1. A computer-implemented method, comprising: receiving, by a data processing apparatus, a request for a map, the request including viewport data indicative of a map zoom level parameter at which the map will be presented at a user device; determining, by the data processing apparatus, a number of available content item spaces to be associated with the map; receiving, by the data processing apparatus, content item bids for the available content item spaces, the content item bids each being associated with a bid zoom level parameter; selecting, by the data processing apparatus, a portion of the content item bids, content item bids in the selected portion of the content item bids being associated with bid zoom level parameters that correspond with the map zoom level parameter indicated by the viewport data; selecting, by the data processing apparatus and based on values of the selected portion of the content item bids, data identifying content items for presentation in the number of available content item spaces associated with the map; and providing, by the data processing apparatus, the data identifying the content items that were selected for presentation in the number of available content item spaces.