Contribution to the Life History of the Swallow-Tanager

Abstract

A 3-yr. study of Tersina viridis occidentalis in the Parque National de Rancho Grande. A sequence of plumage changes during first 4 yrs. is postulated. Adults weigh 29-35 gm. Esophagus capable of great expansion: "stomach" very muscular but small; right testis much larger than left. They are semi-migratory, with broad ecological tolerance, but concentrate in hilly sections in their breeding season and in low, hot, humid areas s. of Lake Maracaibo in winter. They are relatively inactive about half of the day (6 hrs.); forage 2 hrs.; engage in social activity 3-4 hrs. Flight resembles that of finches or Tyrannids. They bathe only during dry weather; have an omnivorous diet, with insects predominating during the rainy season. Insects are caught on the wing in flycatcher fashion. Favorite fruits include large fleshy forms like avocado. A variety of notes of varied significance are descr. Young males have a more varied song than older males. Pair formation is initiated by the female and followed by a gradual formation of a distinct territory which is defended by the male alone. Territories varied from 15 x 30 m. to 50 x 100 m. depending on availability of nesting sites, and were co-inhabited with many other spp. Three spp. appeared to dominate the tersinas. These were Tyrannus melancholicus Thraupsis episcopus, and Tachyphonus rufus. A "curtsy reaction" which dominates the social behavior is descr. It was first seen in a tame male 230 days old, and becomes intensified with age. Mass social insect catching occurs between 6 and 14 days after young are hatched. This may be a transition in social behavior to the post-breeding flocking habit. Expts. with mounted birds show that other tersinas are recognized at a distance of 150 m.; owls are ignored; hawks cause mild "excitement" but not fear; and the bat falcon, Falco albigularis, causes real fear. Nest site selection is initiated by male but actual selection is made by female. Three nests were finished in 5, 6, and 7 days, almost entirely by the female. Nest size varies with altitude (climate?) and second is larger than first. Mating begins after nest is built Three eggs are incubated 80-90% of the entire 13 to 17 days by the female alone. Fledging success is very high. Nestling period is 24 days. Detailed table of changes in nestlings is given. Wt. variation in nestlings is accounted for in part by storage of considerable quantities of food in the esophagus, a trait of adaptive value in areas of frequent torrential rainstorms. Brooding and feeding of young, and nest care are descr. Tersinas flock in groups of 10 to 30 individuals, the flocks not mingling with other spp., and even fighting off spp. which previously dominated them individually. Journal notes on "individual characteristics and exceptional behavior of different pairs" concludes the paper. || ABSTRACT AUTHORS: L. M. Bartlett ZR 0