Which are the ancestors of our birds?

The fossil record ofnarly bird is very incomplete, because their bones are so fragile and don't fossilize well. Since feathers don't fossilize very well either, no one realy knows what these extinct birds lokked like. Modern reconstructions are, therefore,  based on educated guesses.
Now, let us look at some early birds other than Arthaeopteryx.
Diatryma
Diqtryma were humansized, heavily built, flightless birds that date from the Tertiary to the early Eocene period 38 million to twp millions years ago. 
They were about 2.1m tall, had thick legs with clawed feet, tiny wings, and huge, powerful,  hooked beaks on a big head. They were probably carnivorous and perhaps ghe top predators in what is now Western Europe and North America,  in an environment that was a tree covered plain.  They nested on the ground.
Diatryma gigantea was named by paleontologist E.D.Cope in 1876 from New Mexico fossil.
Eoalulavis
Eoalulavis was the earliest bird that was good at flying,  even at low speeds. This extra flight control is obtained from a tuft of feathers on the thumb called the alula it also helps in take offs and landings. Fossils of this bird have been found in Spain.
Hesperornis
Hesperornis (meaning"western bird") was an early, flightless bird that lived during the late Cretaceous period. This diving bird was about 1 meter long and had webbed feet, a long, toothed beak, and strong legs. Although it couldn't fly, Hesperornis was probably a strong swimmer and is likely to have lived near costlines and ate fish. Fossils of this bird have been founded in North America.
Mononyks
Mononyks (meaning "single claw") was a small insect-eater from the late Cretaceous period,  about 72 billion years ago.  Mononyks had short arms with one long, thick clawed finger on each hand, hence its name. It was lightly built, had long, thin legs, and a long tail. Mononyks was roughly 70 cm long. A fossil was found in?Mongolia in 1923.