You've definitely got a lot of good stuff here, along with a fair bit of room for growth and a number of issues I'll be pointing out. I do think that overall you're demonstrating that you're developing quite nicely as far as employing and understanding the principles of construction and your general spatial reasoning go. It's clear that in most cases you're employing the principles covered in the lesson, though just how and where you apply them can use some tweaking.

To start, your organic intersections are very well done. You're developing a really strong impression of how these forms interact with one another in three dimensions, not just as shapes on a flat page.

Moving onto your birds, you're mindful in a number of places of establishing the same kind of relationships between your 3D forms - for example, defining just where and how the neck connects to the torso by use of a contour ellipse on the goose. I did notice however that the feet/legs here come out quite rigid, and that is not helped by the density of contour lines you're adding.

There are different ways in which contour lines can be employed, with some being more effective than others. You've got the ones we introduced the concept with back in lesson 2 - simple contour lines sitting along the surface of an existing form, and only on that form. We see these through the midsection of your goose's legs. They're certainly helpful in certain areas, but their effectiveness is quite limited. No contour line will do much to help a weakly constructed silhouette, but they can certainly help provide visual cues to explain to the viewer how a form that already feels 3D bends and turns through 3D space.

Then you've got the contour lines that define the relationships between two forms, as you've done between the neck and torso of the goose. These do the same thing as the previous kind, and more - specifically, by creating a relationship between forms in 3D space, they establish a recursive link where if one form feels 3D, then the other one too will feel 3D. In turn, the first must therefore also be three dimensional, and on and on. This kind of cycling connection becomes very strong and believable, and once in place, any further contour lines are unlikely to be anything more than superfluous. To that end, more contour lines is not necessarily better - they suffer from diminishing returns, and we'll get less out of each next one we add. It's important therefore to weigh exactly what it is you want to get out of any mark you're putting down (contour line or otherwise), and to think about whether or not it is the best line for the job you want it to do.

Now, you use the sausage method a great deal throughout this lesson, and these early drawings not employing it (and therefore ending up with pretty stiff legs) is not something I'll dwell on. As far as the feet go however, I did feel that the feet on the bottom-right bird (beside the goose) were quite well done.

On the bird with spread wings, you mentioned that you should have constructed the feathers individually - this is not correct. The feathers themselves are a texture, laid out along the surface of the simpler wing structure underneath, and as such they should not be constructed at all. Don't treat the simpler form you've drawn for the wing as though it is a flat shape that must be filled in with feathers. Instead, as shown here, treat it like a solid structure having feathers attached to its surface. Also, to the point of texture itself, you'll definitely want to revisit Lesson 2's newly rewritten texture section, as I explain the differences between trying to explicitly draw each and every instance of a textural form, versus implying their presence by drawing only the shadows of a few.

Moving onto your ferret, you did make a note about choosing not to draw the sagging belly of this one, and that's entirely fine. After all, this one in particular doesn't have a sagging belly - that principle of integrating it into the initial sausage is just what you'd do in most situations, specifically the ones where there is in fact a belly sag. That said, you need to spend more time observing your reference - that's not just for this particular ferret, but in general, I find that you jump into putting your forms down a little too quickly, and as such don't always put down the best reflection of what you see in your reference. Looking at the ferret reference image, we can see that as shown here the ribcage is a lot longer and narrower, and being that the ribcage is bone (and therefore not flexible), most of the bend is biased towards the back. We can also see that the left forepaw is in an entirely different position than it would be in the reference. To that point, keep an eye on the feet and where they touch the ground - if you don't maintain a sense of where the actual ground is, then it's easy to end up with several different ground-levels for each foot/paw, which can stand out quite a bit in a drawing.

Jumping on through, I can see that you're making better use of techniques like additional masses (and wrapping them nicely around the existing construction), using the sausage method, and even doing a better job of maintaining a consistent ground level. I do feel that in some of the feet (like your rhinos), the way the feet touch the ground feel ratherlight, rather than giving the impression of the sheer mass and weight of the beast. When drawing these creatures, think about just how much mass you're dealing with, and how all the components weigh upon one another.

The last thing I wanted to touch upon is in regards to your additional masses, which are by and large well drawn on their own. One thing that can help your use of this technique is to get used to integrating them into one another - that is, treating them more like a puzzle where the mass of the shoulder muscles might fit tightly against the back muscle masses, and so on. In a number of your drawings, you'll wrap a somewhat free-standing form around the main torso sausage, but it'll still feel somewhat like an island in the sea. We can also see how towards the end, your snapping turtle/rhino hybrid has great big protrusions along its back, but they don't actually feel grounded in the rest of the construction. They're definitely three dimensional, but the feel somewhat more like a series of fancy hats that could slip off at any time.

The same concept of being a puzzle with many interlocking pieces goes for the head constructions as well. You'll find that the eye socket, the brow ridge, the cheek bone, the muzzle, etc. all fit very snugly together, rather than as independent elements existing on their own. You don't want the eye socket to feel independent - it's more that you want it to feel like the result of all the forms around it, since it is a cavity rather than a form of its own.

All in all, you're making good progress, but you do have a number of things to work on as you continue to move forwards. You are definitely good to move onto the next step however, so I'll mark this lesson as complete.