For both ethical and scientific reasons, it is essential to minimise disturbance at nests while recording. The basic principle of the Code of Conduct, therefore, is that locating and observing a nest should not jeopardise its safety. Each observer must exercise a sense of responsibility, always putting the bird’s interests first. This section explains how observations can be conducted without putting nests at risk, and indicates things that should not be done.
There are three potential risks to the nest that must be eliminated during nest recording:
Before addressing these potential risks, it is important to note that nests are naturally subject to both desertion and predation and recorders who employ cold-searching techniques in areas of suitable habitat often find nests that have already failed before monitoring has begun. Desertion may arise through adverse weather, food shortage or the death of a parent, or when eggs fail to hatch because they are infertile. Avian and mammalian predators are also responsible for the failure of a high proportion of nests each year, as documented by an increasing number of projects using nest cameras and sensors.
Participants often fear that visiting nests will increase the probability of the attempt failing. While this is a widely-held belief, extensive reviews of scientific studies (Götmark 1992, Mayer-Gross et al 1997) indicate that visits to solitary nesting birds, particularly passerines, have little or no significant effect on the outcome of the breeding attempt as long as the guidelines provided here are followed. The degree of consistency of failure rates reported by nest recorders and in scientific studies, all employing different methods and visiting regimes, lends further weight to these results. Searches of completely undisturbed areas late in the season can reveal far more nests showing obvious signs of failure than those showing signs of successful rearing.
1. Plan your visits
The first step to avoiding the potential risks associated with nest recording is to keep the amount of time spent at the nest to a minimum. On finding a nest:
- Note the location accurately so it can be found quickly on subsequent visits.
- Have all the equipment you need to hand before you approach.
- Only visit when necessary to collect data. For example, once the completed clutch size has been established, the eggs do not need to be counted again before hatching.
2. Take care whilst searching
A surprising number of species nest on the ground (e.g. those of Lapwing, Skylark, Willow Warbler) or low down in rank vegetation (e.g. Blackcap, Whitethroat), and can occur at reasonably high densities. Check carefully where you position your feet and always search ahead before pushing through foliage.
3. Don’t leave any evidence
While predators are unlikely to deliberately follow tracks made in the vegetation (persecuted mammals such as foxes are likely to avoid, rather than follow, human scent) in the hope of finding prey, they will use them as passageways, potentially bringing them into closer contact with nests than would otherwise be the case. Damaging vegetation can also expose the nest to rain and wind.
- Avoid making tracks as far as possible, but if you do, be sure to push the vegetation back into place afterwards (a stick is helpful for this).
- On approaching a known nest, pick a route that disturbs as little vegetation as possible, taking big strides and stepping over or gently parting clumps of undergrowth.
- Use a different return route so that any vestige of a trail does not end at the nest. Begin the approach to roadside nests some yards away along the verge to avoid taking a direct track.
- Always replace any vegetation moved to see into the nest exactly as you found it.
- Do not visit a nest while a predator is in the vicinity.
- If faecal sacs are produced by chicks during your visit to the nest, dispose of them yourself at a distance.
4. Approach a nest carefully, but casually
In case parent birds are watching, it is good practice to approach nests casually, as if by chance, rather than directly and deliberately. Birds are then likely to regard you as harmless (much as they would a passing herbivore), and not as a predator intent on robbing the nest.
5. Check for sitting birds
Ideally, nests should be approached while unattended. This is a relatively simple task during the chick stage, when parents spend the majority of time away collecting food, but during incubation, the female will spend the majority of her time sitting on the eggs, at which time she must never be startled.
- Birds are most sensitive to disturbance at the start of the breeding attempt, during egg laying and very early incubation, when they have invested less in the current attempt and have more time to lay a replacement clutch.
- As you approach an open nest, try to see if a parent is sitting; a bird crouching low on a nest above eye level can very easily be missed. If this is not possible, gently tapping the vegetation on approach will alert the bird to your presence and it will slip off quietly.
- An equivalent approach can be taken for nestbox species by making a reasonably noisy approach and tapping the box before opening it, giving the bird the opportunity to leave. On opening the box, positioning your body across the aperture will reduce the risk of a remaining bird flying out, and the lid can then be lowered again gently.
