Hi Scott,
There are some good points and some not too good ones here. I'm posting this back to you rotated and corrected, I believe, to tonally more resemble your original. It's what I'm looking at as I critique.
First, I have to say that the critique is based on how I would approach this drawing, but I recognise that it's not necessarily your way of doing things. For example, I'd be concentrating on detail and texture, where you might be aiming for a more general tonal representation of form. With that in mind, and starting top left...
The vertical boards of the old door have a lovely feeling of old weathered wood while still retaining their secondary position in the study. You've varied the width of the gap between the boards too - failing to do that is a common fault and results in the appearance of a drawn line rather than a gap. It's very unlikely two old warped boards would mirror each others edges.
The dark interior of the henhouse is sufficiently dark to give depth to the drawing and to simplify the drawing of Robbie, as you can make good use of edge highlighting to separate him fro the background. Do be aware though, that his rear end is deep inside the house. If you make his rear too light you will lose any sense of recession. The same applies to his legs and paws, where you would logically expect his rear paws to be more in the shade than his front ones.
Viewing your drawing at full scale, it would appear that you have shaded Robbie to represent his three-dimensional form. It works quite well. Here I would differ in my approach, as I would recreate his hair and use the darks and highlights to suggest that form. Your Robbie is rather devoid of any sense of hair texture. His screen-left leg is looking a bit wonky

) and don't be too concerned about his rather stark eye. He has a good dense pupil and sufficient white above it to create a bright white catchlight. All that is required is the toning down of his "white" iris, avoiding the highlight. A technique that works for me is to shade the iris slightly too dark and then to gently lighten some areas with Blu-Tack until a true sense of a rounded eyeball emerges. If the highlight is top right, refraction within the eyeball will lighten the bottom left quadrant. It's in that area that I find Blu-Tack very helpful, as I can progressively remove and replace graphite until the correct and subtle secondary highlight appears.
His face is looking a bit flat, but I don't think that will harm your drawing. I'm used to drawing dogs, so I know where the orbital ridge, his cheekbone and other features will appear, but they're not essential. It all depends on how great a sense of realism you want to achieve.
I noticed in your black areas that the tooth of your paper is causing white dots to appear. If you want to achieve a solid black or dark tone, try burnishing with a sharp HB - that should fill those holes and smooth out the softer trade.
The edges of the boards on the side of the henhouse possess a good sense of realism. And I'm glad to see you've retained the groove in the bottom board. I like little details like that - it's obviously a length of tongue-and-groove floorboard that someone has repaired the house with at some time. Not me, incidentally - the house was past repair so after I photographed it I regretfully had to push it onto a bonfire with my tractor.
The lightness of the boards of the wall should make drawing the hen easier - if you use relatively dark tones for her.
The slightly grainy texture of your shading of the wheel works well to suggest a lightly rusted surface. The ellipse is still not quite correct, but it's not far out.
Just continue as you are doing. Don't rush it - take it one small area at a rime, forget it's arising from a reference photo, and try to imagine that area in your mind. Ultimately, drawing from your mind will always inject a reality that copying what you see in a reference will never achieve.
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My plan was to go back and work on a lot of perspective and shading exercises and maybe apply that to some still life work to work on texture. What would you suggest?
I think such exercises will help you a lot. Try drawing something in perspective without a reference, choose a lighting direction and then shade it accordingly - much as you did with the house exercise. To succeed, you have to imagine the object in three-dimensions before you can work out where the light hits it and which areas are in shade. And, a little more advanced, which areas might be receiving light reflected from other planes of the object. You truly cannot draw successfully what you do not fully understand. That's why I keep plugging on about drawing from the mind

As you begin to be happy with the results, begin to introduce textures into your exercises. And finally, think the other way around - use those textures and their lighting to describe the three-dimensional form; the form that you previously suggested with your initial shading. Which brings me back to Robbie, where I would draw his hair and use its highlights to describe his three-dimensional form.