Decomposition stages as a clue for estimating the post-mortem interval in carcasses and providing accurate bird collision rates

Abstract

The estimation of the post-mortem interval is crucial to accurately provide bird collision rates against manmade infrastructures. Standard methodologies recommend initially clearing all carcasses to ensure that subsequent collisions can be attributed to known time intervals. In this study, we propose a more cost-efficient approach aiming to link the decomposition stages as unequivocally as possible to the most likely time elapsed since death. Factors influencing the decomposition stages of bird carcasses were evaluated by means of two experiments. Firstly, we examined carcasses of large birds in three seasons differing in temperature, sun radiation and humidity: summer, autumn and spring. Secondly, we tested the influence of body mass in the same season (spring) using small, medium-sized and large bird carcasses. Results showed that the decomposition score increased monotonically with time, attaining the highest magnitude effect. A carcass with a decomposition score ≥ 4 (skeletal reduction) was in the field for ≥ 15 days, whereas a carcass with a score < 3 (fresh or emphysematous) was exposed < 3 days. Decomposition scores were higher in summer and did not differ among carcass sizes. This study provides an alternative protocol to estimate the post-mortem interval in wild birds in studies in search of bird fatalities.

Figure 1

Decomposition scores of large-sized birds (i.e., hens) throughout time (five sampling events at days 1, 3, 7, 15 and 30 after the fresh carcasses were placed in the field) in three different seasons. The vertical bars show the average ± 95% confidence intervals of the data pooled for the two islands (the main effect of island, and the interactions between Island and the other two factors were not significant; see Table 2 for more detail).

Figure 2

Decomposition scores of three different bird-sizes (large—domestic hens, medium—pigeons, small—chicks of domestic hens) throughout the first week after fresh carcasses placement in the field (three sampling events at days 1, 3, and 7) on the islands of Lanzarote and Fuerteventura. The vertical bars show the average ± 95% confidence intervals of the data pooled for the two islands (the main effect of island, and the interactions between Island and the other two factors were not significant; see Table 3 for more detail).