Car traffic along hedgerows affects breeding success of Great Tits Parus major

Abstract

Capsule In hedgerows near roads with fast and frequent traffic, the mortality of Great Tit Parus major broods was higher than in hedgerows with less traffic and hedgerows with no disturbance.

Hedgerows are non-crop linear areas with trees, bushes and scrub in the open landscape. They have become important for their contribution to conservation objectives, especially in managed landscapes because they provide an increasingly important habitat in which certain species of birds can breed (Hinsley & Bellamy 2000). However, when hedgerows are situated adjacent to roads, birds which use these habitats can be exposed to traffic noise, visual disturbance and the risk of collision with cars and trucks.

Most studies concerning the effects of roads on birdlife have examined road mortality from collisions with vehicles (Hudson & Snow 1965, Hernandez 1988, Erritzoe et al. 2003, Orlowski 2008, Benítez-Lópes et al. 2010). Others show that traffic volume and noise along busy roads reduces adjacent habitat ‘quality’ indicated by a reduction in the density of many bird species (Reijnen et al. 1996, 1997, Fernández-Juricic 2001, Parris & Schneider 2009). Most of these avian studies have been done in woodland, with focus on avian response along a decreasing gradient of disturbance with increasing distance from traffic. Little attention has been given to linear habitats where disturbance is uniform, and still less is given to the effects of traffic on breeding birds and nest mortality (Kuitunen et al. 2003).

Here we test the hypothesis that fast and frequent traffic keeps parents away from nests more than do less heavily used roads and we predicted negative impacts of traffic density on reproductive success. This study was conducted using nestboxes placed in hedgerows along roads which were characterized by two different volumes of traffic and average vehicle velocity. We compared reproductive success in these hedgerows with those of undisturbed hedgerows. Great Tits Parus major were the study species.

The study took place in April–June 2006 and 2007 in Mols Bjerge National Park in Eastern Jutland, Denmark, using 150 Great Tit nestboxes mounted at 50 m intervals in hedgerow trees. The nest boxes were distributed equally between three different areas subject to different levels of disturbance: (1) Hedgerows between crop fields with no human disturbance; (2) hedgerows between crop fields and dirt roads with slow and infrequent traffic (10–30 km/h; 30–50 vehicles per day); (3) hedgerows between crop fields and tarmacadamed roads with fast and frequent traffic (60–80 km/h; 1500–2000 vehicles per day) based on traffic density data from the Municipality Road Administration.

Nestboxes were visited at least once a week, at similar times of day, during the laying season. The date of appearance of eggs in the nest was recorded, together with the clutch size at each visit. First-egg date was calculated based on the assumption that one egg was laid each day. Nest visit frequency and the occurrence of gaps in the laying sequence were such that errors in calculating fifirst-egg dates were trivial, as found by Cresswell & McCleery (2003). After the appearance of eggs, nests were followed daily to determine full clutch size and the date at which incubation started. Any dead nestlings and non-hatched eggs were recorded at each visit. Where the exact age was possible to determine, 15-day-old pre-fledging young were weighed using a 20-g Pesola spring balance and counted to determine the breeding success for each nestbox. Finally, fledging age was recorded.

Data from 2006 and 2007 were pooled to maximize sample size for statistical analysis. A one-way anova was used to analyse differences in the breeding parameters between the disturbance grades of the hedgerows.

There was no significant difference in the mean number of eggs laid per Great Tit nest between hedgerows with no disturbance, slow traffic and fast traffic (F _2,90 = 0.26, P = 0.77). In contrast, there was a significant difference in the number of fledglings between disturbance grades (F _2,90 = 6.50, P < 0.01), the result of a significantly lower mean number of fledglings in the nestboxes along paved roads with fast traffic, compared with those in hedgerows with no disturbance and along dirt roads with slow traffic (Tukey, P < 0.05) (Fig. 1). This was due to a pronounced difference between disturbance grades in the number of broods completely lost, with 50% mortality along paved roads compared to 14 and 13% along hedgerows with no disturbance and along dirt roads, respectively (Fig. 2, Table 1). Fledgling success, i.e. the number of fledglings as a proportion of eggs laid, was significant lower along paved roads with fast traffic, compared with the other disturbance grades (F _2,90 = 7.14, P < 0.01; Tukey, P < 0.05) (Fig. 2).

