Possibly a plum, but the pics are too small to tell for sure. Can you get some larger close-ups of the foliage and fruit?

Resin

and the bark.

Here are some more pics. I tried to make them bigger, I hope it works. This year the tree did not bear any fruit. When it did, the fruit' was the size of tapioca, no bigger and jet black.

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Hope these are more helpful. Marg

Looks more like a buckthorn in these pics!

Resin

Looks like Common Buckthorn - Rhamnus cathartica to me. Native to Europe, dioecious, extremely invasive species in North America. It is prohibited or classified as a noxious weed in at least 6 US states, as well as Ontario (they call it "European Buckthorn").

The birds do like the berries, however they have a laxative effect(hence "cathartica"), so they are pooped out quickly and this is how the species spreads.

Here is a link that might be useful: Ontario Weeds: European buckthorn

I did a search for Buckthorn trees and found good pics. That is definitely what it is. Thanks so much Resin and terrene. Mine doesn't seem to be sending out suckers, perhaps because we keep the grass around it cut. I would like to get another one because I like the shape of the tree. Thanks again. Marg

Buckthorns are an invasive species in that region. Doubtful you will find any for sale and even if you do, you would most likely want to avoid buying from that nursery as selling invasives in unethical.

Here is a link that might be useful: more info

Your tree does indeed have an uncommon tree-like form for a Buckthorn. But if it were me - that sucker would be chopped down! Both the common and Glossy Buckthorns are terribly invasive and I've been removing them from the yard for years. There are many other trees that aren't so weedy and are a lot more attractive than the Buckthorns IMO.

I would like to get another one because I like the shape of the tree. Thanks again. Marg

Please don't.

I must say I did a drive-by and the form confused me - I see the forked veination but I can't say I've seen that form (albeit not too many out here). IME buckthorn burns fairly well and should be a nice addition to the woodpile.

Dan

Common buckthorn spreads by seeds, not suckers. Birds eat the berries and get a nasty case of diarrhea and deposit the seeds far and wide. The buckthorn shrubs then spread by dropped seeds where they create a dense thicket where nothing grows but more buckthorn. The shade in a buckthorn thicket is so dense that hardly any plants can grow. Buckthorn is especially invasive in woodlands, turning large areas in to buckthorn thickets where no new trees grow and where the ground underneath becomes barren. The plant is mildly toxic to nothing eats eat and the birds can't digest the berries, so it is essentially useless to wildlife. Where I live there are acres and acres of of forest where the is nothing but buckthorn growing underneath the trees. I spend a lot of time eradicating this pest plant and would highly recommend getting rid of it! A big one with lots of berries like what you have can make thousands of new plants.
Buckthorn also is a host plant for crop diseases and pests like oat crown fungus and soybean aphids.

Here is a link that might be useful: Buckthorn

Thanks everyone for the info about Buckthorn. I think all the farmers around here have done away with trees or shrubs in the fence row. I haven't seen another Buckthorn around here nor have I seen any seedlings. What I found strange about mine was that it has no thorns. I thought they all did. After seeing pictures of Buckthorns, I thought maybe this one had been pruned to grow into that shape so I went out and checked to look for signs of pruning but didn't see any. Do you think that is what they did to it? Marg

I have cut 1000's of Buckthorns and I can say that their shape and thorns are quite variable. I have seen many that are single trunked and tree-like. Also, they do not always have thorns. I have noticed that older buckthorns often do not have many thorns, and it seems they the thorns are more prevalent on long vigorous shoots on young plants.
You particular Buckthorn looks like an older one, so that might be why it doesn't have many thorns.

The Common Buckthorn (Rhamnus cathartica) growing around here tends to sucker, although it has a more tree-like form than Glossy buckthorn (Frangula alnus), which is very shrubby and multi-stemmed, and also more invasive.

Here is a pic of a trunk of an older Common Buckthorn growing on my property, you can see the suckers. This one is growing in a wetland, so I tried girdling it to kill it, since I don't have any herbicides that are approved for wetlands. Girdling probably won't work though.

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None of the hundreds of Buckthorns (of both species) that I've cut or observed seem to have thorns, from seedlings to 40 year old specimens?

1 more buckthorn won't make any difference. I lived in MN where they have naturalized and although people try to eradicate them, they will always be around. There are non-fruiting forms available too including 'Fineline'.

BTW, so it is "unethical" to sell invasives? I challenge you to think outside the box a little on native vs. non-natives. Aren't we humans vectors of plants just like the birds that drop seeds on a newly developed sandbar somewhere? Maybe this should be another post, I am just getting sick of the invasives dogma.

Salicaceae,

I'm gonna stay out of the overall conversation on this, but just wanted to comment on your post above. Natives vs. non-natives is an entirely different subject than invasives vs. non-invasives! Maybe you know this and your comment above just came across wrong, but, if not, you might want to take another look at what these things really mean.

You are correct that one more individual invasive plant probably won't make a huge difference, in the grade scheme of things, if introduced into an area already heavily populated with that species. However, that doesn't make it any more ethical. It's like throwing garbage out of your car window while driving down an already heavily littered highway (except maybe a little worse, since this type of litter multiplies). It may not make a noticeable difference, but that doesn't mean it's a responsible or an ethical thing to do.

Introducing non-native non-invasives is cool; it makes gardening more interesting. But, I can't think of any good reason to introduce invasive plants; there's too many awesome well behaved choice available.