Record-breaking Long-tailed Tits
Record Breaking Long-tailed Tits
As we touched upon in our last post, one of 2015’s most notable species has been Long-tailed Tits Aegithalos caudatus, with this being a record ringing year for them in Filey. One of the many advantages of bird ringing is that it allows us to census population levels on a local scale, particularly in small passerines. These would otherwise be very difficult to survey using other methods. As these birds all look identical there is no way of determining that the twenty birds in your garden one day are the same twenty birds which visit your garden the following day. Based on our findings this years it is reasonable to assume that there is indeed a very high turnover of birds on a local level, especially during a period of population growth.
Long-tailed Tits unlike their names suggests are not true tits (Paridae) but belong to the Aegithalidae and despite their habit of forming mixed winter flocks with Parid tits and sharing similar habitat types, differ substantially in their ecology. One of the most distinctive features of Long-tailed Tit ecology is the nest, unlike hole nesting Paridae species Long-tailed Tits build perhaps one of the most intricate and complex nest structures of any British bird. The nest is essentially a pouch constructed from lichen and spiders webs filled with a layer of feathers. The webs add elasticity to the external structure allowing the nest to expand as the young birds grow inside it. Nests in Filey are frequently built low (>3m) from the ground within dense thorny stands of Blackthorn Prunus spinosa, Hawthorn Crataegus monogyna and Bramble Rubus fruticosus, making the Top Scrub, Old Tip and Parish Wood all particularly good locations. Occasionally nests are built higher up in the fork of a tree. Nesting occurs in lose colonies, in which it is not uncommon to see birds which have lost their brood to help neighbours raise their broods instead. Nesting occurs earlier than in the Paridae Tits, with approximately 10 eggs being laid, beginning around late March.
2015 has now seen approximately 200 birds rung in Filey, which is 50 more birds than during the previous best year back in 2004. Looking at the graph it is clear to see that the species has become increasingly abundant within the bird observatory over the past 40 years. However a major shift in population has occurred here since 2005 which itself was one of the worst years ever recorded for this species in Filey. It is unlikely that ringing effort in Filey has contributed to the fluctuations in numbers as we can see by comparing our figures with Flamborough and Buckton figures (below). These mirror what we have seen in Filey. Further to this, comparison with BTO figures yet again shows a similar national trend with a notable blip around 2005 followed by a population boom across Britain.
These graphs suggest that the local Long-tailed Tit population is following a national and regional trend and is increasing within the Bird Observatory. It is interesting that looking at Met office figures winter 2004/5 and again in 2012/3 we saw quite a lot of snow including periods of snow and rain late into March and through mid April the following springs, which may have coincided with the start of breeding for Long-tailed Tits. By comparison winter 2014/5 was very mild (as was 2014), we saw a wet March but importantly a relatively dry mild April which coincides with nesting and feeding nestlings. Further to this we have seen a recent local trend of relatively dry April weather and importantly mild winters, which appears to have coincided with the population increase over the last 10 years.
Long-tailed Tits are prone to high levels of winter mortality during periods of cold and wet weather, and extended periods of snow cover. Unlike the Paridae species Long-tailed Tits lack the capacity for feeding extensively on seeds and nuts, due to their relatively small, weak bill. A heavy dependency on invertebrate prey is a downfall in winter not shared with most similar small passerines with a few exceptions like crests Regulus spp. et al. Invertebrates generally suffer greater population declines during cold periods of weather, especially when this is also exceptionally wet. Long-tailed Tits lack the dexterity within the feet compared to other similar species, meaning unlike say Blue Tits Cyanistes caeruleus it cannot hold a food item under its foot and hammer it with the bill to access its soft interior. In general Long-tailed Tits are poorly equipped at dealing with larger items of food, which is reflective of their ecological niche, of feeding on small invertebrates on the outer twigs of trees and bushes. A combination of these factors make them less adapted to cold winter weather compared to similar species.
Winter mortality is notably a big factor on population dynamics in this species as we have seen above, making recent mild winters an undoubted contribution to this populations revival. However for a population to persist and increase it needs to reproduce. Importantly in this case conditions must also be good during the nesting phase. For this the species needs physical nest locations, access to invertebrate prey for feeding nestlings and good weather conditions to allow insect numbers to increase. Additionally mild weather prevents the young naked (Psilopaedic) nestlings from chilling in the nest. Windy and exceptionally wet weather may also wash nests out. Fortunately April and the early breeding period has been relatively mild and dry over 2015 allowing a small window for completing breeding, unlike recent May weather which has been notably cold (especially on a night) and at times wet.
Further to the recent weather which has undoubtedly helped this species, whether climate change or just a recent run of favourable weather long-tailed tits are benefiting from this. However the weather alone is probably not the only determining factor for Long-tailed Tits in Filey. Upon comparing with Flamborough and Buckton ringing sites one can see the Filey population is substantially higher (or ringing data says it is). Buckton consists of relatively open farmland and Flamborough a combination of favourable woodland habitat mixed with open areas of farmland on a relatively isolated outer headland. Filey ringing sites by comparison are dominated by a matrix of recently planted woodland (<20 years old) without any excessive areas of closed canopy mature woodland. Instead dense stands of scrub dominated by Dogwood Cornus sanguinea, Spindle Euonymus europaeus, Willow Salix alba, Salix cinerea etc, Hawthorn, and Bramble dominate over woodland. This all makes for optimum nesting habitat (in deep thickets), foraging habitat on a wide diversity of invertebrates (linked to the high density of bushes), shelter from predators such as Sparrowhawk Accipiter nisus whilst foraging/dispersing and importantly an enclosed, warm micro-climate for both winter foraging and winter roosting. Additionally sites are linked by tree lines and hedge systems and in some cases sub-urban gardens (Long-tailed Tits seldom fly away from cover) allowing the population to access multiple sites easily.
Perhaps the final (key) factor (disregarding predators which are not considered to be a key factor here) to consider is immigration and whether we are seeing birds from outside of the observatory. It would be naive to suggest anything other than this. We undoubtedly experience influxes along the coast into this area from further afield and from inland. The capacity for the immediate areas around the ringing areas to have produced all 200 birds is unlikely, however theoretically this would only constitute 20 pairs having 10 young each. Itself an unlikely circumstance given the high mortality in small birds. It does however highlight that not many breeding pairs would be required to produce 200 individuals given the high fecundity seen in this species. Long-tailed tits are also notoriously sedentary (we dream of a white headed caudatus) but in general British rosaceus birds seldom travel more than a few km from their natal site. This most likely could be concluded as not all birds we catch are from the observatory area but a large portion probably are, hopefully some recoveries of these 200 or so birds will show us exactly where they are going and how long they are surviving.
It is therefore not surprising that 2015 has seen such a rise in the fortunes of this charismatic species within the bird observatory. A combination of favourable weather and a large increase in favourable habitat allows long-tailed tits to utilise the observatory efficiently and it is likely if sufficient ringing occurs and climatic cycles remain consistent 2016 or 2017 may see another record year for this species. This is just another way that bird ringing can help us piece together a picture of local population dynamics within a given bird species. In this case a species which has benefited from habitat creation work by FBOG and EYRG volunteers.