In recent years, temperature data loggers have been used to collect breeding details including the duration of incubation breaks, detect the exact start of incubation, and record unsuccessful breeding in various types of bird nests. Nevertheless, the majority of conventional techniques are inapplicable in subterranean settings. This study presents the results of using a temperature data logger to monitor breeding phases and the transitions between them in the natural nest holes of the Common Kingfisher Alcedo atthis. The first sensor was placed inside an occupied nest chamber (during egg laying or incubation) beside the clutch, while the second sensor was placed in a reference hole near the nest at the same depth and height as the occupied nest hole. The temperature inside the nest chamber and the reference hole was measured continuously at one-minute intervals. Measurements were taken during various time periods, from egg laying/incubation to the successful fledging of chicks or their death. The temperature curves clearly indicate the laying of the eggs, the start of incubation, successful fledging, or the death of chicks in the nest chamber, and even other fine details such as diurnal fluctuations or levels of thermal stability. It was not always possible to determine exactly when the chicks hatched from the temperature data, as incubated eggs and freshly hatched chicks have similar temperature demands and receive similar heat from the brooding parent.

In recent years, thanks to continuous technological development, temperature data loggers have been used to collect data from the breeding progress of birds, which can be used, e.g. to obtain information on the length of incubation breaks, to detect the start of incubation or to record unsuccessful breeding. Temperature data loggers have been used in various types of bird nests, but not in a „closed“ nests such as a burrow or cavity. The study presents the results of using a temperature data loggers in natural nest holes of the common kingfisher (Alcedo atthis). Their nests, which lack substantial insulating structures, are primarily influenced by the relatively stable soil temperature. The temperature of the space inside a nest chamber was measured on a continuous basis with an interval of 1 minute between measurements. The measurement was carried out from egg laying/incubation to a successful leave of the nest chamber/death of chicks (5 nests on 3 different streams in 3 breeding seasons). The temperature curves showed the laying of the eggs, the start of incubation, the successful flight out of the nest chamber or the death of the chicks in the nest chamber. It was not always possible to determine exactly when the chicks hatched from the temperature data.

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.