Handa Island is only accessible during the seabird nesting season, roughly April to early September. The ferry from Tarbet does not run outside these months.
May and June are the peak months for seabirds. The cliffs are packed with guillemots, razorbills, and kittiwakes at maximum density, and puffins are most active around their burrow entrances. Great skuas have chicks and are at their most aggressive. The days are very long this far north — in late June, it barely gets dark.
July is still excellent for birds and tends to have slightly warmer weather. The great skua chicks will be fledging by late July and adult behaviour starts to calm.
August and early September see the birds departing rapidly — by mid-August the cliffs thin noticeably. But the weather can be more settled than in early summer, and the walking is still fine.
The northwest Highlands are notoriously wet and windy regardless of season. Come prepared for rain and strong winds; on rough days the ferry may not run at all. The best weather windows tend to be brief and hard to predict, so flexibility in your itinerary helps.