10-8-63 ( cont. ) simply that Masked boobys, because they cannot easily escape are attentive, and defend the nest, eggs and chick with great vigor, while Browns being able to get airborne do so. I have the impression that both male Browns and Masked are more skittish than the females, and that the females are noticeably larger in both species. Brown Booby males also possess the hoarse voice; while female browns have the deep-full full throated call. While working in the colonies of these two species, I had occasion to note interspecific behavior on a number of occasions. If an adult of either species enters the territory of a family group of the other species, any member of the family group, ♂, ♀, or Chick recognizes and repels the intruder. It seems classically funny to watch a 2,200 gram ♀ Masked " run out of Brown Booby town " by an 800 gram, partially feathered and still downy Brown Chick. The members of family groups also recognize intruders of their own species and attack vigourously. This seems to be particularly true when there is a small chick in the Family group. However, in the Masked Booby I suspect that several pairs of adults may ( on occasion ) share the dutys of raising a single chick. It is too bad that there are not more new nests to test out this suspicion. Arrival behavior over the nesting colony is varied and colorful. A bird taking off ( Masked Booby ) gains altitude slowly and has to pass over nesting territories of other birds. If a defending bird is in the territory it squaks if the indi- vidual taking off is at an altitude below about 12 feet. If the " intruder " is low enough, he may receive a stout peck as sgrains for altitude, and once today I saw an immature ( Juv. ) Masked knocked off balance so as to crash in an undignified manner when he crossed the territory held by a female with a downy chick. Adults passing over often call, and I watched one marked male pass over a female several times, each time she would call as if recognizing him, and he usually responded with his call and soon he landed next to the female: Apparently they were a mated pair for there was no hostile behavior after he landed. Neither was there a recognizable greeting ceremony. I watched several fights on the ground today. One, where I identified from voice two males as the contestants; lasted several minutes. A preliminary to the actual fighting was a series of head shaking movements. The head is moved up and down rapidly ( about 5 -9 times per second ) which the neck is twisted from side to side in a slower cadence ( one complete movement left to right to left to begin again takes about 1 to 1.5 seconds ). Then suddenly the battle is joined