Tropicbird

Molt - Red-tailed Tropicbird streamers molt alternately with each streamer taking about six months to complete growth; each streamer is then retained at full length for approximately six months thereafter (A. C. Veit and I. L. Jones unpubl. manuscript). Thus, when a tropicbird returns to its nesting island, it normally has two fully grown streamers, one recently fully grown and the other having been fully grown for about six months and showing signs of wear (A. C. Veit and I. L. Jones unpubl. manuscript). src

Tropicbirds (Pelecaniformes, Phaethontidae) include three sexually monomorphic,
socially monogamous tropical seabird species, all of which have conspicuous tail
streamers. The adult Red-tailed Tropicbird (Phaethon rubricauda) is pigeon-sized
with satin-textured white plumage, a red-orange bill, and two flexible
elongated red central feathers that form conspicuous tail streamers a body
length long. Males and females perform complex aerial courtship displays prior
to breeding. Groups of vocalizing tropicbirds circle about in the sky above the
breeding area; a pair (or sometimes trio) will separate from the group and begin
synchronized flight maneuvers [Photo: <1> mirror ] in which one bird hovers above the other, the
upper bird is carried backward by the wind, it then glides forward in a downward
curve and the lower bird begins a hover-backward rise. Both birds slowly twitch
their streamers from side to side. The result is alternating vertical circles
(Fleet 1974, Schreiber and Schreiber 1993, A. C. Veit and I. L. Jones pers.
obs). That scene presents a biologist with an improbable sight. What is the
function of the red streamers, and what mechanism could possibly explain their
evolution? Use of streamers in those displays, together with the finding that
adults' brief possession of two fully grown streamers coincides with their
prelaying period (A. C. Veit and I. L. Jones unpubl. manuscript), strongly
suggests a sexually selected function for tail streamers.

The slender lifting-surface theory suggested that only the part of the tail proximal to the point of maximum continuous width is aerodynamically functional and the tail behind that point is relatively free from aerodynamic-selection pressures (Thomas 1993). Because tail streamers extend far beyond the point of maximum width of the rest of the tail, aerodynamic theory indicates they generate drag but no lift , which suggests that they are unlikely to enhance flight performance (Balmford et al. 1993, Thomas 1993). Tail streamer ornaments formed by elongated central rectrices (pin tails) may have no beneficial aerodynamic function but instead have arisen as sexually selected ornaments that function to attract mates or in intrasexual competition (Balmford et al. 1993, Thomas 1993). Allowing for the possibility that the streamers might have some function related to flight, as yet unexplained by aerodynamic research, we looked for an allometric relationship between tail streamer length and body size (PC1), wing length, and body mass. We predicted that if Red-tailed Tropicbird tail streamers have a significant flight function, they should correlate across individuals with other aerodynamically important traits such as wing length. [Result of the study is: Not have a significant flight function.]

We evaluated our findings in relation to two independent hypotheses concerning streamer function:
(1) streamer function is mainly aerodynamic and enhances flight performance, and
(2) streamer function is mainly ornamental and favored by inter- or intrasexual selection.
Within hypothesis (2), we considered two alternatives: (i) streamers are condition-dependent indicators of individual quality (i.e. a "good genes" or "handicap" process; Andersson 1986, Pomiankowski 1988); and (ii) streamer expression is arbitrary because they are the product of a sexual selection mechanism not involving condition-dependence (i.e. a runaway process, Fisher 1930; Kirkpatrick 1982; Lande 1980, 1981; or sensory exploitation, Ryan and Rand 1993).