Effort estimation is a project management activity that is mandatory for the execution of software projects. Despite its importance, there have been just a few studies published on such activities within the Agile Global Software Development (AGSD) context. Their aggregated results were recently published as part of a secondary study that reported the state of the art on effort estimation in AGSD. This study aims to complement the above-mentioned secondary study by means of an empirical investigation on the state of the practice on effort estimation in AGSD. To do so, a survey was carried out using as instrument an on-line questionnaire and a sample comprising software practitioners experienced in effort estimation within the AGSD context. Results show that the effort estimation techniques used within the AGSD and collocated contexts remained unchanged, with planning poker being the one employed the most. Sourcing strategies were found to have no or a small influence upon the choice of estimation techniques. With regard to effort predictors, global challenges such as cultural and time zone differences were reported, in addition to factors that are commonly considered in the collocated context, such as team experience. Finally, many challenges that impact the accuracy of the effort estimates were reported by the respondents, such as problems with the software requirements and the fact that the communication effort between sites is not properly accounted.