BTO - Breeding Birds of the Wider Countryside: Discussion 4.5

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4.5 Changes in breeding performance

Changes in a range of aspects of breeding performance can be measured under the Nest Record Scheme and the Constant Effort Sites scheme. The former provides information on components of breeding performance per nesting attempt. The latter provides an index of breeding performance accrued over all nesting attempts in a particular year, combined with the effect of changes in the survival of fledglings once they have left the nest but before they are caught as juveniles – a period when losses of young can be high.

Breeding performance may be influenced by a variety of factors, including food availability, predation pressure and weather conditions. Variation in breeding performance may help to influence, and may even be the main demographic factor responsible for determining the size of a population. Conversely, the breeding performance of a population may be negatively related to its size, with productivity decreasing as the number of individuals increases, and vice versa. This relationship may be due to the action of density-dependent factors, such as competition for resources: as numbers increase, competition for resources is likely to increase, possibly resulting in a reduction in productivity. Alternatively, increases in abundance may result from range expansion into new, sub-optimal habitats where breeding performance is poorer and the average productivity of the population is thus lowered, whilst declines may result from the loss of individuals from these sub-optimal habitats, leading to a subsequent increase in average productivity.

4.5.1 Changes in clutch and brood size

Those species exhibiting statistically significant trends in clutch and brood size over the past 34 years are shown in Tables 4.5.1.1 and 4.5.1.2. Although the numbers of species showing increases and decreases in clutch size were approximately equal (13 and 14 species respectively) there were many more species showing increases in brood size (26) than decreases (17) over the same period.

Table 4.5.1.1 Significant trends in clutch size measured between 1968 and 2002

Species	Period (yrs)	Mean annual sample	Trend	Predicted in first year	Predicted in last year	Change	Comment
Long-tailed Tit	34	32	Linear decline	7.65 eggs	6.55 eggs	-1.1 eggs
Magpie	34	49	Curvilinear	5.57 eggs	4.68 eggs	-0.89 eggs
Hen Harrier	34	12	Curvilinear	5.56 eggs	4.73 eggs	-0.83 eggs	Small sample
Mute Swan	34	18	Linear decline	5.86 eggs	5.08 eggs	-0.78 eggs	Small sample
Great Tit	34	92	Linear decline	8.23 eggs	7.61 eggs	-0.62 eggs
Peregrine	34	15	Linear decline	3.64 eggs	3.09 eggs	-0.55 eggs	Small sample
Moorhen	34	93	Linear decline	6.51 eggs	6.05 eggs	-0.46 eggs
Treecreeper	34	14	Linear decline	5.4 eggs	5.06 eggs	-0.34 eggs	Small sample
Common Sandpiper	34	12	Linear decline	3.96 eggs	3.73 eggs	-0.23 eggs	Small sample
Rook	34	14	Curvilinear	4.15 eggs	3.93 eggs	-0.22 eggs	Small sample
Pied Wagtail	34	59	Linear decline	5.13 eggs	4.95 eggs	-0.18 eggs
Nightjar	34	15	Linear decline	2 eggs	1.87 eggs	-0.13 eggs	Small sample
Chaffinch	34	85	Curvilinear	4.21 eggs	4.09 eggs	-0.12 eggs
Sedge Warbler	34	39	Curvilinear	4.95 eggs	4.88 eggs	-0.07 eggs
Buzzard	34	30	Curvilinear	2.16 eggs	2.18 eggs	0.02 eggs
Grey Wagtail	34	41	Curvilinear	4.68 eggs	4.79 eggs	0.11 eggs
Lapwing	34	125	Linear increase	3.69 eggs	3.81 eggs	0.12 eggs
Mistle Thrush	34	37	Linear increase	3.88 eggs	4.05 eggs	0.17 eggs
Wren	34	96	Linear increase	5.6 eggs	5.82 eggs	0.22 eggs
Stonechat	34	26	Linear increase	5.02 eggs	5.25 eggs	0.23 eggs	Small sample
Woodlark	34	15	Curvilinear	3.36 eggs	3.63 eggs	0.27 eggs	Small sample
Dunnock	34	99	Linear increase	3.91 eggs	4.21 eggs	0.3 eggs
Redstart	34	50	Curvilinear	5.86 eggs	6.19 eggs	0.33 eggs
Starling	34	78	Linear increase	4.45 eggs	4.82 eggs	0.37 eggs
Skylark	34	39	Linear increase	3.35 eggs	3.74 eggs	0.39 eggs
Barn Owl	34	13	Curvilinear	4.3 eggs	4.78 eggs	0.48 eggs	Small sample
Tree Sparrow	34	93	Curvilinear	4.71 eggs	5.19 eggs	0.48 eggs

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Six species (Nightjar, Yellow Wagtail, Pied Wagtail, Great Tit, Long-tailed Tit and Chaffinch) exhibited decreases in both clutch size and brood size over the period, whilst another eight species (Dunnock, Stonechat, Skylark, Grey Wagtail, Redstart, Tree Sparrow, Barn Owl and Wren) exhibited increases in both clutch size and brood size. For three species, declines in clutch size were partially (Magpie) or fully (Moorhen and Rook) compensated for by increases in average brood size, suggesting that conditions for young had improved for these species whilst conditions for parent birds during egg formation may have deteriorated.

