Most ornithologist say it is not possible. In fact there is no book or article explaining census techniques to be aplicate to wetland for passerine birds.
Luis - please find attached a protocol for surveying wetland birds in North America.
As I understand it, transects surveys through Phragmites are not all that practical unless you have some roads available, such as are found in areas of southern Spain like La Janda. Really, the trick is to use occupancy (like program PRESENCE or package unmarked in R) or distance estimation methods to model the probability of detection for these (mostly) secretive birds. Get the data from areas you have access to, then extrapolate to larger areas. That said, because of their often low detection rates, you would need to make more visits to each site in order to get decent estimates of their detectability and occurrence.
Thank you very much Andrew, Micah and Usman. Thats the way I´m working on.
It´s to me important to know you are agree with the method and to know your suggestions and new ideas. Waiting for you in a next visit to Spain. Yours sincerely.