Parental Care in the Small Tree Finch Camarhynchus Parvulus in Relation to Parasitism and Environmental Factors
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Ibis (2021), 163, 137–149 doi: 10.1111/ibi.12845 Parental care in the Small Tree Finch Camarhynchus parvulus in relation to parasitism and environmental factors † † EILEEN HEYER, ARNO CIMADOM, CHRISTIAN WAPPL & SABINE TEBBICH* Department of Behavioural Biology, University of Vienna, Althanstraße 14,1090, Vienna, Austria The parental food compensation hypothesis suggests that parents may compensate for the negative effects of parasites on chicks by increased food provisioning. However, this ability differs widely among host species and may also depend on ecological factors such as adverse weather conditions and habitat quality. Although weed management can improve habitat quality, management measures can bring about a temporary decrease in food availability and thus may reduce parents’ ability to provide their nestlings with enough energy. In our study we investigated the interaction of parasitism and weed man- agement, and the influence of climate on feeding rates in a Darwin’s tree finch species, which is negatively impacted by two invasive species. The larvae of the invasive parasitic fly Philornis downsi ingest the blood and body tissues of tree finch nestlings, and the inva- sive Blackberry Rubus niveus affects one of the main habitats of Darwin’s tree finches. We compared parental food provisioning of the Small Tree Finch Camarhynchus parvu- lus in parasitized and parasite-free nests in three different areas, which differed in inva- sive weed management (no management, short-term and long-term management). In a parasite reduction experiment, we investigated whether the Small Tree Finch increases food provisioning rates to nestlings when parasitized and whether this ability depends on weed management conditions and precipitation. Our results provide no evidence that Small Tree Finches can compensate with additional food provisioning when parasitized with P. downsi. However, we found an increase in male effort in the short-term manage- ment area, which might indicate that males compensate for lower food quality with increased provisioning effort. Furthermore, parental food provisioning was lower during rainfall, which provides an explanation for the negative influence of rain on breeding suc- cess found in earlier studies. Like other Darwin’s finches, the Small Tree Finch seems to lack the ability to compensate for the negative effects of P. downsi parasitism, which is one explanation for why this invasive parasite has such a devastating effect on this host species. Keywords: compensation hypothesis, Darwin’s finches, food provisioning, insecticidal treatment, Philornis downsi, rain, weed management. Parasites impose strong selection on their hosts by evolution of effective counterstrategies in their impairing their growth, survival and/or reproduc- host (Tschirren et al. 2009), including strategies to tion (Price 1980, Loye & Zuk 1991, Lehmann compensate for the damage caused by the parasite 1993, Clayton & Moore 1997, Tschirren et al. (Clayton & Moore 1997, Lemoine et al. 2011). 2009). This selective pressure will favour the Chicks of altricial birds have limited defences against parasites, which in turn selects for parental parasite control or compensation (Agrawal et al. * Corresponding author. 1999, Tschirren et al. 2009). One option to com- Email: [email protected] pensate for the energy loss of the chicks is to †These authors contributed equally to this work. increase food provisioning (Johnson & Albrecht © 2020 The Authors. Ibis published by John Wiley & Sons Ltd on behalf of British Ornithologists’ Union. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. 138 E. Heyer et al. 1993, Tripet & Richner 1997, reviewed in Tschir- blood and body tissues of the nestlings (Dudaniec ren et al. 2009). & Kleindorfer 2006, Fessl et al. 2006b, O’Connor The parental food compensation hypothesis sug- et al. 2010), causing significant mortality in Dar- gests that parents may increase their food provi- win’s finch chicks ranging from 16% to 100% in sioning rates to compensate for energy loss caused some years, with strong variation within and by parasites (Tripet & Richner 1997, Knutie et al. between host species (Cimadom et al. 2014, 2016). Empirical studies that have tested the par- reviewed in Kleindorfer & Dudaniec 2016). Fitness ental food compensation hypothesis provide con- loss of the hosts depends on parasite load, body flicting results, with supporting evidence in some size and brood size (Dudaniec et al. 2007). species, e.g. Great Tits Parus major (Christe et al. P. downsi abundance per chick decreases with 1996), Eurasian Blue Tits Cyanistes caeruleus (Tri- increasing brood size, indicating a parasite dilution pet & Richner 1997, Hurtrez-Bousses et al., 1998) effect (Dudaniec et al. 2007). Compared with and the Galapagos Mockingbird Mimus parvulus other Darwin’s finches, the arboreal finches are (Knutie et al. 2016), but not in others, e.g. House especially affected by P. downsi due to smaller Wrens Troglodytes aedon (Morrison & Johnson brood sizes. A study by Cimadom et al. (2014) 2002), Darwin’s ground finches: Small Ground suggests that the high mortality in the Green War- Finches Geospiza fuliginosa, Medium Ground bler-Finch may also be explained by reduced food Finches Geospiza fortis (Koop et al. 2013, O’Con- availability due to weed management. The main nor et al. 2014, Knutie et al. 2016), and Eurasian habitat of arboreal Darwin’s finches on Santa Cruz Penduline Tits Remiz pendulinus (Darolova et al. Island is a cloud forest (Dvorak et al. 2012) that 1997). has been heavily invaded by introduced plant spe- One reason for these conflicting results could cies (Renterıa et al. 2012). The Galapagos National be that food provisioning depends on food avail- Park Directorate (GNPD) controls invasive plants, ability, and thus when food is scarce, parents can- leading to the temporary removal of the entire not compensate for the effect of parasites by understorey (Filek et al. 2018). One year after this increasing the rate at which chicks are fed. Food management measure, the breeding success of the availability is influenced by environmental factors Green Warbler-Finch was significantly lower in such as habitat quality and weather conditions areas that had been controlled recently than in (Mason 1944, Hollander et al. 2015, McNew et al. areas without weed management (Cimadom et al. 2019). Management measures that improve food 2014). Cimadom et al. hypothesized that the con- availability may help parents to counteract energy trol of invasive plants led to decreased food avail- loss caused by parasitism. However, management ability, thus reducing parents’ ability to provide measures can sometimes also temporally decrease their chicks with sufficient energy (Chiverton & food availability and thus reduce parents’ ability to Sotherton 1991, Moreby & Southway 1999, Boat- provide their nestlings with sufficient energy man et al. 2004, Morris et al. 2005, Giuliano et al. (Chiverton & Sotherton 1991, Moreby & South- 2018, Cimadom et al. 2019). An experimental way 1999, Boatman et al. 2004, Morris et al. study that aimed to test whether there is an inter- 2005, Giuliano et al. 2018, Cimadom et al. 2019). action between habitat quality and parasitism In the Galapagos Islands, long-term habitat revealed that Green Warbler-Finches were able to restoration programmes have been implemented to compensate for parasitism when arthropod abun- help recover the original habitat of Darwin’s dance was high but not when arthropod abun- finches endemic to the Galapagos Islands. One of dance was reduced due to control of invasive the long-term management programmes on Santa plants. In contrast, in the Small Tree Finch, breed- Cruz Island includes habitat restoration for arbo- ing success was very low independent of weed real Darwin’s finches (Woodpecker Finch Camar- management; only about 17% of the nests with hynchus pallidus, Large Tree Finch Camarhynchus chicks were successful, a rate considerably lower psittacula, Small Tree Finch Camarhynchus parvu- than in the Green Warbler-Finch (53% of nests lus and the Green Warbler-Finch Certhidea oli- with chicks were successful, Cimadom et al. vacea). This species group faces habitat changes 2019). Breeding success in the Small Tree Finch due to invasive plant species and also is strongly only increased when nests were experimentally affected by the invasive parasitic fly Philornis freed from parasites (Cimadom et al. 2019), indi- downsi. The parasitic larvae of this fly feed on cating high virulence of P. downsi in this species. © 2020 The Authors. Ibis published by John Wiley & Sons Ltd on behalf of British Ornithologists’ Union. Parental care in the Small Tree Finch 139 Moreover, Cimadom et al. (2014) found that nests removal of the understorey. To test the influence of Green Warbler-Finches and Small Tree Finches of rain on food provisioning behaviour, we com- that experienced more intensive rainfall during pared food provisioning rates during rainfall and in chick-rearing were less likely to produce fledglings. periods without rain. The authors hypothesized that parents are less We predicted that if Small Tree Finches com- active during intense rain and/or less efficient in pensate for parasitism, parents would show higher foraging, which may lead to lower food provision- food provisioning rates in nests that are parasitized ing rates. A negative influence of