31 May 2013

Marsh Tit population crash

The NERC Centre for Ecology & Hydrology (CEH) Marsh Tit project in west Cambridgeshire was outlined in a previous Demog Blog post in 2010, and is now in its 10th year. But after a decade of stability 2012-13 has seen a dramatic fall in the population, and ringing has enabled us to understand how and why this has happened.


Almost all of the Marsh Tits in the main study population are colour-ringed, and the number of territories in the four main study woods has typically been around 30-32 each spring, with about 15% of those being occupied by lone males. But in 2013 the number of occupied territories has fallen by a third to just 19, and more than a quarter of those did not have a female present.


Adult survival from the previous spring was only 44%, which was among the lowest recorded since the study began and some way short of the average 51%, but this alone could not explain the sudden drop in population. The main problem was juvenile recruitment, linked to the very poor weather during the breeding season in May 2012. In line with results for other tits shown by the Nest Record Scheme (click here for 2012 premilinary results), the Marsh Tits also seemed to struggle to rear young during 2012 as persistent rain hampered foraging. Things were so bad that we had the first ever record of adults abandoning a nest mid-way through incubation.


Reduced productivity, and probably also reduced fledgling survival, was apparent in the much reduced number of juveniles recruited into the population during our autumn trapping period, when I catch and colour-ring most of the new birds. During a typical year I expect to find 50-60 juveniles in the four woods between August to October, but in autumn 2012 the number was just 21. With so little recruitment on top of lower than average adult survival, there were not enough juveniles to replace adult mortality and maintain the breeding population in 2013. The gender imbalance in the surviving birds indicated that mortality was especially severe among females, which are at the bottom of the pecking order.


That extreme weather and a single poor breeding season could almost halve our breeding pairs was quite worrying. Marsh Tit is a Red-listed species that has declined nationally by 72% since the 1960s, and our study shows that random events such as the very wet spring 2012 can be a severe blow to fragmented populations of birds that may already be struggling. With 2013 shaping up to be another relatively poor breeding season for tits, the previously large and stable population of Marsh Tits in the Cambridgeshire study may take years to recover, if it ever does.

Thanks to Richard Broughton for writing this post and for the photos.

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