Density dependence of clutch size: habitat heterogeneity or individual adjustment?

Abstract

1. Two hypotheses have been proposed to explain density‐dependent patterns in reproduction. The habitat heterogeneity hypothesis (HHH) explains density‐dependent reproduction at the population level from poorer quality territories in hetero geneous environments only being occupied at high densities. No effect of population density is expected on reproduction within a given territory. If reproduction and territory quality are correlated, there will be a negative correlation between reproduction and density. Alternatively, density‐dependent responses could be caused by variation in reproduction with population density. This will be referred to as the individual adjustment hypothesis (IAH). 2. The HHH predicts that individuals breeding on the same territory in years with different densities would show no response in reproduction to changes in population density. The response in reproduction of the population to changes in density would therefore be stronger than the within territory response. Under the IAH, the response of individuals on specific territories would be similar to the average response in the whole population. 3. This paper attempts to discriminate between these two hypotheses using data on clutch sizes of great tits Parus major L. from six different populations. In all populations the mean clutch size was negatively correlated with the population density. In four out of the six study populations individual females breeding on the same territory showed a negative correlation between the change in clutch size between years and the change in density. In none of the populations it was found that the response within territories was significantly smaller than the average response for the population. 4. These data suggest that density dependence of clutch size in great tits is not due to poorer quality territories being occupied more frequently at higher population densities. Within territories, individual females adjusted their clutch size to changes in density, and this was sufficient to explain the density dependence of clutch size in five of the six populations. In one population some evidence supporting the HHH was found.