removals. [8]
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If the eggs happen to hatch, the parents will immediately start feeding them fish and keeping them warm. If the eggs don't hatch, the parents will slowly start leaving the eggs a little bit at a time.
The brown-headed cowbird is an obligate brood parasite; it lays its eggs in the nests of other small passerines (perching birds), particularly those that build cup-like nests. The brown-headed cowbird eggs have been documented in nests of at least 220 host species, including hummingbirds and raptors.