Rockablog: Variety is the spice of life!

"So what do you actually do out there?" - Like I said before, this is one of the most common questions we get about working on Rockabill- the term "warden" can seem a bit vague! At the start of the season it was all about habitat management and getting the island ready to host as many nesting Terns as possible. Since the first eggs were laid we've been doing twice-daily nest-checks of a number of sub-sections of the island. We'll be checking these every morning and evening between now and when we leave at the end of the summer, so we'll know every little detail about the timing and number of eggs laid, the hatching success and growth of the chicks, and ultimately whether those chicks fledged and the nests were successful.

At this part of the season we check our study areas for new eggs, twice a day - some are in nestboxes, others outside them.

Spending a couple of hours each day checking nests of thousands of Tern species, you see a lot of eggs!! (understatement alert!) This period is arguably less interesting/fun than when we have chicks running around the place, but the eggs are interesting too. Common Terns lay eggs on patches of bare ground, and so their eggs follow the same basic pattern of most ground-nesting birds eggs - they're coloured to match the ground as closely as possible, with a series of spots, streaks and blotches that help break up the outline of the egg against the ground - so any aerial predators (Gulls, Crows) will have difficulty seeing them and hopefully pass them by. If you take a plain coloured/patterned chicken egg and put it on bare soil, gravel or shingle, you'd have no trouble seeing it from a distance, and knowing that it was an egg too. The Common Tern eggs you'll see below are usually much harder to spot until you're right up next to them.

In a colony like this where you have >2,000 pairs of Commons nesting there's considerable variation in colour and pattern style of eggs. Commons out here nest on bare rock, soil, on top of vegetation, on top of dead vegetation etc. - all of which are different colours, and so the population here needs to be able to cope with that. Essentially, it's a numbers game, as evolution so often is! There's huge variety in both the eggs they lay, and the substrate they lay on, so there'll never be a year where all of the eggs are easily found by predators - at least some (usually most) will successfully last long enough to hatch chicks, who themselves are well camouflaged too. A good proportion of chicks survive, and come back in future years to breed, they lay a large number of eggs with various camouflaged patterns, and the cycle continues!

So here's a sample of the variety in Common Tern eggs that has served them so well over the years:

(All pictures taken under NPWS license - never take pictures of nests or eggs without one!!)

Colours:

Most of our Common Tern eggs are this green-y colour, with brown/black spots and blotches.

....some are a lighter shade, with spots more concentrated at the bottom end.....

...there are also plenty of brown eggs like these. Note that these eggs are more pointed at the top end than those pictured above!

We also get a few like this - bright blue!

Patterns:

Some are spotty all over, others more blotchy.....

.....some have the spots/blotches concentrated at one end....

....or occassionally across the middle......

Shape:

Most Common Tern eggs are a 'normal' egg shape, but there's always a few exceptions....

Occasionally we see some that are more pointed than average (like Roseate Tern eggs), and some shorter and more rounded.

Just Plain Weird.....:

In a colony of thousands of nests, and even more eggs, we see a few that havn't formed right and are generally infertile. In many cases the adults realise this soon after laying and roll them out of the nest, so that they can concentrate their efforts on their other eggs.