Extreme weight overshoot in Common Kingfisher (Alcedo atthis) nestlings

We monitored the growth of 982 kingfisher chicks in 160 breeding events in the Greater Prague district, Czech Republic. The dataset revealed a standard growth pattern for wing length, which reached full grown adult size by fledging (usually 23–25 days). The increase in body weight followed a specific trajectory, approximated by a general additive model (GAM). Chicks attained the average adult weight as early as 9–10 days of age, exceeding the adult average by up to 41% when they were approximately 15–16 days old. This was followed by a prolonged decline in weight back to the adult average by the time of fledging–typically 24 days. This long decrease in weight cannot be attributed to fat resorption, starvation, or dehydration but is possibly related to disorganization of body reserves and the digestive tract after the beginning of thermoregulation. While bill length at fledging remained about 42% shorter than in adults, wing length and body weight reached adult values. Reproductive adaptation to safe and stable but relatively cold nesting conditions underground enabled Alcedo atthis (and also Megaceryle alcyon in North America and M. torquata in South America) to expand to colder latitudes.