yo, congrats on the a1! honestly its a total beast. if you want that 30fps burst, you basically have to stay with sony glass cuz third-party lenses like the Sigma 150-600mm f/5-6.3 DG DN OS Sports are hard-capped at 15fps. i mean, why waste that sensor's potential?? here is what i recommend based on my experience:

• for birding, reach is king. the Sony FE 200-600mm f/5.6-6.3 G OSS is the clear winner. the internal zoom is actually a massive advantage because it keeps the center of gravity stable when youre tracking fast movement.
• the Sony FE 100-400mm f/4.5-5.6 GM OSS is sharper but you'll almost always need the Sony FE 1.4x Teleconverter for wildlife. once you add that glass, you lose a stop of light which kinda hurts for field sports at sunset. basically, go for the 200-600 if you prioritize birds. cheers!

Totally agree with the point about staying native. If you have already invested in an a1, you really gotta stick with Sony glass to get your money's worth out of that autofocus system. Honestly, going with any of their high-end telephoto zooms is basically the only way to go if you want those high frame rates without the glitchy performance you sometimes get with other brands. From a more budget-friendly perspective, I would definitely suggest looking at the used market or even some open-box deals. These lenses are usually built like tanks, so buying a pre-owned one can save you a ton of cash while still giving you that top-tier performance. Just make sure you are getting the official Sony stuff because the compatibility is just way smoother and you wont be capped on your speed from what I have seen. A few practical things to keep in mind:

• check local camera shops for trade-ins or used stock
• maybe rent a couple of different types first to see which weight you can actually carry all day
• dont forget to budget for a decent gimbal head or strap because those things get heavy fast Basically, just stay in the Sony ecosystem and you cant really go wrong. It is definitely more expensive upfront, but it is better than buying twice when you realize the cheaper brands just cant keep up with what that sensor is trying to do.

In my experience shooting sports and wildlife over the years, you really cant compromise on native glass for a body this fast. Ive tried many setups and the Sony FE 200-600mm f/5.6-6.3 G OSS is the more reliable choice for what you're doing. Its actually the more cost-effective route too because 400mm is just never enough for birds. If you go with the Sony FE 100-400mm f/4.5-5.6 GM OSS, you're basically forced to buy a Sony FE 1.4x Teleconverter eventually to get the reach. That combo ends up costing way more and doesnt feel as solid in the hand. The internal zoom on the 200-600 is much safer for long-term use... it doesnt suck in dust like the telescoping barrels do. For field sports, the 100-400 is maybe a bit punchier, but for birding, the 200-600 is the tool you want. Stick to the native lenses tho, dont even look at Sigma or Tamron if you want that 30fps. Its just not worth the risk of missing the action.