We arrived around 11 on a Saturday and it wasn't bad and the animals were still very interested. I would say the earlier the better that way the animals are hungry and it's not too hot for them either. Because by the time we were half way through the animals were starting to just want to hang out in the shade.
It used to be that you had to be there by say 8 45 9 o'clock but the last couple of times I've went I've arrived around 9 30-9 45 and been very close to the front of the line it seemed to me as the day progressed the line got longer so my recommendation would be go earlier in the morning plus it's a better drive and the animals are hungrier and are more likely to come up to your car as the day progresses they've ate from everyone coming through and some animals are less likely to come up and visit
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