30G SURVIVAL OF BLUE & GREAT TIT PULLI AT WOOLSTON - 2006

by Kieran Foster

The Study

In 2003 around ten nest boxes were placed on trees in the centre of No.3 Bed at Woolston Eyes. This number was increased in 2004 to around 35 boxes, ten of which were filled with polystryrene to hopefully encourage Willow Tits to excavate, therefore leaving 25 for Blue and Great Tits. Most of the boxes were placed at around shoulder height to allow easy inspection; a few were placed higher on the trees and require ladders. Uptake of the boxes has been quite successful with six, 13 13 and 11 occupied fron 2003 to 2006 respectively.

Ringing takes place throughout the year and an active feeding station is operated during the winter months so it was decided to look at the number of pulli (unfledged young) Tits being ringed and their subsequent survival based on birds being re-trapped. Young birds ringed as part of the normal mist netting have been excluded as their precise origin cannot be established. This is a short summary of the findings to the end of 2006.

Great and Blue Tits are both caught during most ringing sessions and are common birds at Woolston Eyes. The number of birds caught each year is shown in Table 1.

Table 1: Number of Blue and Great Tits ringed each year from 2003 to 2006. (Numbers of pulli are also shown as a percentage of all newly ringed birds).

                          2003                         2004                            2005                          2006                    2003 - 2006

                     All    Pulli    %          All    Pulli     %         All     Pulli    %          All     Pulli    %     All     Pulli       %

Great Tit      160    35    21.9%      191    77    40.3%      151    80    52.9%      152    52    34.2%      654    244    37.3%

Blue Tit        116      9     7.8%      153    32    20.9%        86    26    30.2%      141    12      8.5%      496      79    16.9%      

Great Tit appears to have a much stronger preference for the nest boxes than Blue Tit, leading to the ringing of 244 pulli compared to 79 of Blue Tit. However, the number of juvenile and adult birds ringed is almost the same for both species (410 and 417). The percentage of Great Tits ringed as pulli is 37.3% compared to 16.9% for Blue Tit.

Two methods pf calculating survival have been used. The first is the survival of birds from each cohort, calculated as a percentage of pulli ringed for a specific year that are re-trapped in subsequent years (see Table 2). The second method used provides survival rates from each year to the next by calculating the number of re-trapped birds from each cohort of pulli that have been re-trapped in previous years (see Table 3).

Birds re-trapped up to the time of writing in October 2007 have been included so that first and second year survival for all pulli in the study can be calculated. However birds surviving into a third calendar year can only be calculated for birds hatched in 2003. 2004 and 2005.

There have been no records of pulli ringed at Woolston and recovered elsewhere from the study period.

The Findings

The number of Blue and Great Tit pulli recaptured at Woolston shows that Great Tits have a greater survival rate than Blue Tits, especially during the first calendar year. As a percentage of birds ringed and re-trapped in the same year, Great Tits have a four year average of 43.65% whereas Blue Tits have a four year average of only 4.34% (see Table 2). Even in the best year, Blue Tit survival in the first year is barely above 10% whereas Great Tit survival is at worst 17%. In the second year 10% of Great Tit pulli are still surviving compared to only 1.6% of Blue Tit pulli. In fact, only three of 79 Blue Tits ringed as pulli in nest boxes from 2003 - 2006 have subsequently been re-captured.

Table 2: Survival of pulli Great and Blue Tits between calendar years (expressed as a % of pulli ringed each year)

                                      2003                2004                2005               2006             2003-2006

Species                        GT       BT        GT        BT         GT       BT        GT       BT       GT       BT

1st year survival      74.0      11.1      55.8      6.25      27.5      0.0      17.3      0.0      43.7      4.3

2nd year survival       2.7      11.1      19.0       3.1      12.2       0.0       3.8      0.0      10.1      1.6

3rd year survival       0.0       11.1       8.9       0.0        5.0       0.0      N/A      N.A       5.7      1.5 

Table 3: Survival of pulli Great and Blue Tits between calendar years (expressed as a % of pulli re-trapped in the previous year)

