Its just there. BUT, when the birds leaves nest, young are available for predation. Foraging strategy may not be the best foraging strategy, but the best compromise given predation risk. Phenotypes as compromises underscores the importance of constraints . These are artificial selection examples but can apply to natural selection as well.
Constraints can be phylogenetic or developmental.
It might be adaptive for certain mammals to be able to breath under water, but their phylogenetic history and developmental program constrains them from evolving gills. An informative means of analyzing adaptations is through the comparative approach . Problem is not how it comes down, but why it takes so long to do so. Gliding must be a result of natural selection. Parsimony demands that we recognize adaptation at the level necessitated by the facts and no higher.