Sunday, 13 September 2015

Common Terns on Rockabill, 2015

Our penultimate species update is about our breeding Common Terns. We have more Common Terns on Rockabill than all of the other species put together, which is somewhat inconvenient for the wardens when you realise that they're the species that attacks us day in, day out! That being said, Common Terns are full of character with their varying levels of cheekiness and aggression and so we grow quite fond of them over the course of the season - a real love/hate relationship! That same aggression also benefits the nesting Roseate Terns all over the island, as the Commons do all of the hard work when it comes to scaring off predators of various sorts. In the 27 years of the Rockabill Tern conservation project the numbers of Common Terns have gone from a few hundred pairs to more than 2,100 pairs in recent years. So how did they get on in 2015.......?

Common Tern on Rockabill, Summer 2015. (picture taken under NPWS license)

Breeding Pairs:
In 1989 there were 109 pairs of Common Terns on Rockabill. Thats the year that the conservation project started here, and  by 2011 there were 2191 pairs. Thats what we call conservation!! Numbers dipped a bit in some of the years since, though we had 2150 nests early in the season last year, so still no shortage of Common Terns! This year we had 1950 breeding Common Tern pairs. In the short term that might not seem great - 200 less than last year. But with a species like this there will be some variation in numbers from year to year for a variety of reasons, including poor productivity several years ago, Commons deciding to breed elsewhere, and there being limited room for all of the birds on Rockabill! Though numbers were good last year, they were around 160 pairs lower the previous year, showing that there is often fluctuation. The fact that the birds on Rockabill have been protected and monitored for over 25 years in a row means we have considerable data and information on them, and can take a long-term view of whats happening. As long as they stay at around 2,000 pairs, and productivity is ok, then there shouldn't be anything to worry about. That being said, we'd certainly welcome a couple of hundred more if they turned up next summer!

Common Tern mating 'dance'. (picture taken under NPWS license)
Common Tern on its nest. (picture taken under NPWS license)

Average clutch size was 2 eggs, which is similar to the last two years, though in other years it has been higher. Like with total pair numbers, a certain amount of variation between years is to be expected. Factors that can influence the number of eggs include food availability early in the season and also the age-structure of the adult population - more 'old' pairs will lay more eggs than younger pairs. 
Around 75% of eggs hatched, which is quite similar to previous years. Whether or not eggs hatched are mostly impacted by weather and predation, though the amount of food available to adults before the egg was laid will also have played a part in determining the viability of the egg.

Common Tern clutch. (picture taken under NPWS license)
Common Tern egg hatching. (picture taken under NPWS license)

 Productivity  - that is the average number of chicks that survived per nest - was a bit over 0.77. This might seem low but its the same as the average over the last five years. I've mentioned it in previous blogs, but when Common Terns lay two or three eggs they're banking on one of those eggs surviving to be a fledged chick at the end of the season, so 0.77 is a bit below that aim for each pair.

Common Tern adult and chick, a day or two old. (picture taken under NPWS license)

Factors impacting chick survival again include weather, predation and age of adults, amongst others. This year some of the worst days of rain and wind hit when many of the chicks were 2-3 weeks old. At this age they were too big to be effectively sheltered by their parents, so Common Tern chicks in areas with little natural shelter were exposed to the elements and many died. A certain number of fledglings later died due to strong winds blowing them around when they were still inexperienced at flying, though this is a regular annual occurrence and is one of the hurdles young Terns will inevitably face in their first few days in the air. Adult Terns that are young usually have lower success rates than more experienced adults, so this might have been a factor this year too. 

Common Tern chick, 1-2 weeks old. (picture taken under NPWS license)
Common Tern fledgling. (picture taken under NPWS license)

If it seems like I'm painting a very negative picture of how our Common Terns got on this year, thats certainly not the case. It was simply an average year for them on Rockabill, which is nothing to worry about, but we'll just be hoping theres a couple of better years to come in the not too distant future to balance things out a bit!

Check out our work on Rockabill on 'Animal Rescue' on TV3 tomorrow (Monday 14th Sept 2015) on TV3 at 8.30pm, and check back in with the blog later in the week to hear how our Roseate Terns got on this year - our top priority species at their biggest European colony! 

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