Sunday, 22 July 2012

Juvenile production

Occupying the thoughts of many a ringer is how is the breeding season going. Not well is the simple answer but what is the evidence?

With this in mind, I thought I'd compare juvenile numbers so far this year with the same time last year (breeding season up until 22nd July both years). This incorporates data collected from Priory Country Park & Sandy Smith Nature Reserve. Effort at Priory CP (11 visits both years) has been the same but effort at Sandy Smith NR has been less (4 compared to 8 last year) due to the weather. Neither site has been flooded (i.e. under water), just extremely damp at times!

2012 juvenile totals are given first, 2011 juvenile totals in brackets:

Blackbird 7 (2)
When I first saw this I was surprised. Now I've thought about it, I'd speculate that worms etc. are easier to find in the wet (they come to the surface). Food is therefore easier to find, resulting in better productivity. Surprise gone.

Blackcap 14 (27)
About half the numbers. The local weather is surely affecting the availability of food and ability of the adults to keep eggs warm/feed youngsters as well as themselves.

Blue Tit 24 (43)
Reports I've heard from nest recorders say many breeding attempts showed much reduced brood sizes or failures. Though I have ringed 24 juveniles so not a complete disaster. Those that did fledge seem to be finding food inbetween the deluges!

Bullfinch 1 (1)
Perhaps a little early to tell. 2 very different seasons weather wise and totals come out the same. Mmmm?

Chaffinch 2 (12)
Adults and youngsters alike are hard to come by. Mainly caught at feeders at Sandy Smith NR. Weather must be having an affect.

Chiff Chaff 12 (15)
A small reduction. A small change such as this is too small to draw any conclusions from as small changes in weather, food availability etc on any given day can change what we catch.

Dunnock 10 (29)
Significantly down on last year. Again, the weather must be having an affect.

Garden Warbler 2 (5)
Again, a small reduction but I've caught fewer adults this year (13) compared to last year (22). Productivity could be down because there are fewer adults around to breed and/or the weather.

Great Spotted Woodpecker 1 (1)
No change.

Great Tit 36 (47)
A decline here also, but fairing better than Blue Tits. Great Tits share a very similar breeding  season with Blue Tits but have faired a little bit better.

Long Tailed Tit 2 (10)
I just don't seem to be catching many Long Tailed Tits at all.

Reed Warbler 5 (9)
As relatively late breeders, the picture is not entirely clear however there ought to be more juveniles about by now if they were having a good year. Effort this year has included a session by the reedbeds at Priory CP (where as last year didn't), proving that the decline is significant. Brood patches on adult females trapped are showing they are still breeding and/or beginning to stop.

Robin 13 (25)
Robins have had several good years in a row at Priory Country Park. Indications are that this is a significant decline & it's easy to blame the affects of weather on food availablity.

Sedge Warbler 3 (4)
Similar to last year. All caught at Sandy Smith NR, effort here has been less in 2012 due to the weather.

Song Thrush 3 (1)
An increase, but is it significant?

Treecreeper 3 (0)
Another species showing better in 2012. They are present in small numbers at Priory CP, and 3 juveniles caught within 1 week represents good value for the breeding season.

Whitethroat 4 (77)
A massive decline in 2012. 2011 was a significantly good season for Whitethroat - as an example a visit on 2nd July 2011 to Sandy Smith NR, 37 juveniles were caught in one session. So far, this year, only 4 juveniles were caught there and the adults are now moulting & have more than likely given up breeding.

Wren 12 (10)
A slight improvement in 2012. No evidence here that the weather has caused them any difficulties.

This time last year I had also caught juveniles of Coal Tit (1), Goldfinch (4), Grasshopper Warbler (1), Greenfinch (1), Marsh Tit (1) and Willow Warbler (1). The increased diversity of species caught supports the theory that last year was a much better breeding year.

To put this in a bit of context:

a) This is not official scientific fact and is just my own speculation based on ringing data & totals that I have collected (with some help from other ringers).
b) The effect of weather on breeding can be highly localised (it could be raining at Priory CP but dry at Sandy Smith NR for example!). This data may or may not reflect national trends in juvenile productivity.
c) I am not making any comparison between the sites. Habitats are different for a start!
d) The breeding season is not yet over. August (traditionally when we catch the most juveniles) will really show how well the breeding season has gone. If the weather turns good, it could still turn out well for the birds.... well, we can hope!

1 comment:

Phil said...

As you say, not conclusive but not good either. V Intersting your decline in Whitethroat which mirrors ours so far.