Further thoughts--scans of bird photos with a lot of open sky shot on 35mm E-6 are going to be tough. I went through three submissions of one slide trying to figure out what they were objecting to, and it turns out that tiny dark circles that look to me like dye clumps viewed at 100% are classified as "dust, scratches or other blemishes," which means lots of spotting, and to get a scan that's large enough to look sharp when downsized to 48Mb uncompressed 8-bit, it's hard to avoid turning up such "blemishes." For this purpose, something like an out of focus bird in the distance against a blue sky is a "blemish" and should be spotted out.
I also discovered that Alamy uses QC ranking, which will affect turnaround time on submissions. If you have repeated rejections, your QC rank drops, and instead of reviewing your submission within 37 hours, it may take five business days, and if you submit anything else in the meantime and the one problematic image is rejected, they'll reject the whole batch so you can check it again and resubmit, which requires reuploading.
So all-in-all, it doesn't look so good for 35mm film for this purpose. My current tally on Alamy: film-1, digital-85.
Once I make sure my QC ranking is solid (turnaround has been quick lately, so I don't want to risk it just yet) and when I have a lull, I'll try scanning some medium and large format images and see if they do better.