                                     2003                2004                2005                2006            2003 - 2006

Species                        GT       BT        GT        BT         GT       BT        GT       BT       GT       BT

1st year survival      74.0      11.1       55.8      6.25      27.5      0.0      17.3      0,0      43.7       4.3

2nd year survival       3.7     100.0      34.0      50.0      44.4      0.0     22.2      0.0      23.1      37.5

3rd year survival       0.0      100.0      46.7       0.0      37.5      0.0      N/A      N/A      28.1      33.3

No Great Tits ringed as pulli in either 2003 or 2004 have been re-captured in a fourth calendar year up to the writing of this report in October 2007. A single surviving Blue Tit from 2003 was however re-captured.

Sample sizes for calculating second and third year survival from the previous year are very small but Table 3 appears to show that once Blue Tits are past the first year, they have as reasonable a chance of surviving to the next year as Great Tit. For example, the four year average for second-year survival is 23.1% for Great Tit and 37.5% for Blue Tit, although the latter is based on only two birds.

Comment

Interestingly, in a study of Blue Tit and Great Tit pulli survival based on birds ringed and re-trapped at Fox Howl, Delamere from 1993 - 2005 by David Norman (pers. comm.), comparable figures of pulli being re-trapped during the first year are 3.1% for Blue Tit and 4.7% for Great Tit. David only netted at Fox Howl during winter months and so the birds would need to survive for at least six months in order to be re-trapped and only around five or six sessions were done in these months, so again there was less chance for a bird to be re-trapped. Looking at the figures from birds re-trapped from November onwards in the year of hatching the Blue Tits at Woolston show almost exactly the same percentage as David's at Fox Howl, the four year average being 3.56%. Great Tits on the other hand show a much higher percentage with a four year average of 11.33%. This is quite a drop from the 43.65% of birds being re-trapped during the first year and shows that the number of birds being lost to the local population is particularly high during the initial few months from fledging.

David commented that he noticed during his study that there were incidences of high mortality in the boxes and many birds failed to fledge. If the majority of Great Tits fledge successfully but the Blue Tits do not this would account for the difference between the species. Peter Coffey (pers. comm.) suggested that from his experience at Prion, in North Wales, Blue Tits are usually several days later in breeding compared to the Great Tits and this can have an effect in some years on the survival of pulli in the nest. I try to limit the number of visits to the boxes at Woolston to a maximum of two visits per box. The first is around the end of the first week of May to check which boxes are occupied and to ring any early broods and a second visit to ring any birds not ringed during the first visit. I do not re-visit to check success or failure to fledge. However, when emptying the boxes before the breeding season I have not found any dead birds or rings in the nests.

Neil McCulloch, writing in The New Atlas of Breeding Birds in Britain and Ireland; 1988-91 (BTO/SOC/IWC) notes that the ecological requirements of Blue Tits and Great Tits overlap considerably, resulting in competition between the species for nesting sites, including nest-boxes. It has been shown that when nest sites are in short supply, the Great Tit is the superior competitor, although neither species defends territories against the other. The Blue Tit is considered to be more efficient in exploiting food resources shared by both species and tends to enjoy better breeding success than Great Tit when both species occur at high density and there is no shortage of nest sites.

Summary

At Woolston, Great Tit pulli consistently survive the first year much better than Blue Tit pulli - and much better than Great Tit pulli at Fox Howl. It is not clear why this is so; perhaps more information over the next few years will provide the answers.

This study of the survival success of the nest box pulli at Woolston also shows that surviving as a juvenile is the hardest part and that if an individual can survive those first few difficult months it stands a reasonable chance of living until the next year.

Ringing of Great Tit pulli at Woolston has thrown up an unusually high local survival rate which we otherwise would not have known about. Whilst it may be questionable whether ringing Blue Tits and Great Tits in boxes is worth the time, effort and expense, if you are regularly mist netting at the site where you have the nest boxes it might be worthwhile.

Kieran Foster