I never thought that I would write a blog post with a title like that, but I read a very interesting paper recently in Bird Study - Volume 70, Part 1 - 2, February-May 2023 entitled Influence of nest box design and nesting material on ectoparasite load for four woodland passerines by Thomas Blunsden and Anne E. Goodenough. If you check nest boxes every year like I have since 1985, you too would find this paper very interesting. I'll have a go at summarising the findings of the paper, so bear with me!
I suppose the first question, is what is an ectoparasite, and what effect can they have? An ectoparasite is an organism that lives on the skin of a host, and from which they derive sustenance. And nest ectoparasites constitute a threat to the fitness and survival of chicks and parents. These parasites can cause anaemia, a lack of weight gain for chicks and weight loss for adults. The authors go on to say that some ectoparasites can also cause disease, either because they act as vectors, or because puncture wounds or scratches become infected. This can potentially reduce post-fledging survival of young, or post-breeding survival of adults. So, they can have quite a negative impact.
For passerines nesting in woodland in the UK, the two most important nest ectoparasites are Hen Fleas, and parasitic Blowfly spp. I had never heard of Hen Fleas, and was aware of Blowflies, but didn't really have any understanding of their relationship and impact on nesting passerines. What is important, is that both of these ectoparasites undertakes key stages of their life cycle within nest material alongside their avian host. Hen Fleas synchronise their reproduction with that of birds to enable them to complete two generations during the birds' nesting period. The parasitic Blowflies lay their eggs when the chicks are approximately 30% grown, so that the larvae can take direct meals of blood from the growing chicks, and then pupate within the nest. Because of the potential cost of these parasites on breeding success, Great Tits in Switzerland, in an experiment, actively chose nest sites without Hen Fleas, when they were avialable! When only infested nest sites were availiable, the laying of clutches was delayed, desertion was higher, and hatching success was lower.
The authors stated that a number of different parameters can influence the presence and abundance of nest ectoparasites. Bird species is important, as understandably some ectoparasites are host specialists e.g., Hen Fleas are particularly abundant in the nests of tits. Nest type plays a key role as well: open nests generally have a low ectoparasite load but a high predation risk, but birds using natural cavities and nest boxes tend to have a high parasitic load, but low predation risk. Within nest boxes, the fact that birds tend to use the same nest site over successive years can be a major determinant of ectoparasite load, especially for ectoparasites that pupate within the nest or use nest sites to overwinter.
Nest composition and the amount of nest material can also influence ectoparasites. The materials used to construct a nest varies between bird species, with climate conditions, geographic location, and local availability of materials also having an impact. Vegetative material, twigs, bark and dead leaves, is often used to form a structural layer to provide the shape of the nest, while dry grass, feathers, wool and animal hair can provide an insulative lining. I found it interesting that fresh plant material, such as green leaves, are also used by some bird species, possibly because they contain chemical aromatic compounds that fill the air of the nest box, and act as repellents or natural fumigants. A study by Rendell & Verbeek in 1996, found positive correlations between the volume of nest material used by Great Tits and the number of Hen Fleas.
Nest boxes are often used to provide a safe place to nest for certain bird species, and they provide the opportunity to monitor the populations of the bird species utilising them. However, the nest box construction, maintenance or management could affect ectoparasite loads e.g., older nest boxes could have higher ectoparasite loads if parasites overwinter in the box. Also, nest box dimensions can affect nest volume, as larger nests are built in larger boxes, and again this could affect parasite loads, which tend to increase with nest volume. The authors looked at the interrelationships between ectoparasite abundance, bird species, nest box design, amount of nesting material and nest composition.
They stated that their aim was to advance the understanding of bird and ectoparasite relationships and inform the optimization of nest box design for conservation and management, especially for declining woodland specialists such as the Pied Flycatcher, of which Gail and I are very interested in. The nest boxes studied were 'old' wooden nest boxes, 'new' wooden nest boxes of the same dimensions (like ours for Pied Flycatchers and Tree Sparrows) and 'new' deep wooden nest boxes (thought to be predator proof as the extra depth increases the distance between eggs/chicks and the entrance hole).
