 Because light usually indicates a day's passing to birds, exposure to light pollution caused a disruption in the birds' ability to measure the length of a full day.
<clarity>  Because light usually indicates a day's passing to birds, exposure to light pollution caused a disruption in the birds' ability to measure the length of a full day.
 Because light usually indicates a day's passing to birds, exposure to light pollution caused a disruption in the birds' ability to measure the length of a day.