Annual and large-scale variation of breeding output of Greylag geese (Anser anser) in Iceland

Markmið verkefnisins var að afla gagna um breytileika í varpárangri (varptíma, fjölda eggja og fjölda uppkominna unga) grágæsa milli mismunandi landshluta og mögulega milli ára. Þessar upplýsingar gagnast fyrir nýtingu og verndun grágæsastofnsins. Á árunum 2012-2013 voru 360 grágæsahreiður rannsökuð og þættir sem hafa áhrif á varpárangur mældir. Að auki voru 888 grágæsafjölskyldur taldar til að rannsaka breytileika í ungafjölda milla landsvæða. Mikill munur var á upphafi varptíma milli landsvæða og byrjaði að meðaltali 30. apríl á Vesturlandi og Suðurlandi, en hófst mun síðar á Norðurlandi (10. maí) og seinast á austurlandi (20. maí). Urpt var nokkuð breytileg milli svæða og reyndist minni urpt á Austurlandi en á Vesturlandi og Suðurlandi. Breytileiki var lítill í rúmmáli urptar milli landshluta og var það óháð ári. Á Austurlandi voru að meðaltali fæstir unga á par en ungar á par voru flestir á Vesturlandi og Norðurlandi. Breytileiki í mældum þáttum var tiltölulega lítill milli ára en þegar heildarmeðaltöl voru skoðuð reyndist aðeins vera marktækur munur á meðalfjölda unga á par og reyndust þeir vera færri 2013. Líklegt er að munur milli landshluta og ára tengist hitastigi en skilyrði til varps virtust almennt hagstæðari á Suðurlandi og Vesturlandi. Breytileiki í æxlunarárangri grágæsa og nýting og vernd grágæsastofnsins er rædd.

In this study differences in the breeding output of greylag geese across Iceland were assessed over two years. The aim was to produce a large-scale comparison of parameters that relate to breeding output (timing of breeding, reproductive investment and brood sizes) and identify large-scale variation in these within Iceland and possibly between years. During the two years of sampling a total of 360 Greylag goose nests were visited across Iceland and factors relating to breeding output were measured. In addition 888 Greylag goose pairs were surveyed to estimated large-scale variation in brood sizes. Incubation phenology varied significantly between parts of the country and started on average on the 30th of April in West- and South Iceland, but significantly later in North Iceland (10th of May) and East Iceland (20th of May). Clutch size did vary between research areas as East Iceland had a smaller clutch size than South and West Iceland. Little to no variation was found in clutch volume between areas and it was independent of year. East Iceland was found to have the smallest brood size on average while West and North Iceland had the largest brood sizes. Annual variation was relatively little and when annual means for the whole country were inspected the only significant difference found was in the mean number of goslings, where brood size was smaller in 2013. Regional variation in start of incubation is most likely related to mean temperatures as the south and west parts of the country have the most temperate climate. It is therefore likely that South and West Iceland, generally have more favourable breeding conditions. Further potential causes for differences in breeding output between regions in Iceland are discussed and what implications this has for a sustainable utilization of the Greylag goose population.