Table 4.5.1.2 Significant trends in brood size measured between 1968 and 2002

Species	Period (yrs)	Mean annual sample	Trend	Predicted in first year	Predicted in last year	Change	Comment
Great Tit	34	158	Linear decline	7.44 chicks	6.56 chicks	-0.88 chicks
Blue Tit	34	135	Linear decline	8.32 chicks	7.5 chicks	-0.82 chicks
Great Spotted Woodpecker	34	15	Curvilinear	3.21 chicks	2.52 chicks	-0.69 chicks	Small sample
Long-tailed Tit	34	26	Curvilinear	6.76 chicks	6.26 chicks	-0.5 chicks	Small sample
House Sparrow	34	95	Curvilinear	3.33 chicks	2.84 chicks	-0.49 chicks
Yellow Wagtail	34	13	Linear decline	4.84 chicks	4.37 chicks	-0.47 chicks	Small sample
Greenfinch	34	112	Curvilinear	4.02 chicks	3.66 chicks	-0.36 chicks
Chiffchaff	34	29	Linear decline	5.12 chicks	4.78 chicks	-0.34 chicks	Small sample
Raven	34	56	Linear decline	3.22 chicks	2.89 chicks	-0.33 chicks
Carrion Crow	34	82	Curvilinear	2.86 chicks	2.61 chicks	-0.25 chicks	Includes Hooded Crow
Pied Wagtail	34	110	Curvilinear	4.43 chicks	4.21 chicks	-0.22 chicks
Wheatear	34	64	Curvilinear	4.73 chicks	4.58 chicks	-0.15 chicks
Jackdaw	34	78	Curvilinear	2.6 chicks	2.49 chicks	-0.11 chicks
Whitethroat	34	59	Curvilinear	4.25 chicks	4.15 chicks	-0.1 chicks
Nightjar	34	23	Curvilinear	1.81 chicks	1.73 chicks	-0.08 chicks	Small sample
Turtle Dove	34	17	Curvilinear	1.82 chicks	1.81 chicks	-0.01 chicks	Small sample
Chaffinch	34	136	Curvilinear	3.57 chicks	3.56 chicks	-0.01 chicks
Blackbird	34	117	Curvilinear	3.32 chicks	3.36 chicks	0.04 chicks
Collared Dove	34	68	Linear increase	1.76 chicks	1.84 chicks	0.08 chicks
Spotted Flycatcher	34	131	Curvilinear	3.61 chicks	3.69 chicks	0.08 chicks
Swallow	34	282	Curvilinear	4.07 chicks	4.16 chicks	0.09 chicks
Magpie	34	86	Curvilinear	3.06 chicks	3.16 chicks	0.1 chicks
Linnet	34	122	Curvilinear	4.07 chicks	4.2 chicks	0.13 chicks
Yellowhammer	34	67	Curvilinear	2.96 chicks	3.09 chicks	0.13 chicks
Grey Wagtail	34	85	Curvilinear	3.93 chicks	4.08 chicks	0.15 chicks
Reed Bunting	34	62	Linear increase	4.05 chicks	4.21 chicks	0.16 chicks
Stonechat	34	53	Linear increase	4.69 chicks	4.87 chicks	0.18 chicks
Barn Owl	34	69	Curvilinear	3 chicks	3.19 chicks	0.19 chicks
Dunnock	34	104	Linear increase	3.45 chicks	3.67 chicks	0.22 chicks
Willow Warbler	34	134	Curvilinear	5.24 chicks	5.5 chicks	0.26 chicks
Merlin	34	57	Linear increase	3.5 chicks	3.78 chicks	0.28 chicks
Kestrel	34	110	Linear increase	3.85 chicks	4.14 chicks	0.29 chicks
Skylark	34	68	Linear increase	3.14 chicks	3.48 chicks	0.34 chicks
Corn Bunting	34	12	Curvilinear	3.09 chicks	3.44 chicks	0.35 chicks	Small sample
Wren	34	96	Linear increase	4.78 chicks	5.16 chicks	0.38 chicks
Redstart	34	88	Curvilinear	5.07 chicks	5.47 chicks	0.4 chicks
Tree Pipit	34	29	Linear increase	4.35 chicks	4.75 chicks	0.4 chicks	Small sample
Dipper	34	141	Linear increase	3.52 chicks	3.96 chicks	0.44 chicks
Rook	34	90	Linear increase	2.33 chicks	2.81 chicks	0.48 chicks
Sparrowhawk	34	78	Curvilinear	3.1 chicks	3.59 chicks	0.49 chicks
Tree Sparrow	34	104	Linear increase	3.85 chicks	4.52 chicks	0.67 chicks
Moorhen	34	77	Curvilinear	3.3 chicks	4.43 chicks	1.13 chicks
Nuthatch	34	57	Curvilinear	4.03 chicks	5.26 chicks	1.23 chicks

