Thursday, 19 April 2018

Oxwich Marsh: investigating diet and competition for prey in Oxwich warblers

This post has been written by Sarah Davies, a PhD student in Cardiff University. Sarah has been ringing at Oxwich for approximately 14 months, and has been using it as one of the sites for data collection to inform her research on dietary composition and prey partitioning. An overview is below.

Reed warblers are a common sight at Oxwich Marsh in the summer months, and have been the subject of many studies in ecology and animal behaviour. I have always been fascinated by these warblers and was lucky enough to be given the opportunity to study them for my PhD at Cardiff University. 

Reed warblers in the autumn (a worn adult left and a pristine juvenile right)

Focus of Research

My research is about the dietary choices of reed warblers and how this is affected by the availability of prey. Reed warblers are generalist insectivores, so they can consume a wide variety of invertebrate prey from snails, to tiny midges, to spiders. In short, they are not fussy eaters! They also show dietary plasticity which means that they adjust their diet opportunistically, depending on what is available in the environment. In reedbed habitats, the abundance of different prey groups fluctuates with local emergences over the season. Thus, unlike woodland habitats, wetlands such as reedbeds are non-seasonal and prey is available throughout the summer.

Reed warblers may not always have their pick of the best invertebrates due to dietary competition from other warbler species in the marsh. Two other reedbed species are commonly ringed at Oxwich in the summer: the Cetti’s warbler and the sedge warbler. Little is known about to what extent these birds partition their diets (i.e. choose different prey) to avoid competition and exclusion. In addition, overlaps in diet may change depending on prey availability. If prey is very abundant, coexisting bird species may be able to eat many different prey items and dietary overlap will be low, but if prey is less abundant, birds may only be able to eat select prey groups and their diets will overlap more. 

As well as the differences in diet between different species I am also interested in whether age influences dietary choice. Juvenile birds may not have access to the best foraging sites due to dominance of older individuals; they may be inexperienced hunters or may have different nutritional requirements.

Data Collection and Analysis

One way to analyse diet is to collect faecal samples from birds in the field and use molecular techniques to extract DNA from the prey remains in the droppings. Using high-throughput sequencing, identification of prey to species level can be achieved. Once prey in the diet has been identified for a given number of samples, it is possible to estimate dietary overlap between different species.

Last summer, I worked with my fellow ringers at the Gower Ringing Group to collect faecal samples from reed warblers, sedge warblers and Cetti’s warblers ringed at Oxwich Marsh. Ringing took place at least once a week between mid-April and early September, so there was plenty of opportunity to catch warblers. Samples were collected non-invasively when birds defecated into clean bird bags while waiting for processing. Generally, this was a very straightforward task with most birds leaving a sample for us after a few minutes! Each was assigned its own ID so that the sample could be traced back to the bird and its biometrics. The samples are currently stored at Cardiff University for analysis and I am in the process of extracting the prey DNA in the lab.

To determine prey availability, I monitored invertebrates in the marsh three times over the summer to compare emergences of different prey groups. I wanted to include invertebrates from the reedbeds and the surrounding scrub, since warblers are known to use a variety of foraging habitats in the marsh. Once I know what is available at each time period I can compare this to what was eaten by the birds  which will allow me to detect dietary preferences for each species over the summer.

Sedge warbler (Keith Vaughton)
Next Steps

My fieldwork with the Gower Ringing Group was a lot of fun and great practice for my bird ringing! Following on from last summer I am planning to do a similar, smaller scale study this summer to look at the between year changes in diet at Oxwich Marsh.

Thanks to everyone at the Gower Ringing Group for their invaluable help with this project!

Sarah Davies
19/04/2018

Tuesday, 17 April 2018

Oxwich Marsh: Early April totals

April is an odd month at Oxwich. Blustery wind, rain and sea mist all hamper our efforts, and if we don't connect with a good passage of finches it can be hard going. 

During the month we typically capture a higher proportion of re-trapped to new birds than at any other time of year, and it is often a good time for recapturing some of our old stagers.

