How do ticks affect Golden Plover chicks? By utilising data from an existing study, David Douglas and James Pearce-Higgins have discovered that Golden Plover chicks that carry more sheep-ticks Ixodes ricinus have a lower chance of survival. Their findings are written up as a paper in Bird Study. The work is only based on a small sample and the data don’t identify the mechanism that leads to increased mortality but, given the current interest in the biological effects of ticks, the findings are interesting.
Costs of carrying ticks
Carrying ticks has three potential effects on wader chicks
- Ticks suck blood, which could be costly.
- Ticks can introduce diseases, via tick-borne bacteria and viruses.
- There may be effects on feeding efficiency, via impaired vision, hearing etc.
Sheep ticks act as vectors for a variety of pathogens, including the louping ill virus (LIV) which can aﬀect a range of domestic and wild mammals, as well as wild birds. LIV is known to cause high mortality of Red Grouse chicks but there has been no previous assessment of the eﬀects of sheep ticks on other moorland birds, such as breeding waders. It should be noted that wader chicks eat ticks – so they are not ‘all bad’.
All sorts of things could influence the probability of tick infestation in birds:
- Ticks may be able to survive better in warmer conditions.
- Tick numbers can be affected by the number of mammalian hosts.
- Mammal to bird transfer could be affected by land management and habitat structure.
Ecological interactions between ectoparasitic ticks and waders are not well understood. Given possible increases in tick abundance with climate change, the authors of the new study felt that it would be useful to test whether ticks have detectable eﬀects on the Golden Plover chicks that carry them.
Spotting an opportunity
The Golden Plover chicks that provided the data used in this paper were caught as part of a wind farm study at Gordonbush in northern Scotland, a site made up of 33 km2 of blanket bog. There is more information about the study in these two papers:
- Negative impact of wind energy development on a breeding shorebird assessed with a BACI study design. Alex Sansom, James W. Pearce‐Higgins & David J. T. Douglas
- Relative importance of prey abundance and habitat structure as drivers of shorebird breeding success and abundance. David J. T. Douglas & James W. Pearce‐Higgins
Tick numbers on Golden Plover chicks were collected at the time of ringing (within 24 hours of hatching) and during subsequent recaptures. Recapture was facilitated by locating tagged birds using radio-location. On each capture, chicks were weighed and the number of ticks visible on the bare parts of the head (around the eyes and bill) were counted. Most ticks attach themselves to the bare parts of the head and neck.
Variation in tick loads on Golden Plover chicks
The number of sheep-ticks found on these Golden Plover chicks was higher than those previously reported for waders but were within the range of those found on Red Grouse on moorland. Previous wader studies had been focused upon areas with sheep, which were routinely treated to reduce tick infestations. In the current study it was found that:
- 90% of chicks were carrying ticks, with between 1 and 12 ticks being found on each affected Golden Plover chick. The highest tick-load was found in mid-age chicks.
- Tick loads were higher during periods with warmer maximum temperatures and when chicks were estimated to have moved through taller vegetation between recaptures.
- Chick growth rates were depressed by high tick-loads, especially when temperatures were warmer.
- Of the 21 chicks, 4 fledged, 13 died and the outcomes for the other 4 were unknown. Half of the deaths appeared to have been due to predation and half to starvation/exposure.
- Chicks that were heavier (for their ages) were more likely to survive. Those with higher tick loads (for their ages) were less likely to survive.
With the small sample size, it was not possible to detect a correlation between tick load and chick growth rates but low survival was correlated with high tick-loads. This had not previously been documented for waders.
Implications for wader conservation
Gordonbush is an area where there is no grazing by domestic animals so the likely mammalian tick-hosts are Red Deer in particular and also Mountain Hare. The correlation between warmer weather and tick numbers, found in this study, could be explained by increased tick activity, while the link to taller vegetation may well be explained by ticks seeking damper microhabitats. In their discussion of the results, the authors suggest potential ways that ticks and waders, of different ages, might interact. Anyone looking to expand the work, in order to understand the mechanics of tick infestation, is likely to spend more time looking at ticks than waders!
The authors of this paper were not able to test for the presence of disease (such as LIV) in their Golden Plover population but this is a plausible cause of the increased probability of mortality. In a study in Yorkshire by Newborn et al. (2009), no evidence was found of LIV in wader chicks, whereas it was present in 3.6% of a sample of Red Grouse chicks at the same sites. Newborn and colleagues report that a single Eurasian Curlew chick has previously been recorded to be seropositive for LIV. In the Newborn study, the lowest incidence of ticks among waders was in Lapwings (6% of broods), followed by Golden Plover (47% of broods had ticks) and Curlew (91% of broods). There is a hint, in these data, that ticks might more commonly attach themselves to wader chicks that are found in taller vegetation.
Despite high tick loads on chicks, and the correlation with lower chick survival, the overall percentage of Golden Plover chicks known to ﬂedge in the Gordonbush study (19%) is comparable with other studies. Perhaps ticks are only causing the deaths of chicks that would have died anyway?
The authors suggest that no case should be made for tick-control, to help breeding waders, until it is clear whether tick-based chick mortality limits Golden Plover and other wader populations on moorland. In the paper, they argue that previous attempts to reduce moorland tick abundance and tick loads on Red Grouse have failed to detect convincing evidence of improvements in grouse survival, breeding success or post-breeding densities.
A range of methods have been deployed in attempts to reduce moorland tick abundance and tick loads on Red Grouse, including reducing densities of mammalian tick hosts (Mountain Hare and deer) and deploying acaricides (as used in sheep dips). See this link to material from GWCT on ticks and Red Grouse.
Work at a single site over two years appears to have documented a level of tick infestation in Golden Plovers that is associated with chick mortality. It is not clear how chicks are being affected, particularly given that there is insufficient evidence thus far that ticks affect chick growth rate.
The authors collected the data analysed in this paper for other studies – the focus was not on tick effects – and they hope that funding might be found for future research focusing upon the associations between ticks and waders, other birds and other animals. Until that happens, it would be useful if shorebird biologists who repeatedly handle wader chicks, in order to measure growth and survival rates, could routinely record the presence or absence of ticks.
Given that warming temperatures could lead to increased tick abundance, this seems to be a good time to discover more about tick behaviour, the importance of ticks as a food source for wader chicks and whether tick-loads are reducing growth rates and fledging success in other wader species.
This research is published in the BTO journal Bird Study. Click on the details below to link to the full paper:
Variation in ectoparasitic sheep tick Ixodes ricinus infestation on European Golden Plover chicks Pluvialis apricaria and implications for growth and survival. David J. T. Douglas and James W. Pearce-Higgins. Bird Study. June 2019.
WaderTales blogs are written by Graham Appleton, to celebrate waders and wader research. Many of the articles are based on previously published papers, with the aim of making wader science available to a broader audience.