Friday, 2 June 2017

Starling RAS round-up

Sorry for doing another post about my Starling RAS project but it is virtually all I do in May on the birding front. It has been particularly busy this year with 193 different adult Starlings coming to the feeders in the garden between 21st April and 24th May. This is roughly double the number of adults recorded over the same period in the previous 2 years. This big increase has probably been caused by the dry spring which will have made it a lot more difficult for the adults to find enough soil invertebrates to feed to their young and made them more reliant on garden handouts.

A large proportion (118) had been ringed prior to the start of this year's RAS period with 99 having been ringed during the previous two breeding seasons or earlier. Looking at these known age adults 66% were at least 2 years old and 34% were ringed last year, as juveniles, so were breeding for the first time. This is a good sample size and is probably representative of the age structure of the population in this area. As the majority of adult Starlings are faithful to their breeding sites the age structure of the population is largely a function of the adult survival rate so the survival rate for adults in the population will be at least 66%. The actual survival rate is likely to be slightly higher as two adults, both at least 2 years old, have been recorded since the RAS period ended and a few others could have been missed or moved territory outside of the catchment area for my garden. The actual adult survival rate for my population is probably closer to 70%. This fits well with some other studies and shows the RAS project is producing good data for monitoring the annual survival rates of adults in this area.

It is harder to say how the breeding season has gone for Starlings for various reasons; not least because juveniles become very mobile and disperse over a wide area shortly after a fledging. I have ringed 266 juveniles this May compared to 215 in May last year and that is despite broods fledging around a week later this year. However, there is no indication there will be a doubling of the number of juveniles coming to the garden this year, as seen with adults, so productivity may actually be lower than last year as might be expected in a very dry spring when natural food is much harder to come by. The recent rain hasn't improved soil moisture levels all that much so I would also expect juvenile mortality to be higher than it would otherwise be if the spring rainfall had been nearer to average levels.

P45, just one the 266 juveniles ringed this May
My interest in Starlings doesn't end when my RAS period finishes and I will continue to ring and record sightings of colour-ringed birds over the course of the summer. Yesterday I caught a new adult that had already started to moult and is the earliest moulting adult I have ever recorded. It shouldn't really be a surprise that adults are starting to moult earlier as they moult soon after they have finished breeding and the breeding season is averaging earlier now than it used to. There is a chance it could be a failed or non breeder but even if that is the case it is still relatively early for it to be in moult and it is yet another a tiny piece of evidence that birds are responding to changes in our seasons and climate. If nothing else it is a reminder that it is never too early to start checking for moult these days.

Moulting adult Starling photographed 01/06/2017. Both wings were symmetrical.

No comments:

Post a Comment