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Long-term changes in clutch or brood size are associated with long-term population trends in a number of species. Here we highlight those changes that are both statistically significant and likely to be of biological importance.

Declines in population size and brood size were recorded for Yellow Wagtail and House Sparrow. Both species show reductions of about half a chick per nesting attempt. The BTO project on Yellow Wagtails, initiated in 2002, aims to investigate the influence of decreased brood sizes on the abundance of this species. In the case of the House Sparrow, population modelling based on BTO data has shown that declines in rural areas were caused by reduced survival rates but that these declines were mainly halted due to improvements in breeding performance (Crick et al. 2002). The apparently accelerating reduction in brood size is therefore of some concern. Work by Kate Vincent at the University of Leicester has suggested that insect food for the chicks may be limited in certain situations and recent brood size reductions may be a manifestation of this at a wider scale. However, it should be noted that over the long-term some of the reduction in brood size may have been compensated by reduced nest failure rates at the egg and chick stages.

Several increasing species show increasing brood sizes, particularly Sparrowhawk, Wren, Nuthatch and Rook. The return of Sparrowhawks into eastern areas of the UK, where populations of songbird prey are greater, may be a factor in this increase. The UK Nuthatch population, which has been expanding northwards and has increased considerably in size, has exhibited an increase in average brood size of more than one extra young per nesting attempt. It would seem likely that this has helped to drive the population increase of this species.

Inverse associations between clutch or brood size and population trend are found in some 25 species. Such relationships may arise through density-dependent processes where increased competition leads to reduced clutch or brood sizes at higher population densities. Totals of nine increasing species and 16 decreasing ones show such associations. Notable examples amongst increasing species include Mute Swan (clutch size), Great Spotted Woodpecker (brood size), Long-tailed Tit (clutch and brood size) and Magpie (clutch size). Amongst declining species the examples include Barn Owl (clutch and brood size), Skylark (clutch and brood size), Tree Sparrow (clutch and brood size) and Corn Bunting (brood size).

4.5.2 Changes in nest failure rates

Statistically significant trends in the daily nest failure rates at the egg and chick stages over the past 34 years are shown in Tables 4.5.2.1 and 4.5.2.2. The number of species exhibiting declines in failure rates at the chick stage (21) was more than double the number exhibiting increases (7), as was the number of species exhibiting declines in failure rates at the egg stage (38 vs. 12). Thus the general picture is one of improving nesting success.

Table 4.5.2.1 Significant trends in egg-stage daily failure rate of nests

Species	Period (yrs)	Mean annual sample	Trend	Predicted in first year	Predicted in last year	Change	Comment
Jay	34	10	Linear decline	0.0528 nests/day	0.0203 nests/day	-0.0325 nests/day	Small sample
Tree Pipit	34	12	Linear decline	0.0401 nests/day	0.0114 nests/day	-0.0287 nests/day	Small sample
Long-tailed Tit	34	51	Curvilinear	0.0321 nests/day	0.0069 nests/day	-0.0252 nests/day
Magpie	34	56	Linear decline	0.0283 nests/day	0.0032 nests/day	-0.0251 nests/day
Redshank	34	33	Linear decline	0.0411 nests/day	0.0171 nests/day	-0.024 nests/day
Dipper	34	104	Curvilinear	0.0252 nests/day	0.003 nests/day	-0.0222 nests/day
Woodlark	34	18	Linear decline	0.0355 nests/day	0.015 nests/day	-0.0205 nests/day	Small sample
Snipe	34	17	Linear decline	0.0326 nests/day	0.0154 nests/day	-0.0172 nests/day	Small sample
Yellowhammer	34	63	Curvilinear	0.0503 nests/day	0.0338 nests/day	-0.0165 nests/day
Wheatear	34	21	Linear decline