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.
You have requested a machine translation of selected content from our databases. This functionality is provided solely for your convenience and is in no way intended to replace human translation. Neither BioOne nor the owners and publishers of the content make, and they explicitly disclaim, any express or implied representations or warranties of any kind, including, without limitation, representations and warranties as to the functionality of the translation feature or the accuracy or completeness of the translations.
Translations are not retained in our system. Your use of this feature and the translations is subject to all use restrictions contained in the Terms and Conditions of Use of the BioOne website.
Vol. 125 • No. 2