Assessing the energetic costs of nest building in birds

Nest building plays an important role in the life of birds and it is assumed to be costly for builders. Although data from estimates of the costs of flights or the outcomes of reproduction are mixed, that nest building is costly remains prevalent. To my knowledge, there have been no direct tests of the costs of nest building on building birds. In the first experiment I pilot-tested a method for tracking mass loss in laboratory-housed zebra finches Taeniopygia guttata and found that at end of nest building the weight of males had decreased. The birds did not lose weight over a similar period when they were not building. In the two experiments that followed I attempted to determine possible causes of the costs. In the first experiment half of the birds built their nest with material 3x heavier than the material provided to the other birds. In the second experiment, I attempted to manipulate the costs of flights during nest building by providing the nest box at ground level or hung on the cage wall. I expected males that built with heavier material, and birds that had to fly up to their nest, would lose more weight than the males which did not. However, although all builders lost around 5% of their body weight, neither manipulation caused birds to lose more weight. I also found no relationship between the amount of material and weight lost. Furthermore, the birds did not spend more time feeding across the experimental periods. These data appear to suggest that the measurable energetic costs experienced during nest building were not treated by the males as detrimental. This suggests that the costs of nest building may not be quite as high as implied in the literature, although this may not reflect the costs in the wild.