Twite questions and answers

Some time ago Paul Talbot posted the following on the blog in response to an update detailing some of the results of the fieldwork undertaken by Nick Wilkinson (and Judit) - see http://calderbirds.blogspot.com/2009/07/twite-update.html


Assuming that Twite being upland birds the almost certainty of a poor spring and summer in the South Pennines should not affect them that drastically one would have thought ? Is there any online data one can view on "normal" nesting success in the South Pennines? It seems rather important that surveys not only record what is happening to a species but why. If there is no data for this then it seems we are simply collating facts after the event and not attempting to change the factors involved. My thoughts are, is disturbance by walkers, sheep, farmers having any effect on nesting success, what foods could be in short supply during the key seasons of females devloping the eggs and once young are born. I am not trying to critical of the excellant work that been carried out so far but it would be useful for more details on the "whys" were published in such reports or are they ?

In response Nick W has sent the following and asked me to post it on here


Hi Paul

Thanks for your comments and apologies for the late reply.

We don’t know the ultimate reason for the decline of the Twite population in England, although three major reasons have been suggested. These are, the loss of saltmarsh wintering habitat; the loss of seed food sources in the breeding season; and the loss of suitable nesting habitat.

Twite can have multiple nesting attempts in one season but Andre Raine’s recent study in the south Pennines suggested that only a small proportion of pairs made more than one attempt after fledging a first brood, while he found no confirmed cases of second attempts from colour-ringed pairs. This contrasts strongly with a population studied in the Outer Hebrides. Additionally, his work on twite use of the supplementary feeding stations indicated that there is a shortage of natural seed food at the beginning and end of the breeding season. Thus, the low rate of repeat nesting caused by a lack of late-season seed food may mean that current productivity is insufficient to maintain the population.

Consequently, the focus of the Twite Recovery Project is to increase the availability of summer seed sources throughout the entire breeding season. However, it aims also to secure the suitable management of nesting habitat and to ensure the provision of suitable salt marsh habitat (they feed mainly on Salicornia) in managed coastal re-alignment schemes. Monitoring is an integral part of the project to assess the twite population response to the intervention work.

Three previous studies provide data on Twite nesting success in the south Pennines, only one of which is currently available online (Brown et al. 1995 The distribution, numbers and breeding ecology of Twite in the South Pennines of England. Bird Study 42, 107-121 http://pdfserve.informaworld.com/480048__912692041.pdf). This study analysed data from the BTO’s Nest Record Card scheme collected between 1944 and 1991. The other two studies collected data in 1989-1994 (McGhie et al. 1994) and 2003-04 (Andre Raine’s PhD study 2006).

A quick explanation about calculating nest success since it is not simply the percentage of nests monitored that are successful. Studies of nest success rarely, if ever, find every nest before egg-laying has started, most being found at the incubation or brood stages. Consequently, our sample of nests is biased towards those that have survived to this point since we will have missed those nests that had failed already. To correct for this bias, we take account of the period that a nest has been monitored until it is no longer active (fledged or failed), i.e. between the first and last visits to the active nest. This period, usually measured in days, is the length of time that a nest is exposed to failure, and is known as the ‘exposure period’. This then allows us to calculate the daily nest failure rate (number of nests failed divided by the total exposure period of all nests) which, when subtracted from 1, gives us the daily nest survival rate (failure rate + survival rate = 1). To calculate nest success, the daily nest survival rate is multiplied to the power of the length in days of the whole nest period from laying of the first egg to fledging (e.g. 30 days for twite with a clutch of 5 eggs). This method is commonly known as the Mayfield method.

Interestingly, all three studies recorded similar levels of nest success: 1944-91 = 58%; 1989-1994 = 48%; 2003-04 = 49% (all estimates calculated using a nesting period of 30 days). The errors around these three estimates of nest success overlap with each other indicating that there has been no significant change in overall nest success over the period of these studies. The Nest Record Card (NRC) data found that nest failure rates during the brood-rearing stage had increased over time, although there was no such trend for the whole nesting period.

Although the causes of nest loss are given for only one of these studies, together with our own monitoring, it suggests that the main cause of nest loss is depredation. This is unsurprising for an open-cup nesting species. Other causes include nest collapse (for those built above ground attached to live bracken stems), clutch infertility, weather and trampling/disturbance by livestock. Raine’s study found no evidence for an effect of disturbance on nest success, even though one of his study colonies was located around a popular footpath with some pairs nesting extremely close to the path.