Blog by Dr Ian Johnstone, Senior Conservation Scientist, RSPB Centre for Conservation Science
Yellowhammers are strikingly coloured buntings that nest in well-grown hedgerows and shrubby habitats, and winter where they can find seed food, often in arable fields. They used to be a bird many were familiar with, but they have undergone an alarming decline over recent decades, particularly in Wales, where abundance has fallen by 62% since 1995.
Luckily, yellowhammers have been well studied in the arable and mixed-farming landscapes of lowland England, where evidence suggests their abundance is limited by winter seed food, particularly grain. This used to be widely available on cereal fields from after harvest and into the autumn and winter, and where outdoors livestock were fed, but as farming has become more efficient and livestock often wintered inside there is less and less left for the birds.
Thanks to agri-environment schemes farmers have been able to grow sacrificial cereals specifically to provide seeds for such birds during winter. However, cereal crops have a major limitation: the seed rapidly falls to the ground and is largely gone by late January. This results in a late winter ‘hungry gap’ – a period of general food shortage for seed eating farmland birds.
Photo: Yellowhammers declined by a significant 21% across the UK between 1995 and 2017.
Previous RSPB research identified a novel solution for the hungry gap that is best suited to mixed farming and livestock-dominated areas. The key crop in these landscapes is ryegrass rather than cereals. Ryegrass is usually grazed throughout the summer or harvested and turned into silage, often in the form of the familiar plastic wrapped big bales, that are fed to stock during winter.
But ryegrass has a secret. If you protect it from grazing and mowing it produces abundant seed which has the useful property (for birds) of remaining attached to the plant late into the winter, and even when the stems collapse making them accessible to birds like yellowhammers. What we don’t know is whether providing plots of seeded ryegrass, in combination with sacrificial cereals, is enough to fill the late-winter hungry gap and reverse the long-term population decline. So, with funding from Defra we set up a replicated landscape-scale experiment to find out.
Photo: Unharvested ryegrass seed heads can provide food for Yellowhammers in late winter. By RSPB
New experiment with seeded ryegrass for yellowhammers
Our study took place over three years in North Wales, where yellowhammers occur in landscapes dominated by sheep farming.
We set up three experimental treatments;
Weedy and seed-rich habitats were rare in the wider landscape.
The extent of seed crop delivery matched or was close to recommended values for supporting seed eating birds based on other studies.
Sacrificial cereals held abundant seed in October but were heavily depleted by January. Ryegrass (and especially the more vigorous Italian species) also produced abundant autumn seed but retained much more of that seed into January.
Yellowhammers and reed buntings fed on both the cereal and the ryegrass crops during autumn, but usage of ryegrass plots continued much later into the winter. To see how yellowhammers responded to different source of food we captured birds and collected faecal samples. We found the heaviest yellowhammers captured on our seed plots were those that had more grass seed remains in their droppings suggesting this was a good food source. Surveys of wider landscapes around our plots showed that farm-yards and gardens were the only other habitat used by feeding birds (where livestock feed and garden bird feeding stations occurred), confirming the importance of seeded ryegrass in filling the late winter hungry gap.
However, when we compared changes in yellowhammer breeding population size among our three treatment types after three winters of providing seed plots, we found that populations had continued to decline regardless of the presence, type, extent and yield of the seed crops we provided.
This suggests that filling the late winter hungry gap using sacrificial cereals and seeded ryegrass together was still not enough and something else was limiting populations in north Wales.
Photo: In Wales, ffridd habitat mosaics can provide good nesting areas for yellowhammers. By RSPB.
One potentially limiting factor is the condition of the breeding habitat. Our breeding surveys showed few pairs nested in hedgerows (which were scarce), with most choosing steep rocky marginal upland habitat mosaics of bracken, gorse and grass known in North Wales as ffridd. This habitat mixture offers rather low-quality grazing, and consequently gets under-grazed in many parts of Wales (there are no ffridd management options in the Glastir agri-environment scheme). Natural succession towards scrub and woodland increasingly renders these habitat mosaics unsuitable for nesting yellowhammers.
Our study demonstrates that seeded ryegrass can indeed fill the late-winter hungry gap. However, it also highlights two further important points: first, we cannot assume that nature conservation fixes developed in one landscape will necessarily work in others; second, that successful species recovery requires all limiting factors to be addressed. Recovery of the Welsh yellowhammer population may require the provision of winter seed crops and the restoration of livestock grazing to ffridd habitats.
Reference:
Johnstone, I. Dodd, S. & Peach, W. (2019) Seeded ryegrass fills the late winter ‘hungry gap’ but fails to enhance local population size of seed-eating farmland birds. Agriculture, Ecosystems & Environment 285, https://doi.org/10.1016/j.agee.2019.106619