But statistically speaking, nearly every factory-farmed land animal is a bird.
So why do advocates seem to ignore chickens, relatively speaking, even going so far as to publicize studies that actually promote eating chickens (see paragraph seven here)? There are many factors, including a natural affinity for mammals and the relative clarity of the health and environmental problems of red meat. Furthermore, it is easy -- almost irresistible -- to grasp onto anything and everything that seems to be disparaging to animal products; it is certainly easier to do that then think strategically about our advocacy such that we actually prevent as much suffering as possible.
Related to this is basic human nature -- we're generally outraged and focused on what is currently in front of us. This article -- Why We Haven’t Seen Inside a Broiler Chicken Factory Farm in a Decade -- goes into why the brutality chickens endure gets so little coverage.
Given all this, if we really care about helping animals rather than praising veganism, we must be constantly aware of the full consequences of what we say and what we promote.