Figure 1. The average number of eggs laid per nest and the average number of fledglings per nest from nests in hedgerows alongside roads with three different disturbance grades. Zero values, i.e. complete brood failures, are included in the fledgling average. 95% confidence limits are shown.

Figure 2. Percentage of dead broods and fledging success from nests in hedgerows alongside roads with three different disturbance grades. 95% confidence limits are shown for fledging success.

Table 1. Great Tit nest breeding variables and number of nests that reached clutch, brood and fledging stages, classified according to hedgerow disturbance grade.

	Undisturbed	Slow traffic	Fast traffic
Laying date of first egg	26 April + 1.79 days	29 April ± 3.49 days	25 April + 1.95 days
Weight (15 days old)	15.78 ± 0.40 g	14.97± 0.65 g	15.43 ± 0.56 g
Fledgling age	17.97 ± 0.27 days	17.85 ± 0.35 days	18.0 ± 0.0 days
Number of clutches	35	30	28
Number of broods	34	29	28
Number of nests with fledglings	31	26	14

There was no difference between the disturbance grades for fledging age (F _2,42 = 0.08, P = 0.92), pre-fledging weight (F _2,174 = 1.86, P = 0.16) or egg laying date (F _2,90 = 1.99, P = 0.14) (Table 1).

The three different hedgerow locations in this study were selected to be equally suitable for breeding Great Tits, and we found no difference in laying date, number of eggs, pre-fledging weight and fledging age. We therefore suggest that the significantly lower number of fledglings and the high mortality of broods near roads with fast and frequent traffic were most likely caused by the death of one or both parent birds. If both parents are killed, the nestlings will starve and die. If only one of the parents die, and the other parent cannot compensate by increasing its foraging rate, the nestlings will almost certainly starve and die as well. Furthermore, if death of the female occurs while the young still require brooding, this is highly likely to cause a complete failure, because the male will not brood the young. It is known that high traffic volume along hedgerows has a marked impact on bird mortality based on roadkill rates (Orlowski 2008) and that traffic velocity seems to be related to bird mortality (Benítez-Lópes et al. 2010). A study of the seasonal distribution of roadkills in a Polish rural landscape showed that 20% of all roadkills in a year occur during May–June, i.e. the nesting period, when birds are very active in finding prey for their nestlings. The only period in which roadkills were higher was in July–August, which coincides with the appearance of young birds. Furthermore, 38% of the roadkills was found to be among species regarded as hedgerow specialists (Orlowski 2008).

A few studies reviewed by Erritzoe et al. (2003) suggested that in situations where hedgerows are close to major roads, birds fly higher and avoid getting hit by cars. This may be the case for non-resident birds flying or migrating over the hedgerows, but our personal observations in this study showed that birds mostly flew below 3 m when crossing the roads in search for food for their nestlings and thus were at high risk of being hit by vehicles, as confirmed by Orlowski (2008). The frequency of dead broods is similar to a study by Kuitunen et al. (2003), who investigated breeding Pied Flycatchers Ficedula hypoleuca in mature woodland crossed by a paved two-lane highway with speed limits of 80–100 km/h. They found that the number of dead broods was greatest close to the road and decreased with distance from the road.

For abundant birds and mammals, only a few percent or less of a population are killed annually by cars, and this loss is readily replaced by reproduction, especially in populations where density dependence operates (Forman 1995). Therefore roadkills may not be a problem for Great Tits at the population level. However, locally, hedgerows adjacent to roads with fast and frequent traffic can be considered ecological traps and a sink for the population. This has been documented for Willow Warblers Phylloscopus trochilus in a study in the Netherlands, in which road zones acted as sinks for males and where roads reduced the population size of Willow Warblers in the whole study area of 165 ha (Foppen & Reijnen 1994).

We found numbers of fledglings per breeding attempt to be significant lower adjacent to fast and frequent traffic and the most likely explanation is traffic-related mortality of the parent birds. In Denmark, road traffic has nearly doubled in the last 25 years (data collected from The Danish Road Directorate) and here the Danish Point Count Census has shown a significant demographic effect on common hedgerow species such as Yellowhammers Emberiza citrinella (Heldberg & Eskildsen 2010). This poses the question whether the increasing road traffic along hedgerows might be a factor contributing to the declines of some species which frequently use hedgerows for their nest sites.

ACKNOWLEDGEMENTS

We thank Tony Fox and Chris J. Topping for valuable comments and proof reading of the manuscript.