Birds are fairly simple shapes to model in miniature. You can make these seagulls from a range of materials, in several scales. The seagulls shown here are 1:12 dollhouse scale made from air dry clay. You can also use two part epoxy putty or polymer clay. The techniques are very similar for all the modelling materials. All the different materials have some advantages.
Lightweight Air Dry Clay - I used Delight Clay - which has the advantage of being very light weight but needs a protective painted coating. Birds made with air dry clay can nest in branches or hang on tiny trees without pulling the tree branch down. The best tips for light weight air dry clay are to work on a piece of glass to keep the clay clean, only remove very small amounts of clay from your carefully sealed stash for each stage of the build, and to use a soft paint brush to blend the joins between sections. You need very small amounts of this clay. Keep the open package in a resealable bag with the air removed and only take out tiny amounts for each step.
Two Part Epoxy Putty - Air dries or cures to a very hard finish, can be sanded and painted, can have sections added on as you build. This makes birds which are heavier than the ones made from air dry clay, but they are less prone to breakage. As with the air dry clay, only mix small amounts of putty for each step. Blend two part epoxy putty sections together with a bit of water on the surface using modelling tools or a dampened finger.
Polymer Clay - Easy to sculpt and blend, colors should be mixed and added for the various sections, rather than painting the finished model. Polymer clay is heavy enough to bend small branches, so you may need sturdy trees for polymer clay birds.
It is far easier to model animals if you have a photo in front of you. Check out the Birding About.com site for photos of your chosen gull. The birding site features profiles of birds including their photos, such as the Ring-Billed Gull.