Unique birds captured over four sessions between 2 and 15 April were:

Species
Total
Blackbird
3
Blackcap
9
Blue tit
22
Brambling
6
Bullfinch
2
Chaffinch
7
Chiffchaff
5
Coal tit
1
Dunnock
3
Goldfinch
26
Great spotted woodpecker
4
Great tit
19
Greenfinch
1
Long-tailed tit
1
Mute swan
1
Reed bunting
16
Robin
1
Siskin
50
Willow warbler
2
TOTAL
179

Of particular interest were


  • A chiffchaff ringed in May 2016, and recaptured on 13 April 2018. We know this bird is a male, and that it breeds on the marsh, as when it was captured in May (and again in June) 2016, it showed a cloacal protrusion. It was also captured at Oxwich in April 2017.
  • Reed buntings originally ringed in March 2014 and in September and October 2015. There was a noticeable influx into the marsh in April, but we have not yet captured a female with a brood patch.
  • Goldfinches ringed in March 2014 and in April 2015 (2). 
  • Six brambling. The latest of these was captured on 14 April. All of the birds were carrying reasonable fat deposits (scores of 4 or more using the British Working Group system), indicating preparation for movement.
  • The mute swan, originally ringed at Goodrington Park, Torbay, Devon in January 2010, and resident on the marsh for the past few years (ring-read by Steve in the field).
Looking back at previous years' data, it is clear that the first half of April 2018 hasn't been as slow as it has felt at times .... The 238 birds processed compares to 163 (2017), 137 (2016), 127 (2015) and 126 (2014).  Maybe it is the increasing number of capable pairs of hands that we have that has made things feel quieter.

The first grasshopper warblers of 2018 were noted reeling in the marsh on 13 April, but the first capture of the year remains elusive. A marsh harrier (female) is making regular use of the reed bed, and there are still a few snipe around. Reed and sedge warblers should be back at Oxwich by the next ringing session.

Thanks to all who have attended over the past two weeks: Heather Coats, Keith Vaughton, Wayne Morris, Kirsty Franklin, Richard Dann, Alex McCubbin, Edward O'Connor, Amy Schwartz, Stephen Vickers, Jo Conway, Sarah Davies, Bethan Dalton and Paul Aubrey.

Owain Gabb
17/04/2018

Male bullfinch (Bethan Dalton)

Female bullfinch

Male reed bunting (Bethan Dalton)

Tuesday, 10 April 2018

Gower Trainees on Tour: a visit to Calf of Man Bird Observatory

In order to gain the experience necessary to secure a ringing permit, it is often advantageous for ringing trainees to seek opportunities to go out with other groups. This article has been prepared by Stephen Vickers and Kirsty Franklin summarising two weeks of volunteering on the Calf of Man.

We arrived on the Isle of Man by plane in the early hours of March 26 and headed straight to the Calf of Man by boat. The entire journey was very easy; with shopping ordered in advance and delivered to the island on a weekly basis, we had no worries about bringing food. We left on April 9 after 14 days on the island, which was as much time as we could squeeze around work.

Ringing takes place on a daily basis on the Calf when weather permits. In fact, we both had a bird in our hands less than 10 minutes after stepping off the boat, so not a bad welcome! The ringing mostly consisted of mist netting and the use of two Heligoland traps, where we were treated to good numbers of goldcrests, goldfinches, robins, and more. This provided us with plenty of ringing and extraction experience, a great help on our way to our ‘C’ permits. The weather wasn’t always on our side, but we were still lucky enough to process  435 and extract 621 birds between us in that time.

Below is a summary of numbers processed by the pair of us, and does not include birds processed by others at the obs while we were there:

Species
Kirsty
Stephen
Blackbird
3
7
Blackcap
4
6
Black redstart
1
0
Chaffinch
9
14
Chiffchaff
17
23
Dunnock
1
1
Great black-backed gull
0
1
Goldcrest
87
84
Goldfinch
38
37
Greenfinch
0
1
Manx shearwater
7
7
Meadow pipit
3
2
Redwing
0
2
Reed bunting
2
2
Ring ouzel
1
0
Robin
13
15
Shag
2
2
Siskin
3
6
Starling
1
1
Wheatear
2
2
Willow warbler
16
9
Wren
1
2
Total
211
224

Unsurprisingly at a bird observatory, we were treated to some nice (uncommon) birds. Kirsty was lucky enough to ring both a female ring ouzel, and a male black redstart. Whilst Stephen lost the coin toss on both uncommon birds, he got a chance to ring in his words “the best seabird in the world”. 

After finding a recently deceased rabbit, Stephen convinced the warden to let him use the large 1m2 spring trap with the rabbit as bait. Two days later, success struck in the shape of an adult great black-backed gull. As reward for the work he put towards catching it, Stephen was the lucky person to ring the bird. He hasn’t stopped speaking about it since!

We also had a huge amount of fun visiting several breeding shag colonies, to make assessments towards measuring breeding productivity and attempt to catch some adults on the nest. With 27 nests found, some already at hatchling stage, it proved a big success. To top it off, we both got to ring two adult shags each. A great species to ring, that we wouldn’t get to do with Gower RG, and which also gave experience with a larger ring size (K ring).

