1. A method comprising: receiving, at one or more data processors, a first content item request and a first plurality of values received from a respective plurality of sensors of a mobile device, the first plurality of values combined with the first content item request in a first URL string, the first plurality of values indicating a first ambient condition of the mobile device; selecting, using one or more data processors, a first content item based on a predictive model for selecting content items and the first plurality of values, the predictive model selecting the first content item from a plurality of candidate content items using associations between i) response data received from the mobile devices in response to the plurality of candidate content items presented on the mobile devices ii) values corresponding to sensors of mobile devices that are included in requests for content responsive to which the plurality of candidate content items are transmitted to the mobile devices, the predictive model maintaining, for each content item of the plurality of candidate content items, a performance metric based on the response data corresponding to the content item; outputting data to effect presentation of the selected first content item; receiving, at one or more data processors, first response data transmitted by the mobile device in response to presentation of the selected first content item at the mobile device associated with the selected first content item; associating, at the one or more data processors, the first response data corresponding to the selected first content item to the first plurality of values included in the first content item request corresponding to the selected first content item; updating the predictive model by associating the first response data corresponding to the selected first content item and the first plurality of values included in the first content item request corresponding to the selected first content item; receiving, at one or more data processors, a second content item request and a second plurality of values received from a respective plurality of sensors of the mobile device, the second plurality of values combined with the second content item request in a second URL string, the plurality of values indicating a second ambient condition of the mobile device; and selecting, using one or more data processors, a second content item from the plurality of candidate content items based on the updated predictive model and the second plurality of values.