Here is a link to the cameras manual. //www.manualslib.com/manual/554976/Canon-Eos-620.html Just copy and paste it into your browsers address bar and it will take you to a downloadable copy of the manual.
ISO is the film speed or digital image sensor setting. F stops are the aperture settings. The smaller the number the larger the opening the more light the lens lets in. f/2.8 is fast, f/1.8 is faster, f/22 is slow. A lens with a faster maximum aperture say f/1.2 will be larger and heavier than a lens with a maximum aperture of f/1.8 and the faster the lens the more expensive it is. A f/1.8 85 mm Canon EF lens costs around $350, an EF 85 f/1.2 costs around $1900.
Would you possibly be interested in an older used Digital camera. The advantage to the digital camera is, no film needed therefore no developing costs. You just loaded the digital image files to your computer.

Thank you and you are very welcome. I'm also an animal person as is my wife. We currently have 4 indoor pets, 2 Coton De Tulear dogs, a mouse and a Cockatiel. We also have many, many free range pets that come to our bird feeders year round. One big advantage to the free range pets is they keep the local insect population in check.
Last year I came across a young BlueJay that was injured. It could barely fly and it had a laceration under one of its wings. We took the bird to the only person licensed in the county to care for wild animals. She set the broken wing and treated the cut and nursed the bird back to health. Eventually the bird was released back into the wild. I know it doesn't sound like a big deal but BlueJays are one of my favorite small wild birds and I don't like seeing any animal suffer.

It will be fine for doing portraits. Like I stated previously, be sure to use a remote shutter release, preferably wireless. The less you have attached to the camera that moves, the less likely you will get the nasty camera shake. One thing I do to increase stability on mine and reduce shake is to hang a sand bag loaded with pennies under the tripod. It works quite well...

OK, that's the original 100-300 5.6 push pull zoom Canon introduced in 1987. New it sold for around $520 and today they are fetching anywhere between $40 and $100 depending on condition. That lens weighs about a pound and a half coupled to the EOS 620 your holding just over 3 pounds of camera (with lens). Have you thought about an decent inexpensive tripod instead of a smaller lens?

OK, you mean EOS, not OES. I remember the 620, it was the big brother to the 650. It's kit lenses were the EF 35-70 and the nifty fifty (EF 50 1.8). I'm confused, you say you have a EF 100-400 then you say you have the 100-300. Which is it, the 100-400 is an off white L series lens. There are at least 3 versions of the 100-300 lens, all black in color and one is an L series lens. The L series lenses all have a red band painted around the front of the lens. Either lens would be good for back yard birding, far better than a 50mm lens. How about a small light weight zoom lens. I have a few older Canon lenses in my collection that may help you out. Let me know and I will take a look to see what's in the cabinet.

I have that tripod. It's a decent lightweight tripod. It came with the 3 way head but I often use a gimbal head on it. It is not some big hulk of a heavy duty tripod that can support a smaller version of the hale telescope. The heaviest lens I've mounted on that tripod is the Sigma 150-600 Sport attached to the matching Sigma 1.4X teleconverter attached to a Canon 5DIV without any problems. I've also used it to take long exposures of the night sky. It's best if you use a remote shutter release. It's a decent tripod for the money if you are looking for a good lightweight tripod.

I know what a Canon SX620 is but I have no idea what a Canon OES 620 lens is and I have a lot of Canon stuff.

There's also something exhilarating but frightening when 50 thousand of them take off simultaneously and fly directly over your head, circle around and land again. Be sure to have a hood on your lens and be wearing a hat.

We get a lot of them around here going south but the big gathering of the snow geese and tundra swans is in late winter when they are headed north. The Atlantic flyway passes over eastern PA. Last March there were over 80,000 snow geese and thousands of tundra swans, at Middle Creek wildlife management area in East central Pennsylvania, on one of the days I went to take pics.

Very nice pics...

I have the Nikkor 200-500 that I use on my D500 with excellent results. The closest equivalent lens I own is a Canon EF mount Sigma 150-600 Sport that I use on my 50 megapixel EOS 5DSR with pretty darn good results on my 13 by 19 Pixma Pro 10 printer.
Welcome to the forum...

Define Philly area. Are you talking Philadelphia County, Delaware County, Bucks County, Montgomery County, Chester County, Camden County, Burlington County, Gloucester County.

There's Allens Camera in Levittown, The Camera Shop in Bryn Mawr, Jacks Camera in Philadelphia, Jacks Camera in Exton to name a few.

Good choice. Unless you are a pixel peepers who is more concerned with statistical analysis and impressive numbers, you will not be sorry.
I usually use the EF 100-400L II with the EF 1.4X teleconverter on my 7DII when shooting handheld and the Sigma 150-600 Sport with matching 1.4X teleconverter when mounted on a tripod.

You talking 7D or 7DII?
There's really no campairing the much newer D500 to the much older 7D.
I own a 7DII and a D500. They are both excellent cameras for shooting birds in flight and glass is also a major factor.
My opinion is based on my own experiences using both cameras.
The 7DII has an amazing AF system once you figure out how to set it up to your liking. You will get excellent results with the default settings but even better if you fine tune it.
The AF system on the D500 is also second to none but it may be easier to fine tune the Canon.
Under the right conditions both cameras will crank out up to 10 frames per second.
I believe the Canon is easier to operate and much easier to learn.
As far as the end results, I'm not going to go into the nonsensical nitpicking dissertation about dynamic range and hi ISO because in reality it means little. You may make lots of adjustments to both cameras to taylor the images or just do what I do and shoot raw, which you're going to have to in order to get 10 fps.
They are both excellent cameras but the D500 does have slightly newer technology.
I personally feel the Canon may be a bit sturdier in the build department but then the D500 is very well built also.
As I already said, I own both and I use both cameras. I carry both into the field when I can.
My personal preference when I'm carrying only one camera is dictated mainly by location and glass and that would be the D500. My recommendation to someone without my level of experience would be the 7DII. It is just easier to learn and use. (Nikonistas will disagree)

Hopefully he hasn't been scared off...