I have been in this exact spot and honestly its frustrating how much everything costs just to take a clear photo of a bird. I bought the Canon RF 100-400mm f/5.6-8 IS USM thinking it would be the perfect solution for my R6 but unfortunately the f/8 aperture at the long end is a real problem. If youre under some trees at the park or its slightly cloudy, your ISO just spikes and the grain is worse than I expected. Its light, sure, but the image quality drops off way too fast in anything but bright noon sun. I also tried adapting the Canon EF 70-300mm f/4-5.6 IS II USM using the Canon Mount Adapter EF-EOS R. Its definitely a bit more reliable in slightly dimmer light because of that f/5.6, but the bulk is annoying. Adding the adapter makes the whole setup feel front-heavy and unbalanced on the R6 body. Plus, you lose 100mm of reach, which really matters for birds. If you need it by Friday, your best bet is probably finding a used Sigma 150-600mm f/5-6.3 DG OS HSM Contemporary for Canon EF if you can find a deal, but even then, the autofocus isnt as snappy as native glass. Honestly, none of these options are perfect and Ive had issues with all of them being either too dark or too heavy. If I had to pick, Id go with the RF 100-400 just for the native AF speed, even though the low light performance is disappointing... just keep your expectations low if the sun goes behind a cloud.