In many species of raptors and owls, breeding pairs are faithful to a home range. In some resident species such as Red Kite, Buzzard, Golden Eagle and Raven, pairs can remain together throughout the year and for at least part of the day will be on their home range. In migratory species such as Honey-buzzard, Marsh Harrier and Osprey, pairs break up at the end of the breeding season. If they survive the rigours of migration, the majority of adults will return to the same location the following year and might pair up again. In long-lived species, the same pair of birds will typically occupy the same home range, and use the same nesting locations, over many years. For relatively short-lived species such as Hen Harrier, Sparrowhawk and Merlin, if the habitat remains unchanged, home ranges may be occupied by a succession of breeding pairs, with some individuals breeding with several partners over the course of their lives.
In general, raptor workers try to visit known home ranges and other suitable habitat several times before and during the breeding season with the aim to establish whether they are occupied or not. Please click on the following link for an overview of the best practice of monitoring occupancy of home ranges which we would encourage all raptor fieldworkers to follow.
Not all home ranges will be occupied by a breeding pair and there are a variety of reasons why a pair of raptors may not breed in a given year. For example, one or both birds may be immature (not yet of breeding age) or food may be in short supply. In some years, only a single bird may be present, caused by the death of or separation from a mate, or recruitment to a vacant territory, particularly if the population is undergoing expansion. Some home ranges may be occupied only when the population reaches a certain level and others stay vacant for long periods, sometimes because of human interference. Others may suffer irreversible habitat changes, or be subjected to increased unintentional human disturbance, e.g. through a change in land use activities, and may never become regularly occupied again.
Once occupancy has been established and an active nest located, the raptor worker monitors the breeding attempt. Please click on the following link for an overview of the best practice of monitoring breeding outcome which we would encourage all raptor fieldworkers to follow.
Ideally, all breeding attempts should be monitored from the start of pair formation to either breeding failure or the successful fledging of young. In a national scheme of this size, using data from a wide range of fieldworkers, this ideal is typically not achievable. For example, the timing of survey visits may bias estimates of raptor breeding success. Individual fieldworkers often cover large geographical areas, so first visits to different parts of the study area must necessarily be staggered. First visits to an area that occur later in the season may miss breeding attempts that failed early and overestimate nesting success. Non-breeding territorial pairs are common in raptor populations and can be easily overlooked, exacerbating the problem. Therefore, there is a bias in favour of detection of nesting attempts that have a longer period of survival. In particular, nests are most likely to be found and examined at the chick stage, placing a strong positive slant on estimations of breeding success, as failure is more likely to occur at the pre-lay stage or during incubation. In the early years of the SRMS, it was not always possible to determine from data submitted at what stage in the breeding cycle individual nests received their first visit, nor in many cases of nest failure, what caused this to happen. The nest recording spreadsheet, introduced at the start of 2005 (updated in 2009), and now widely adopted by raptor workers, is helping to address these issues, and raptor observers are encouraged to submit information on the dates that they carry out every monitoring visit. Recording of visit information will be further improved by the SRMS online data entry system which we will be launching in 2019.