16 April 2013

What a whopper!

A Blackbird weighing 125g is not particularly unusual... in winter. Catching three females at this weight in the second week in April is unusual.

My husband Jeff and I have been studying the Blackbirds in our Thetford garden since we moved here in 1996. Our local breeders are joined by their continental cousins in mid-November each year and they co-exist.  In the mid-winter, particularly when there is snow, their weights peak as they put on fat to survive the cold. In January of this year we caught 52 Blackbirds, seven were over 125g
in weight, indeed one weighed a massive 150g. As Spring approaches, so weights drop.  For the local breeders weights continue to fall – typically a breeding Blackbird will be under 100g with some getting below 90g. For the Continental birds it is a different story – having dropped to close to 100g they will start to increase again, this time putting on fat to fuel their migration. We have had two Norwegian ringed birds in our garden giving a good indication of where many of these birds are off to. Typically we think birds depart our garden at weights of 110g to 120g.

Male Blackbird - Allison Kew

Like most people we look forward to the Spring.  For us one of the features of Spring is the departure of the Continental Blackbirds, allowing us to focus on colour-ringing and re-sighting our breeding birds for our RAS project. We know when the Continentals have gone as the Blackbirds behave differently and we stop catching heavy birds. The date varies from year to year but is normally in the last two weeks in March.

Female Blackbird - Allison Kew

This year we’re still waiting – departure for many of our Blackbirds is at least a couple of weeks later than ever before, the strong, cold, easterly winds not providing good migration conditions.  Over the weekend of 6 and 7 April we had more birds over 117g than in all of the other Aprils combined and one was still here on the 13 April.  Indeed we’ve never had females at 125g in April before – this year there have been three.  All signs of delayed migration. We’re expecting that, with the more favourable weather conditions, all the Continentals will have gone by the coming weekend.

Meanwhile, our local breeders are also slow to get going - by now we’d expect most females to be feeding young and have some doing that, but nowhere near as many as normal.
Just goes to show how useful it is to collect measurements of birds. Without recording wing-length and weight we’d not understand what the Blackbirds were up to as migrants and residents can’t be distinguished by eye. 

Thanks particularly to Jo Lashwood who has done a lot of the recent hard work on this project and actually caught the 150g Blackbird.

Allison Kew
CES & RAS Organiser

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