Mortality and other determinants of bird divorce rate

Abstract

Pair bonds can end in two different ways: either death of the partner or divorce, where the frequency of divorce varies enormously among bird species, from 0% to 100%. To better understand this variation, we created and analyzed the largest dataset so far, consisting of 158 species and 20 variables that quantify the species’ body size, life history, diet, and other characteristics. Our results suggest that species with a high divorce rate have a high mortality rate, tend to be ornamented and sexually dichromatic, live colonially, and form part-time rather than continuous partnerships. Traits quantifying body size or life history are often nonlinearly related with divorce rate. These nonlinearities might (partly) be caused by nonlinear relationships of the same traits with mortality rate which is, in turn, linearly related with divorce rate. In contrast to what has been found previously, mortality rate appears as a key factor for divorce rate in our study. This makes sense, as the likelihood that a partner survives from one year to the next decreases with increasing mortality rate, diminishing the likely success of a partner that attempts to locate its previous partner. We also found taxonomic differences in divorce rate. Specifically, Anseriformes have an exceptionally low divorce rate, which, however, can be explained by the generally important determinants of divorce rate: They also have a lower mortality rate, lower degree of ornamentation and coloniality, and more continuous partnerships than members of other orders.