Sunday, 3 December 2017

Oxwich Marsh early December 2017: a year on year comparison

The ringing site at Oxwich Marsh is approximately 500 m from the high tide mark; rides are located in a mixture of reed bed, ruderal and scrub habitats set behind coastal dunes and foreshore. 

Ringing at the site is often weather affected, and between-year results can be very different for this and other reasons. This is illustrated in the table below, which provides data on the totals of unique birds (newly ringed and the first recapture of previously ringed birds) for 2016 and 2017 to date.

Species
2016
2017
% Change
Significance
Mute Swan
 7


Sparrowhawk
2



Water Rail
3



Jack Snipe
14
9
-36%

Snipe
19
48
153%
**
Kingfisher
1
2
100%

Great Spotted Woodpecker
21
33
57%

Sand Martin
33
19
-42%

Swallow
595
709
19%
**
House Martin
5
104
1980%
**
Tree Pipit
37
19
-49%
*
Meadow Pipit
14
45
221%
**
Yellow Wagtail
 3


Grey Wagtail
1
7
600%

Pied/White Wagtail
44
101
130%
**
Wren
76
70
-8%

Dunnock
39
72
85%
**
Robin
49
76
55%
*
Redstart
1



Stonechat
21
14
-33%

Wheatear
1



Blackbird
29
55
90%
**
Song Thrush
10
16
60%

Redwing
42
92
119%
**
Cetti's Warbler
26
40
54%

Grasshopper Warbler
19
13
-32%

Sedge Warbler
177
142
-20%
*
Reed Warbler
227
192
-15%

Lesser Whitethroat
1



Whitethroat
36
23
-36%

Garden Warbler
16
8
-50%

Blackcap
71
97
37%
*
Yellow-browed Warbler
16



Chiffchaff
145
101
-30%
**
Willow Warbler
146
72
-51%
**
Goldcrest
106
81
-24%

Firecrest
4
2
-50%

Long-tailed Tit
42
21
-50%
**
Willow Tit
1



Coal Tit
8
5
-38%

Blue Tit
235
399
70%
**
Great Tit
135
145
7%

Nuthatch
1



Treecreeper
7
3
-57%

Magpie
1



Starling
2


Chaffinch
208
132
-37%
**
Brambling
1
2
100%

Greenfinch
244
136
-44%
**
Goldfinch
479
305
-36%
**
Siskin
150
165
10%

Lesser Redpoll
2
2
0%

Bullfinch
2
14
600%
**
Yellowhammer
 1


Little Bunting
1



Reed Bunting
117
91
-22%

* change significant at 5% level, ** at 1% level

The data contained in the table can be easily generated using IPMR (Integrated Population Monitoring Reporter). The table indicates that some of the between year changes are significant - so what are the reasons accounting for them? Some interpretation of the numbers is below:
  • Some species, such as sparrowhawk, water rail and starling, are captured relatively randomly. They are around the marsh all year, sometimes (in the case of starling) in very large numbers. However, we don't deliberately target them at present.  
  • Snipe numbers are well up in 2017. This partly reflects less water in the marsh, which may in turn reflect both less rainfall locally, and changes in water level management by Natural Resources Wales (NRW). When water levels are low in autumn, winter and early spring, it is more feasible to put net in areas of wet reed bed. Snipe day roost in these areas. The snipe favour areas that have been trampled down by cattle (at least for dropping into at or before dawn); in 2017 cattle have been absent in the autumn, and we have been able to manipulate snipe distribution by strimming some suitable areas (to create open patches surrounded by tall reed). Jack snipe also day roost in these areas. It follows that the decrease in jack snipe numbers in 2017 may, in fact, reflect a real local decline in 2017, or it may be coincidental. From less than 2:1 snipe to jack snipe in 2016, we are at a ratio of approximately 5:1 in 2017.
  • For house martin, swallow, meadow pipit and all species of wagtail, the higher ringing returns in 2017 reflect increased effort to target these species. House martins will sometimes come to a tape lure in late summer / autumn (and we had one very good day in 2017); we completed more roost visits for swallows and wagtails in 2017; and, due to having more trainees (who would benefit from the experience), we also looked to capture more meadow pipits this year during quiet periods.
  • Some resident species seem to have had a more productive year at the local level in 2017 than they did in 2016. These include dunnock, blackbird, blue tit and robin. This is reflected in far higher numbers of these species.
  • Some changes are likely to have been due to relatively short term weather patterns. The number of tree pipit and garden warbler we capture clearly reflects the effort we put in, but also the amount of optimum weather for capturing them between mid August and early September (the peak passage period for these species in Gower). Sedge warbler and willow warbler numbers also appear quite markedly different, but a few big autumn catches typically significantly influence overall totals of these species (e.g. we captured 44 willow warblers on 14 August 2016 and had two other dates on which 20 or more were captured - the maximum day capture in 2017 was 23 birds). Finally were also unable to get a session in in early October, when an influx of firecrest occurred locally, and have captured our only two individuals of this species in 2017 in late November and early December respectively (far later than in previous years).
  • For some species, influxes are variable between years; redwing (more present in 2017) and yellow-browed warbler (more present in 2016) are the most obvious examples of this in the data presented. 2016 and 2017 have both been modest years for goldcrest. In 2015 we captured over 160 birds during a substantial late autumn influx.
  • A few species may actually be declining at the local level. Greenfinch numbers have dropped each year for the past few years, suggesting trichomoniasis (which is rife in Gower) is causing a population-level impact. Effort to capture them has been a little variable, but not enough to account for the changes in numbers observed. Chaffinch and goldfinch numbers have also dropped in 2017 (albeit a good December may see us catch a good few more of both), which may simply be between year variation; this is something we need to monitor. 
Returning to the current, a session on 2 December 2017 was curtailed by weather (persistent light rain). We are catching a lot of blue and great tits at present, which is providing good extraction practice for our trainees. A proportion of both species were initially ringed in 2013 and 2014 (the years in which ringing recommenced at Oxwich and in which we started more intensive effort respectively). Highlights of the session were a first winter female firecrest, a female bullfinch, what may be our last new Cetti's warbler of 2017 (taking us to a record total of 40 unique birds for the year), and five reed buntings (the latter appeared to be drawn to a tape).

Thanks to Heather Coats, Keith Vaughton, Paul Aubrey, Emma Cole, Sarah Davies, Stephen Vickers, Kirsty Franklin, Joanne Conway, Sophie de Grissac, Alex McCubbin and Edward O'Connor for company and assistance during recent sessions.

Owain Gabb
03/12/2017

Photos (taken by Kirsty Franklin) are below.

Bullfinch (our 14th of an above average year for the species).

Our latest firecrest to date (captured on 2 December 2017)