Thanks for your replies.
First of all, using a sound card won't work because the recorder will be outside far away from any buildings/electricity/
Secondly, I don't know too much about electronics thus will not be able to solder my own automated system or similar. In the scientific literature (e.g. the article: "Using Automated Digital Recording Systems as Effective Tools for the Monitoring of Birds and Amphibians", 2006) people have used automated recorders by connecting an audio recorder with microphone to a microprossor-based controller (in this case the MSP430-P-1121M) which triggers a relay that closes a circuit in the wired remote jack of the recorder making the recorder start recording when the circuit is closed and stopped when the circuit is opened. The microcontroller is programmable by the free KICKSTART software from Texas Instruments. However, as stated above I have not the smallest clue where to begin with this.
Another Idea I have had is to attach servo's to my recorder that at certain times will 'press' the record and stop buttons. The recorder will then be connected to a large external battery and the servo's will be connected to a batterypowered timer and a batterypack. For the marantz PMD660 I am now using I would need 1 for on/off and a 2nd for record. The idea is that say at 6am the timer turns on the servo 1 which then automatically moves 90 degrees, thereby moving the on-switch to on. A second servo will be triggered after that to move over the record button. At 7.30am the timer makes the first servo move back again and the marantz is turned off. Problem 1 is that I cannot find a simple timer that can do this. Problem 2 is that the servo can be set to move one way, but moving back is hard. Problem 3 is that I don't know how to make the timer start the servo to move the on-switch off.
A problem with external power is that I try to connect an external USB battery to my marantz. My marantz needs a 5V DC plug in and says 5V 0.8A on the back. USB is known to have 5V 0.5A. By trying this out I found the marantz will turn on and record, but after a certain time the screen lids and the marantz does not respond anymore untill I turn it off and on again. I don't know how to get a 0.8A current from the USB battery...
A final idea I had is using my iPhone with a recording app I have which can record as suggested in the replies when a certain amplitude treshold is reached. I would connect my iPhone again to a large external usb battery (with iPhone the amps will not be a problem), turn everything off on my iphone except the recording application and connect an external microphone to the iPhone using a converter cable (
http://www.kvconnection.com/product-p/km-iphone-mic.htm), this works well. However, expect but don't know if the quality of using my iPhone as a recorder will be much lower than using a Marantz or similar recorder.
I don't know or don't understand what is meant with iRiver with rockbox, even after reading their website. I looked at the frogloggers and must say that looks promising for 250 pounds (
http://www.frogloggers.com/Order%20Frogloggers%20Page%201.htm). However, reading about it it seems they exactly did what I wanted but than with a different recorder that has an automated recording function apparently. It is just a weather proof case with usb battery connected to a 5V regulator to the recorder. I'll try and see if I can make this myself.
Well, I thought I type out all my ideas in case other people who need this information. However, as you can see I am still in need of some very good suggestions and advice! Thanks.