The Azores bullfinch Pyrrhula murina is an endangered endemic bird species restricted to the east of the island of São Miguel, Azores. In the past decades, the native habitat of the bird was gradually invaded by exotic plants and less than 30% of its range is currently occupied by native vegetation. The late winter has been suggested as a critical period for bird survival as a consequence of food shortage. We developed a model to understand how yearly and seasonal variations on the type and abundance of food items affect the winter energetic budget of the Azores bullfinch. The model is at steady-state as it simulates the winter-early spring period, when Azores bullfinches attain maximum weight, with non-significant fluctuations. The costs of basal metabolic rate (BMR), thermoregulation and other activities are included in the model. The potential energy available to Azores bullfinch between January and April in three different years was estimated based on day length, diet, pecking rates on different food items, their energetic content and assimilation efficiency. Simulations were run with probability distributions of the estimated available energy and the energy left for activities of the Azores bullfinch. Results indicate that between January and April, the bird's energetic requirement for BMR and thermoregulation range between 118 and 123kJd −1 , peaking in March due to thermoregulation constraints. If the bird's energy with activities represent 36% of the energy spent with BMR plus thermoregulation, the estimated daily energy requirements of the Azores bullfinch fall within 160–167kJd −1 . Results indicate that January and February are months when the bird may experience some kind of energy constraint due to low density of energy-rich items (<200kJfeedingarea −1 ). However, from March onwards and particularly in warmer years, the Azores bullfinch will have plenty energy for BMR, thermoregulation and activities, mostly due to the increasing consumption of Ilex perado flower buds. In relation to altitude variations, simulations indicate that thermoregulation costs increase 8 and 13% at mid- and high-altitude, respectively, compared to low-altitude. In terms of habitat management, we can infer that removing the exotic tree Clethra arborea should be undertaken with caution because, during winter C. arborea seeds constitute an important food item to the bird. Alternatively if C. arborea is replaced by other energetically rich items, particularly at lower altitudes, the Azores bullfinch will potentially benefit from it.
Financed by the National Centre for Research and Development under grant No. SP/I/1/77065/10 by the strategic scientific research and experimental development program:
SYNAT - “Interdisciplinary System for Interactive Scientific and Scientific-Technical Information”.