Using GPS and accelerometer data to precisely record egg laying, incubation and chick hatching of Cinereous Vultures () in-situ.

The Cinereous Vulture () is a near-threatened species, making detailed monitoring of its breeding behaviour crucial for effective conservation. Traditional methods are often invasive and logistically challenging. This study presents a novel, non-invasive approach utilizing GPS and accelerometer data from tracking devices to precisely record key breeding parameters: egg-laying, incubation onset and duration, and chick hatching. This method allows for continuous, remote monitoring, minimizing disturbance to the breeding pairs. The method was derived after a study on Cinereous Vultures in Bulgaria, using data from Ornitela GPS-GPRS transmitters (OT-30 and OT-50 models) deployed with leg-loop harnesses. Transmitters were configured to acquire data at intervals of 600 or 1200 seconds. Shorter intervals are recommended for optimal graph resolution. The methodology was developed based on observations of accelerometer graph patterns during the 2023 breeding season and validated through ground-truthing observations of GPS-tracked individuals (n=12, n=21 datasets) and laboratory experimental modelling and calculations in 2024. The study also retrospectively analyzed historical data, revealing previously undocumented hatching events and refining incubation timelines. Accelerometer data derived from the tagged with GPS transmitters Cinereous Vultures, particularly along the X axis, provided distinct patterns corresponding to different breeding stages. During incubation, the X-axis values exhibited a specific, relatively broader range than the regular pattern, reflecting the bird's posture and limited movements while laying in the nest. During the incubation period, the birds are laid in the nest under a banking angle of +/- 10°-15°, alternating to the left and to the right. After hatching, the banking angle increases in the range +/- 15°-20°, when a significant increase in the X-axis range was observed, typically by a factor of 1.2 to 1.9, with most cases falling between 1.4 and 1.7 compared to the incubation period values. This increase reflects the parent's altered movements associated with chick care, such as warming, feeding, and nest maintenance. The intensive brooding and rearing period, lasting 16-23 days post-hatch, is characterized by the same high-amplitude X-axis pattern observed immediately after hatching. As the chick grows and requires less brooding, the accelerometer graph returns to pre-incubation values, indicating the adult's return to a more typical posture on the nest. This methodology offers advantages over traditional monitoring techniques, providing continuous, precise, and non-invasive data acquisition. Its application can enhance conservation programs by enabling timely interventions, improving breeding success rates, and contributing to a deeper understanding of Cinereous Vulture reproductive biology. The potential for adapting this methodology to other vulture species, including Griffon and Bearded Vultures, warrants further investigation. Preliminary findings from limited 2024 data and historical data from Griffon Vultures suggest comparable accelerometer response patterns. This study demonstrates the power of combining GPS and accelerometer data to gain valuable insights into the breeding behavior of threatened avian species, ultimately aiding in their conservation.