The research took place on a reserve in Gloucestershire, and before the start of the 2019 breeding season, the authors erected 100 new deep nest boxes, and 100 new wooden nest boxes of standard size alongside pre-existing old wooden nest boxes, giving 200 trees with a choice of two nest boxes on them. Territoriality would prevent both nest boxes on a single tree being used. All of the boxes had a 32 mm hole, the same size that we use for both our Pied Flycatchers and Tree Sparrows, and the deeper boxes were 55 mm deeper than the standard size. The rationale for using the deep nest boxes was to mitigate potential increases in predator pressure due to the imminent reintroduction of Pine martens, as well as the existing predation pressure by Grey Squirrels.
Post-fledging, 78 nests were collected for analysis. The nests were weighed, frozen at minus 18 Celsius to kill parasites and preserve them until the nests could be processed. The collected nests were then subdivided between species and nest box design. All the parasites were identified, as well as mosses used in the nest construction, and all tree material was recorded. In addition to this, any animal hair used in the lining of the nests was identified to species level.
I'm going to skip over all the data analysis bit, even though it is hugely important, as it's tricky trying to explain it an understandable way, and that's assuming that I understand all the non-parametric U tests and Poisson distribution, to name but a few. So, I am going to jump straight to the results and discussion.
Where there was a choice of two nest boxes of standard size, there was no significant difference between the number of new nest boxes selected compared to the number of old nest boxes selected. However, when there was a choice of either a deep nest box (predator proof?), or an old nest box of standard dimensions, there was a significant avoidance of deep nest boxes. Given that both new nest boxes of standard dimensions and deep nest boxes were added immediately before the breeding season, it suggests that the dimensions of the nest boxes, rather than age were driving the bird's choice. Very interesting!
In their 'discussion' the authors reported that the abundance of Hen Fleas and Blowfly in the nests of the cavity-nesting bird species that they studied, was influenced by bird species, amount of nesting material, and nest composition. Both ectoparasites were found at their highest loads from deep nest boxes, nests from old nest boxes had intermediate loads, and nests from new nest boxes had the lowest loads. Greater amount of nest material increased the abundance of both ectoparasite species.
It was interesting that the authors noted that nests in deep nest boxes had high ectoparasite loads, and the positive correlation between nest mass and ectoparasite abundance, is likely to be partly connected. They observed that birds nesting in deep nest boxes continued adding nesting material until the top of the enlarged nest was a similar distance to the entrance hole as it would be in a nest box of standard dimensions. A greater amount of nest material increases the habitat available for ectoparasites!
Also, the difference between old and new wooden nest boxes showed that the former had a significantly higher abundance of ectoparasites compared to the latter. This could mean that the ectoparasite load increases with nest box age. Connected to this, was that the authors results suggested that cleaning the nest box was ineffective, perhaps because pupating hen Fleas, for example, are hard to remove if they are within the feather dust at the bottom of the box, rather than within the nest itself.
At the end of the paper, a couple of key recommendations are made, and they are ones that I will certainly consider in relation to our nest box scheme, and these are that boxes are regularly replaced to keep ectoparasite loads down, and that deep nest boxes are not used, as they result in larger nests, and are associated with increased loads of two ectoparasites. And the behaviour of birds in building up the nest structure in deep boxes, negates the reason for using them in the first place in terms of reducing predation risk, as the nest contents are within reach of mammalian predators.
I was particularly interested in the recommendation to regularly replace boxes, as I tend to replace mine based on their condition. What the authors didn't say, unless I missed it, is how old is an old box, and at what stage does the ectoparasite start to increase, whereby it is having a negative impact on the outcome of the breeding attempt. Mmm......
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