Despite intensive research, the factors driving spatial patterns in life-history traits remain poorly understood. One of the most frequently documented, and paradoxically, least understood patterns, is the latitudinal gradient of increasing avian clutch size at higher latitudes. These gradients are less marked in the southern hemisphere, thus clutch sizes tend to be smaller at southern latitudes than at equivalent northern ones. We exploited a natural experiment provided by the introduction of European passerines to New Zealand (NZ) to test three widely proposed ecological drivers of this pattern, i.e. the nest predation, Ashmole’s seasonality, and the breeding density hypotheses. We focus on the blackbird Turdus merula and the song thrush T. philomelos as founder effects do not have a major influence on the reproductive traits of their introduced populations. Both species laid smaller clutches in NZ than in Europe. These reductions had stabilised within one hundred years and were not associated with a compensatory increase in investment in individual offspring by laying larger eggs. In contrast to the nest predation hypothesis, daily nest predation rates were lower in NZ than in Europe. Smaller southern hemisphere clutches were associated with higher conspecific population densities and a relaxation of seasonal clutch size trends. These findings thus provide some support for both Ashmole’s seasonality and the breeding density hypotheses. Analyses across 11 European passerines introduced to NZ suggest, however, that neither of these hypotheses provide general explanations of smaller clutches in the southern hemisphere. We suggest that reduced seasonality and lower nest predation promote increased breeding densities and adult survival in the southern hemisphere. The later may drive smaller southern clutch sizes by generating spatial variation in the outcome of the trade-off between reproductive investment and longevity.