The Welcome Swallow was first recorded breeding in New Zealand in 1958, but is now common and widespread. Using nest record cards collected throughout New Zealand between 1962 and 1996 we investigated whether measures of breeding success showed temporal trends consistent with the effects of density dependence due to increased population densities. Controlling for potentially confounding factors, we found that: (i) breeding has occurred earlier in more recent years, (ii) nests in more recent years were more likely to produce fledglings, and (iii) there are no temporal trends in clutch size or the probability of at least one offspring failing to fledge. These results suggest that population densities have not yet reached the threshold at which density dependence limits breeding success. Earlier laying in more recent years is compatible with a response to global warming. As predicted for multiple-brooded species, clutch size exhibited a mid-season peak, the timing of which is consistent with the decline in the second half of the season being due to reduced foraging time. There was no evidence for a latitudinal gradient in clutch size within New Zealand.