Based on observations in Panama, the male orchard oriole is shown to be the most effective pollinator of Erythrina fusca. It is hypothesized that the identical color of the male oriole's body plumage and the floral parts surrounding the nectar indicates a coevolved relationship. Erythrina fusca Loureiro is a large and abundant tree in the Panama Canal Zone and occurs pantropically (Krukoff & Barneby, 1974). I observed nectarfeeding birds visiting a pure stand of E. fusca growing in swampy ground near the Pedro Miguel locks of the canal (Table 1). The avian activity in 4 tree crowns was observed for a total of 16 hours during January and March 1974 and March 1975. The orchard oriole (Icterus spurius) vastly outnumbered all other species combined both in numbers present and in total time spent nectarfeeding. This species's outstanding importance to the effective pollination of Erythrina fusca here is underlined by four facts that form the basis for the hypothesis that E. fusca and this oriole may have coevolved. 1. Of the 13 bird species observed feeding on Erythrina fusca nectar, only the male orchard oriole correctly opened the flowers (see below) to obtain nectar and to receive pollen for transfer to other flowers. The others either robbed nectar by piercing the calyx or standard base, by using only extrafloral nectaries, or by visiting only flowers previously opened by the orchard oriole. 2. The orchard oriole, a migrant that breeds in North America, occurs in large flocks of up to several hundred, far exceeding the numbers of other potential avian pollinators. This species robs nectar from other species of trees, such as Tabebuia guayacan (Seem.) Hemsley, easily piercing the corolla with its long sharp bill, but not from Erythrina fusca flowers. 3. Only adult male orchard orioles were observed to open Erythrina fusca flowers. Females and immature males, which have greenish olive plumage, were excluded by the black and burnt orange adult males. The burnt orange color of the adult male oriole is exactly matched by a portion of the E. fusca flower that becomes visible only after the standard has been pushed back. 4. Erythrina fusca flowers during a period when the orchard oriole is most abundant. These facts will be discussed together with corollary information on the bird and the plant to support the hypothesis that the plant is manipulating the behavior of the male oriole to most effectively affect pollination, particularly outcrossing. 1 I thank the National Geographic Society for a grant to study neotropical migrant birds, the Smithsonian Tropical Research Institute for support, and Drs. Neal G. Smith, Herbert and Irene Baker for their kind help. 2 National Zoological Park, Smithsonian Institution, Washington, D.C. 20008. ANN. MissouRI BOT. GARD. 66: 482-489. 1979. 0026-6493/79/0482-0489/$00.95/0 This content downloaded from 207.46.13.176 on Mon, 20 Jun 2016 06:19:33 UTC All use subject to http://about.jstor.org/terms 1979] MORTON-ORIOLE POLLINATION 483 PHENOLOGY OF ERYTHRINA FUSCA Erythrina fusca begins to show mature inflorescences nearly synchronously on both Pacific and Atlantic slopes of the Canal Zone at the beginning of the dry season. On 29 December 1974, trees at both the Pedro Miguel locks site, which is about 10 km from the Pacific coast, and trees near Gatun, 4 km from the Atlantic coast, had an estimated 1% mature flowers. Flowering lasted from then until 4 March 1975 (66 days), when the smaller trees still had unopened flowers. Figure 1 shows the manner in which flowers are arranged. The flowers are arranged radially around the thick stem which provides a strong perch for the oriole. There are from 2 to 12 inflorescences in each cluster, a flower measureing about 50 mm from the calyx to the tip of the standard. Leaves are shed before flowers mature as is typical for many Erythrina (Standley, 1922). The flowers are highly visible and appear uniform dull yelloworange until opened when the rich burnt-orange color of the wing petals is uncovered. Shortly after the flowering period ends, leaves appear coincident with the onset of the rains in late April. The next season's flower buds form at this time but remain in an immature stage throughout the rainy season. The calyx attains a mature size early and its tough texture no doubt protects the immature parts within. Significantly, an extrafloral nectary located at the tip of the calyx (see Fig. 1), secretes nectar during this 8 to 9 month period between bud formation and flower maturation. Probably these nectaries function to nourish ants (species undetermined) which live in the tree's hollow twigs (pers. obs.; see also Feinsinger et al., this symposium). However, several bird species were observed to use this extrafloral nectar source (Table 1). Some pods attained mature size even while flowers were maturing, but they dried and opened when flowering stopped in the late dry season (April). ORCHARD ORIOLE BEHAVIOR