Scaling up animals just doesn't work. You'll need to adjust proportions as things change. Notice that an elephant has very thick, stocky legs that go straight up and down, while insects have spindly legs that are off to the side. You can make big things fly, they just can't look like the little things.
This is due to a phenomena called the "Square-Cube Law." If you double something in linear scale, you quadruple it in area scale, and you octuple it in volume scale. Take a cube 1cm on the edges. It has side areas of 1cm2 (6cm2 total) and a volume of 1cm3.
If you double it in the linear scale, the edge lengths are now 2cm. This means each face is 4cm2 (24cm2 total) and the volume is 8cm3. The area went up by a factor of 4 (2*2) while the volume went up by a factor of 8 (2*2*2). Mass will scale with volume, unless you change materials to have a lower density.
That is the math behind the square cube law. Now, what does this mean for scaled-up birds?
Well, first off, you'll notice that mass is doubled per area when length is doubled. This means the pressure on anything supporting the object, like a wing or leg, scales with linear measures (aka length). This means that scaling up without adjusting proportions wouldn't work, since the increased force requirements to stay in the air are not countered by increased wing area.
Also, here's a thing about muscles - their force scales with cross-sectional area, not volume. So your scaled-up birds will have scaled-down force, relative to the masses of their body parts. Like wings. Not only would they not have enough force when flapping at their normal rate, but their muscles couldn't even handle flapping at that increased rate because of the increased mass.
The strength (amount of force they can take) of bones is also dependent on the cross-sectional area, so it'll scale down as you scale something up. If you scale things up too much, they'll collapse.
This is why big animals are so stocky: they need to have much more muscle, relatively speaking, than smaller animals because they need stronger bones and muscles, and the only way to do that is through making them bigger. (Well, there are other ways, but they come with their own tradeoffs.)