Cannot read XBEE output using UART

As the title says, when I hook up an Xbee Pro S1 to the Tx/Rx pins I cannot get the Arduino Uno to register any serial data sent to it, although sending data from the Arduino works fine. When I attach the Xbee via software serial there is no issue at all and all data gets though. I am using the "Physical Pixel" Sketch as a test, and sending commands from the X-Ctu software. I have tried two different Xbee modules and two different arduinos to the same effect. The adaptor board I am using is this one. Any thoughts on this, the Serial.println(Serial.available()); line only ever prints zero.

const int ledPin = 13; // the pin that the LED is attached to
int incomingByte;      // a variable to read incoming serial data into

void setup() {
  // initialize serial communication:
  Serial.begin(19200);
  // initialize the LED pin as an output:
  pinMode(ledPin, OUTPUT);
}

void loop() {
  // see if there's incoming serial data
  // and print how many bytes
  Serial.println(Serial.available());
  delay(500);
  if (Serial.available() > 0) {
    // read the oldest byte in the serial buffer:
    incomingByte = Serial.read();
    // if it's a capital H (ASCII 72), turn on the LED:
    if (incomingByte == 'H') {
      digitalWrite(ledPin, HIGH);
    }
    // if it's an L (ASCII 76) turn off the LED:
    if (incomingByte == 'L') {
      digitalWrite(ledPin, LOW);
    }
  }
}