Fat deposition in the scissors-tailed flycatcher (Muscivora t. tyrannus) and the small-billed elaenia (Elaenia parvirostris) during the austral migratory period in northern Venezuela
The scissors-tailed flycatcher (Muscivora t. tyrannus) and the small-billed elaenia (Elaenia parvirostris) are two migrating bird species which come to Venezuela during the austral winter from the austral regions of South America. A quantitative analysis of the fat deposits in specimens of both species collected in northeastern Venezuela showed a considerable increase in the amount of fat at the end of the molt, before the beginning of the spring migration. The data indicate that Elaenia parvirostris arrives at its winter ground before initiating the molt, still having a high amount of fat (50% of lean dry weight). On the other hand, Muscivora t. tyrannus seems to arrive at the winter ground after the initiation of the molt with a low fat level. While the lean dry weight remained constant during the whole austral winter, the total dry weight fluctuated in a parallel direction with the fat deposition. The lean fresh weight rose at the time of higher molt intensity, parallel with an increase of water content. The fresh weight first increased at the time of higher molt intensity in proportion to an increase of water and a decrease of fat, and secondly at the end of the molt in proportion to a considerable increase of fat and a decrease of water. We suggest that the inverse correlation found between the water index and the fat index could allow the bird to load itself with more fuel at the beginning of the migration, without increasing its total weight too much, an important factor to consider since the bird is fighting gravity during flight. The estimation of the potential flight-ranges showed that any of both species could fly more than 1500 km in a non-stop flight. We suggest that both species begin the migration making short flights and accumulate more and more fat on each stop-point, in such a way that they could make longer and longer flights as they come nearer to the breeding ground. A gradual increase in fat reserves may have an adaptive value.