The evolution of terrestrial birds

Flight, the defining characteristic of birds, is typically only seen as an advantageous strength and I suspect this is related to human bias given it took our race thousands of years to achieve this feat through technology. However,  there are several species of birds that have appeared to sacrifice their ability to fly, yet they have retained similar vestiges in the from of their underdeveloped wings. This is suggestive that these birds did not struggle to every evolve flight, but instead, evolved flightlessness. Again, human bias may lead one to ask why this would ever occur as flying appears to be such a strong advantage. While true, flight is also associated with several costs including the high relegation of energy during wing development, the morphological trade offs necessary to have a body that can efficiently flight and the increased metabolic needs to sustain flight. In other words, for flight to be an advantage, the species in question must be able to achieve a net positive after factoring the massive expenses that are necessary.  This most often occurs on island settings with either low levels of predation or competition as well as in environments with open apex predatory niches. These roles require different developmental priorities to fully exploit resources they allow one to access.