Common Terns seem to be obsessed with one thing at this time of the year, an activity that takes up nearly all of their time and energy and gives them little chance to do anything else. They’re almost constantly out catching fish, dancing with them in their beaks and presenting them as food offerings to each other. But what’s this fixation all about and why do they do it? There is a simple answer behind it, it’s all about mating. But when you take a closer look at this behaviour you realise just how complex it actually is.
This tern and fish relationship starts in May (on our shores at least) with males using them as a token gift in their courtship feeding. This behaviour has a variety of purposes with Common Terns, playing an important role in finding a suitable mate, helping females to form their eggs and then providing them with energy to incubate the eggs.
The courtship ritual kicks off with the males performing either a so called low flight or high flight, or sometimes both, above the colony. These flights trigger females to fly up in pursuit of the male, although no actual fish presentation will occur during these flights. Low flights usually involve several females pursuing a single male whilst high flights are much more exclusive and will be a one on one pursuit. Many females may beg for fish from a single male, especially during low flights, but it’s not until they’re back on the ground that the male will decide who the lucky female is going to be.
It’s back on level ground when all sorts of tricks can be played and things start getting complicated. Males will sometimes pretend to be females and beg for fish from other males, not actually eating their prize but instead using it to help attract their own female. The females, not wanting to be outdone, can be just as devious. With their first egg fertilised by an initial partner and then laid, they will often take an additional fish offering from a neighbouring male whilst their original mate is out fishing. This inevitably leads to copulation and ultimately a brood of juveniles with different fathers.
All this foul play aside, when a pair of terns finally decide to devote their time to each other the male will expend all his energy on catching fish for his mate. He will continuously present her with fish for up to ten days before hatching (just one of the behaviours you can see in the video below), with little time being set aside for anything else. But the hard work doesn’t stop there, when the eggs have hatched the male must continue to provide for his family. The male will appear with fish around 15 times each day, and considering only one in three to five dives is successful, this equates to a lot of hard work. After the first four days the female will join in too and their combined effort means the chicks are brought a meal up to 35 times a day. But even within this period of investment things aren’t as straightforward as they may seem. Desperate, or perhaps just lazy, parents may fall into the habit of robbing their neighbours of these hard-won fish, often from the mouth of chicks just as they’re about to swallow them.
Here at Montrose Basin we’re in the lucky position to be able to view this action as it unfolds on our specially constructed tern raft. This week has seen around 40 Common Terns counted on the raft, and although they seem to have temporarily deserted, they are still on the reserve and we’re hopeful of them returning soon. So next time you’re in the Visitor Centre watching the Common Terns coming and going take a moment to realise just how complicated their ‘fishy’ lives can get.
Visitor Centre Assistant Manager.
Source; Couzens, D (2014) Dominic Couzens on Common Terns, Bird Watching Magazine May 2014.