Spring stopover of intercontinental migratory thrushes along the northern coast of the Gulf of Mexico

Abstract

The narrow woodlands and wooded barrier islands along the northern coast of the Gulf of Mexico provide the first available landfall for Neotropical landbird migrants after their spring trans-Gulf flight. We studied the timing, volume, fat stores, length of stay, and fat deposition of thrushes (n = 1,903) during stopover at a coastal Louisiana woodland in 1987, 1988, and 1990. Peak migration occurred around mid-April in Wood Thrushes (Hylocichla mustelina) and was one to two weeks later for Catharus thrushes. More than one-third of the thrushes carried no observable subcutaneous fat stores upon initial capture, and only 5% of the birds had estimated fat stores that exceeded 20% of lean body mass. Body mass and condition index (mass/wing length) were negatively related to time of capture, suggesting that birds captured later in the day arrived with lower fat stores. Wing size varied negatively with capture date, suggesting differential migration between sexes and among age classes and/or populations. Recapture probability was related to the amount of fat stores upon initial capture, and length of stay was predictable from estimated fat stores, rate and amount of fat replenishment, and capture date. Flight range estimates suggest that most thrushes captured in spring along the northern coast of the Gulf of Mexico are unable to reach breeding destinations in a single flight. A series of short flights may represent a better migratory strategy than attempting to cover the same distance in one long flight.