Time shows overall success for nest box breeding birds

2022 was the 68th year of monitoring nest boxes at Yarner Wood, and the tally of monitored Pied flycatcher nests has now surpassed 3000 !. As previously blogged about https://wordpress.com/post/eastdartmoorwoods.org/15286, nest boxes were originally provided in 1955 to research nest height choice by tits. The boxes attracted pied flycatchers to breed at the reserve for the first time, and so have been maintained and monitored ever since. Today, weekly nest monitoring is carried out by a group of Natural England staff and volunteers. These visits are supplemented by extra visits to collect more detailed data for the various science projects that Malcolm Burgess leads.

Changes in populations

In 2022 52 pairs of Pied flycatcher bred in nest boxes at Yarner Wood, as well as 36 pairs of Blue tit, 11 Great tit, 4 Coal tit, 3 Common redstart and 1 Nuthatch. These species also breed in natural tree cavities and so there are many more pairs in Yarner Wood than these totals, although nearly all Pied flycatchers choose nest boxes to breed in and so our Pied flycatcher estimate does reflect their true breeding population size.

Our long term monitoring enables us to look at how populations have changed over the 68 years -Figure 1 shows this for the four most common nestbox using species since 1955. All these species breed in greater numbers now compared to the 1950s. The Pied flycatcher population increased rapidly through the late 1980s, before declining through the 1990s. Over the past twenty years the proportion of the nest boxes occupied by Pied flycatchers has been stable at around 20-30%, but because the number of boxes provided increased in 2011 the number of pairs at Yarner has actually increased from around 40 pairs in the 1990s to a little over 50 pairs on average over the past decade. 2022 was therefore an average year for Pied flycatchers. Redstarts were most common in the 1980s, due to rainfall patterns in the Sahel where they winter being especially favourable for them at that time. Blue tits have become much more common at Yarner, no doubt from the supplementary food provided at the bird feeders year round since the 1980s, and warmer winters, meaning more survive the winter. Blue tit numbers have been lower in recent years however, and 2022 had the fewest number of pairs using the boxes since 1999, and the lowest occupation rate (the proportion of boxes occupied) since 1988. Blue tits have experienced a run of six years prior to 2022 with low breeding success which has probably contributed to this.

Figure 1 – The proportion of nest boxes occupied at Yarner Wood annually since 1955.

Changes in timing

Monitoring nests over decades enables us to see changes in the timing of breeding. The season starts in late March, when I start searching for the first arriving male Pied flycatchers. We have records of the first male seen at Yarner each year since the 1960s. In 2022, despite fine conditions for finding them, none arrived during the sunny mild period in March (we have had arrivals in March before). We now know from tracking devises that some were fitted with that they were further south on their migrations at this time. Weather systems were less favourable for northward migration for a few more weeks, and it wasn’t until April 13 that they started arriving. 2022 was the latest arrival date since 2013. But despite this, the timing of egg laying closely followed the long term trend, which is for earlier laying since the 1950s due to climatic change advancing the onset of spring. Egg laying dates of Blue and Great tits also track this change in the timing of spring.

Changes in nest success

The success of nests varies considerably year to year which is why we need long term monitoring to detect change. The most common cause of nest failure in the nestboxes is wet weather at the chick stage. On wet (and cold) days food takes longer to find and so adults are away from the chicks for longer periods of time and spend less time brooding them (sitting over them to warm them up), and as young chicks cannot thermoregulate they chill and eventually stop begging for food and perish.  In 2022, while we had some wet days, it was mostly dry and rain didn’t take much of a toll on nests of any species and so we had good nest success for Blue tit (6 chicks per nest – the highest rate since 2011) and Great tit (3 chicks per nest), with average success for Pied flycatchers (4 chicks per nest). Pied flycatchers were knocked back a little by nest predation which isn’t common in our oak boxes.  This can include predators like Great Spotted woodpecker but this year the predator was actually a mammal species we like to see doing well at Yarner Wood. Common dormouse! Dormice predate nests when they are laying eggs, the birds lay one egg a day and don’t use the box during the day until they start incubating once the entire clutch is laid. This gives dormice an opportunity to use the largely undefended nest box as a day roost as they emerge from hibernation, and if the nest happens to contain eggs, well then that is a handy meal. We recorded this annually, and it happens most often in years when the timing of egg laying coincides with when dormice are coming out of hibernation. As flycatchers lay eggs a little later than the tits their timing is more likely to coincide with this dormice activity. Five pairs of Pied flycatchers lost their nests in this way in 2022, although two of these pairs tried again (both successfully) in another nearby nest box.

Helped by funding from Natural England we have fitted some adults with geolocator tracking devises over the last 10 years to learn about their migration. This work has revealed that our Pied flycatchers migrate to a fairly limited part of Western Africa centred on Guinea, Sierra Leone and Liberia, which they reach via stops in Portugal and brief stops after crossing the Sahara Desert. We have learnt that they usually cross the Sahara in a single non-stop flight that takes hours. You can read more about this work in this blog. This year have been fitting tags that additionally record air pressure to give us more insights. We successfully retrieved 7 of these multi-sensor tags this season that we fitted in 2021, and fitted another 20 in 2022 – look out for some results of this work which will reveal for example flight altitudes as they cross the Sahara desert amongst other things!

Malcolm Burgess, PiedFly.Net

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