Due to the strong seasonality of their preferred habitat, thorn forest and tropical dry forest, white-throated magpie-jays are somewhat predictable in their breeding phenology. In northwest Costa Rica, breeding activities (female begging, nest building, courtship, etc.) begin in early February, probably in concert with the onset of dry weather. (Reports suggest that nesting can start in November in the same region (Nicoya, Guanacaste; Skutch 1953). By early March, most groups have started a nesting attempt. This continues, depending on their success rate, through June; if a nesting attempt ceases before mid-June, either due to successful fledging or failure, the group will often try again (Skutch 1953; Innes and Johnston, 1996; Langen 1996b; Berg 2004b, 2005) . One pair was observed building a nest in mid-July, but the nest was not used (Ellis. pers. obs).
Magpie-jays nest in trees. Their preferred nest site is either in a tree isolated from continuous canopy, or if such is not available, then one in the canopy edge. They do not nest in areas of continuous tree canopy. They do not seem to demonstrate a preferred tree species, using trees with a variety of forms, including those with spreading canopies (e.g. Enterolobium cyclocarpum, Samanea saman, Crescentia alata, Mango trees) to more upright trees and large shrubs (Glyricidia), and even in palm crowns and bamboo (Skutch 1953). No effort is made to conceal the nest in the dry season, though the visibility of nests from the ground often decreases with leaf-out.
The nest itself is a bulky mass of sticks with a well-made cup inside. The outer-most layer of nest material is consistently made of large twigs of trees or shrubs with spines, often including Acacia cornigera and A. collinsii in Costa Rica. The inner cup is woven of much finer vegetable material; one preferred material for this prupose is strands of palm frond. Soft and cottony materials are not used. In some cases the nest may be flimsy enough that the contents can be viewed from below, although this is rare. Whole leaves are not used, and the presence of leaves in a nest is enough to determine that the nest is old.
Eggs are a light brownish blue, heavily speckled with dark blotches.
3 to five is usual. Occasionally clutch size appears larger due to intraspecific brood parasitism (Berg 2004a; pers. obs. JME) which can lead to over 10 eggs in a nest.
The adult breeding female preforms all incubation duties. She will also shade the nest during hot periods, although her social mate may be allowed to perform this duty as well when the female is feeding. Females may incubate for over an hour. The incubation period is approximately 20 days.
Nestlings are highly altricial.
As noted, White-throated Magpie-jays are cooperative breeders, and as such, non-parental individuals provide care to both nestlings and fledglings. This primarily includes retained adult daughters, who are therefore sisters or half-sisters to the young receiving care, but can also include young males who have not yet dispersed (Innes 1996, Langen 1999, Berg 2005). Care consists of feeding and predator defense, and group members also help defend territory boundaries. Group members seem to increase group success largely through taking over care of fledglings, allowing the primary pair to renest.
Intraspecific brood parasitism is genetically documented in white-throated species, and observations support this. It occurs when other group members lay eggs in the nest of the primary female; egg dumping by extra-group females has not been documented. In a multi-year study, up to 22% of eggs tested were not those of the primary female (Berg 2005). This is corroborated by observations of over ten eggs in a single nest, presumably reflecting rampant brood parasitism by group members. Behavioral responses to this pressure include incubation by the dominant female only, and fierce nest defense, which can include chases and fighting, the the dominant female catches a subordinate at the nest. It is not certain if dominant females abandon parasitized nests if they suspect egg dumping; theory would predict that this might not occur, given the close genetic relationships between female group members.