A couple of evenings were spent dazzling Manx shearwaters. Brown rats decimated Manxie population numbers on the island, and even now after many years of rat eradication and conservation efforts the population only stands at approximately 400 pairs. Whilst these numbers don’t quite match the populations on the Welsh Islands (360,000+ pairs), we had a very successful time processing 14 individuals between us and were able to get more practice of dazzling. It was also nice to ring a Manxie on the island where it got its name!

Whilst ringing was a big part of why we went to the Calf, there was also plenty of other work to get stuck into to help with the running of the observatory. Jobs included drainage ditch digging to create new wetland scrapes, wildlife walks to contribute to the daily log of species recorded on the island, scrub clearing, and Manx shearwater nest box building. 

The Calf of Man was a great place to spend a few weeks ringing and volunteering, and whilst it was a cold delayed start to spring, we had great fun and saw some great species.

Calf of Man Bird Observatory does not currently run a blog, but frequent updates on their ringing and other observatory activities can be found via Facebook and Twitter @CalfObs.

Ornithological warden Aron Sapsford can also be contacted via email.

Photos are below.

Stephen Vickers & Kirsty Franklin
10/04/2018

Male black redstart

Dazzling Manx shearwaters

Taking biometric data from the great black back.

Female ring ouzel
Ringing an adult shag

Male wheatear

Making Manx shearwater boxes

Clearing scrub to create net rides
Digging drainage ditches

Male eider

Monday, 2 April 2018

Oxwich Marsh 30 March & 1 April: winter visitors and a sub-Saharan migrant

A short period of fairly settled weather, with light easterly and north-easterly winds, allowed us to get in two sessions. The results are below:

Species Name
Ringed
Recaptured
Grand Total
Blue Tit
4
10
14
Brambling
5

5
Chaffinch
4
1
5
Dunnock
1
5
6
Goldfinch
7
2
9
Great Spotted Woodpecker

2
2
Great Tit
2
8
10
Nuthatch

1
1
Reed Bunting
2
1
3
Robin

3
3
Siskin
7
18
25
Snipe
2

2
Treecreeper
1

1
Willow Warbler
1

1
Wren
1

1
Grand Total
37
51
88

The features of the combined catch were:
  • What were probably the final two common snipe of the spring (one in both sessions). Jack snipe were seen on the marsh on 1 April, but we did not manage to capture any. Both species are likely to be down to single figure numbers at present.
  • A continued trickle of bramblings. Of the five birds captured, four were females and one a cracking male. We had to release another male brambling that showed evidence of developing Fringilla papillomavirus.
  • Our first willow warbler of the spring (and the first sub-Saharan migrant captured in 2018). This bird was carrying reasonable fat deposits, suggesting it was still en route to its breeding grounds.
  • Evidence of breeding in dunnock, great spotted woodpecker and siskin. Individuals of these species either showed defeathering or well defined brood patches (females), or cloacal protrusions (males).
  • The first treecreeper of the year, caught on 1 April. The nuthatch trapped during the same session would have been more notable had it not been initially ringed during a March 2018 visit.
A breakdown of birds captured in the year to date is below. It is very nice to have siskin as our most captured species; greenfinch (conversely) is largely notable by its absence from the working totals at present, while it has also been a slow start to the year in terms of goldfinches. The number of new brambling suggests it has been an exceptional winter locally for the species, while it is also our best first winter period for common snipe.

Species Name
Ringed
Recaptured
Grand Total
Blackbird
3
1
4
Blue Tit
31
97
128
Brambling
17
3
20
Cetti's Warbler

4
4
Chaffinch
45
4
49
Chiffchaff
1

1
Coal Tit
2
3
5
Dunnock
3
32
35
Goldcrest
5
1
6
Goldfinch
36
9
45
Great Spotted Woodpecker
1
3
4
Great Tit
11
45
56
Greenfinch
3

3
Jack Snipe
5
1
6
Marsh Tit
1

1
Nuthatch
1
1
2
Reed Bunting
10
4
14
Robin

15
15
Siskin
61
75
136
Snipe
26
4
30
Song Thrush

1
1
Treecreeper
1

1
Willow Warbler
1

1
Wren
2
2
4
Grand Total
266
305
571

Thanks to all who managed to attend one or both sessions: Heather Coats, Keith Vaughton, Wayne Morris, Bethan Dalton, Alex McCubbin, Edward O'Connor, Sophie de Grissac and Richard Dann.

Owain Gabb
02/04/2018

The last snipe of spring?

Female brambling

Male brambling

Female brambling

The first willow warbler of the year

Blue tit with deformed bill (3 years old)