- Do not approach nests of diurnal species late in the day, when temperatures are low, or when it is raining.
- Special care must be taken when visiting the nests of tree- and crag-nesting raptors and Raven soon after hatching, as small young can be inadvertently carried to the edge of the nest, or even out of it, by brooding birds as they leave.
Certain species are more prone to disturbance than others – there is a lot of regional and individual variation, but we would ask any observers thinking of monitoring the following species during incubation to nrs [at] bto.org (subject: Advice%20about%20nest%20monitoring) (contact the Nest Records Unit) beforehand so that we can put them in touch with an experienced recorder:
|Manx Shearwater||Take special care not to damage burrows when inspecting.|
|Storm Petrel||Sensitive to disturbance when laying and incubating.|
|Shelduck||Sensitive to disturbance when laying and incubating.|
|Black Grouse||Do not disturb leks in the mornings; search for nests later in day.|
|Grey Partridge||Sensitive to disturbance when laying and incubating.|
|Pheasant||Sensitive to disturbance when laying and incubating.|
|Sandwich Tern||Particularly sensitive to disturbance early in season; whole colony may desert for a new site. More tolerant at incubation onwards.|
|Puffin||Take special care not to damage burrows when inspecting.|
|Turtle Dove||Risk of desertion when laying.|
|Tawny Owl||Risk of desertion when laying. Also, eggs can also chill when female off, so visits best in late afternoon.|
|Long-eared Owl||Sensitive to disturbance during site selection and early incubation.|
|Grasshopper Warbler||Risk of trampling when searching nest site.|
|Wood Warbler||Risk of trampling when searching nest site.|
|Chiffchaff||Risk of trampling when searching nest site.|
|Willow Warbler||Risk of trampling when searching nest site.|
|Jay||Sensitive to disturbance when laying and incubating.|
|House Sparrow||Risk of desertion when laying.|
|Tree Sparrow||Risk of desertion when laying.|
|Bullfinch||Sensitive to disturbance when building.|
|Hawfinch||Sensitive to disturbance when laying and incubating.|
|Yellowhammer||Risk of desertion when laying.|
6. Take care at the nest
To avoid needless disturbance, remain at the nest for as short a period as is necessary to make observations. Avoid leaning on vegetation containing or surrounding the nest as this may damaging foliage, and the sudden jerk or release of a branch may tip the nest contents.
7. Nest contents can be moved but not picked up
Only licensed bird ringers are legally permitted to pick up and handle eggs and chicks for the purposes of taking measurements and ringing. Other observers are permitted to touch eggs and chicks within the nest cup in order to count them and to check whether eggs are warm, but this must be done very gently to avoid damage.
8. Watch out for large chicks
Parents become progressively less sensitive of disturbance as the nesting attempt continues, but the chicks’ awareness increases. As with sitting adults, large chicks must never be startled. When partially feathered, the young of some species may instinctively scatter from the nest on the close approach of a predator, a process known a ‘exploding’. This gives a chance of survival for at least part of the brood, but once out of the nest the survivors are vulnerable to chilling and to ground predators.
- Sylvia warblers, wagtails, Dunnock, Linnet, Song Thrush and Blackbird are the species most prone to explosion, which can occur any time after the chicks reach about 8-9 days of age (usually c. 6 mm of primary feathers extending from the sheath).
- Be particularly careful when approaching nests of these species at this time, avoiding any sudden movements toward the nest and keeping as far away as possible while counting the contents.
- Chicks may only legally be handled by licensed bird ringers possessing the appropriate endorsement.
Ibáñez-Álamo, J. D., Sanllorente, O. and Soler, M. (2012) The impact of researcher disturbance on nest predation rates: a meta-analysis. Ibis, 154: 5–14.
Götmark, F. (1992) The effects of investigator disturbance on nesting birds. Current Ornithology 9,63-104.
Mayer-Gross, H., Crick, H.Q.P. & Greenwood, J.J.D. (1997) The effect of observers visiting the nests of passerines: an experimental study. Bird Study 44, 53-65.