People might be surprised, but this eating their own chicks has clear evolutionary advantages. Seagulls have no pockets or ability to create and carry bags or backpacks in able to carry provisions. Thus rather than spend time, energy and points evolving those traits, they instead opted for another solution. The seagulls get pregnant and thus can carry more nutrition during a long journey. After challenging and long travel, at the destination, or even midsea provided they find a suitable piece of floating styrofoam or driftwood, they give birth to a chick and ingest their essence. This also has the extra advantage of being more aerodynamic compared to other solutions.