Cooperative Feeding, Defense of Young, and Flocking in the Black-Faced Grosbeak

Abstract

The following observations on Black-faced Grosbeaks (Caryothraustes canadensis poliogaster) made in a Costa Rican rainforest extend those made by Skutch ten years previously (Skutch 1972). My findings, although limited and dealing with unmarked birds, serve to affirm the likelihood that these birds breed cooperatively and to suggest the possibility of a complex social organization differing from that reported for other cooperative breeders (cf. Brown 1978). On 3 and 4 July 1978, small, noisy, monospecific flocks of Black-faced Grosbeaks periodically visited a pejibaye (Bactris gasipaes) grove at the Organization for Tropical Studies field station, Finca La Selva. At each visit the birds flew together from tree to tree uttering chu-weet call notes for several minutes and then left together, flying out of earshot. On July 4th I saw four adult grosbeaks land on the dried flowering stalk of a bromeliad attached about 6 m above the ground to the trunk of a pejibaye palm. One by one the birds then flew about 1 m down to an open nest in the top of another bromeliad on the same trunk. Each of the four birds fed two young in the nest; each flew away before the next bird descended. Afterwards, the entire flock of grosbeaks left the grove. On July 5th, J. Dillon also saw four adult grosbeaks feed young at the nest. Later the same day, I saw a flock of six grosbeaks enter the grove but observed only four adults visiting the nest. This time only one perched on the dead flowering stalk before approaching the nest. The other three flew directly to the nest from nearby trees. Two days later I witnessed group defense of newly fledged young. By 7 July one young had left the nest and was perched less than 1 m above the ground in a low bush. Two or more adults flew near the fledgling, perched next to it and fed it. When I approached within 30 m of the fledgling, the adults flew to a perch about 5 m above me calling loudly (presumably a mobbing call), following which the flock departed. After many minutes six grosbeaks returned, the bills of several being filled with food. They all perched about 6 m above me, near the nest tree calling loudly (chu-weet call). A second fledgling then fluttered down, accidentally colliding with me, at which time the adults' calls increased in intensity. For the next several minutes I captured and released the fluttering fledgling several times. During this period, four and only four of the adults flew to within 3 m of the ground, giving loud seet-seet calls incessantly and flying back and forth within 2 to 3 m of me on both sides.