An intratropical migratory passerine can quickly improve its physiological condition during post migration, reproduction and departure phases on the breeding site in the Cerrado

Abstract

Migration and reproduction are energetically expensive processes that migratory animal species must confront during their life-cycle. The relationship between hematocrit and mass in birds highlight their ability to invest energy in activities such as reproduction and migration and face costs linked to all subphases. We tested three hypotheses to evaluate the relationship between hematocrit and mass for an intratropical migrant bird, the Lesser Elaenia (Elaenia chiriquensis): (1) In the arrival phase, we expected that there would be a decrease in mass accompanied by a drop in hematocrit levels in a negative linear relationship; (2) during the reproduction phase we expected, at most, a weak relationship and a decrease in mass and hematocrit, due to the physiological complexity of this phase; (3) on the departure phase, we expected an increase in mass complemented by an increase in hematocrit as a positive linear relationship. We found that the Lesser Elaenia has average mass and hematocrit levels in the arrival phase similar to those in the other phases, thus not showing the expected trend as predicted above. On the other hand, during the reproduction phase, as expected, there was no significant relationship between mass and hematocrit levels, whereas a strong relationship between these variables was detected during the departure phase. Overall, the Lesser Elaenia arrives in good condition in their breeding sites, then it undergoes some physiological stress during the breeding period, but later is able to rapidly recover optimal physiological conditions upon departure to wintering grounds.