resource allocation by female birds into their eggs may have important consequences on nestlings' life. female's body condition, food abundance, environmental conditions, clutch size and lay date are some of the variables described to affect egg mass in passeriformes. we studied the effect of those variables on egg mass of white-rumped swallow (tachycineta leucorrhoa). aerial insect abundance was estimated using cone and acrylic traps. egg mass was positively related to female body condition and to mean ambient temperatures during egg formation. these results are in agreement with those found for tree swallow (tachycineta bicolor), a north hemisphere congeneric species.