Geographic patterns in reproductive parameters among Nearctic herons (Ardeidae).

Avian clutch size has been reported to show a poleward increase, a pattern that is well supported in landbirds but less so in waterbirds. We analyzed geographic variation in clutch and brood size among seven species of Nearctic herons and egrets (Ardeidae) over a latitudinal range spanning from 25° to 52°N. We included in the analysis data from 71 published references that reported information on both clutch and brood size. We found a strong and coherent latitudinal increase in clutch size in all species, but no unequivocal latitudinal variation in brood size. This resulted in a decreased index of productivity (brood size:clutch size ratio, excluding nest failures) with increasing latitude. In addition, we detected habitat-related variation in clutch size, which was, on average, smaller in salt-water than in freshwater habitats. The latitudinal decrease of the productivity index, which was not associated with latitudinal variation in nest-failure rates at either the clutch or brood stage, contrasts with previously proposed explanations for gradients in clutch size, which invoke latitudinal differences in seasonal variation of food abundance or nest-predation intensity. We suggest that factors other than those proposed in the case of landbirds may explain the latitudinal variation in clutch size in herons. In any case, our results suggest that herons may pay higher reproductive costs to obtain similar benefits when breeding at higher latitudes