It is unclear whether developmental trends in total leukocyte (WBC) and differential lymphocyte (PropL) counts in peripheral blood of altricial birds typically mirror the known ontogenetic increase in immunocompetence. We documented the development of leukocyte and lymphocyte numbers in peripheral blood of wild, altricial Welcome Swallow (Hirundo neoxena) nestlings. Nestlings had a mass-overshoot-recession growth profile. Hatchlings' mean WBC (7.94 x 10(9) cells/l) and PropL (0.65) were respectively 4x and approximately 1.7x the mean adult value. Both variables declined at a steady rate throughout nestling development and were 1.3x the mean adult value at fledging. Hatching WBC values that substantially exceeded those of adults could have reflected the parasite- and pathogen-rich nest environment of this species. The developmental declines in peripheral blood WBC and PropL were not inconsistent with an ontogenetic increase in specific immunocompetence; they are likely to have resulted mainly from an increase in the rate of leukocyte trafficking to vulnerable tissues and organs.